Bluejacket Boats

Planing boat theory

First, a disclaimer.  I do not consider myself an expert on the subject of boats planing, but more of an informed amateur with an engineering background who is interested in understanding how boats skim along on top of the water rather than plowing along through it.  There might need to be a definition of planing but I have never found a consensus agreement among different designers, writers or other experts on just what constitutes the act of planing.  Many books use speed/length ratios to determine if a hull is planing or not, but that is not satisfying since it only holds for boats that have nearly the same characteristics such as hull shape, weight, etc.   For instance, at one end of the hull shape spectrum, the 16’ Hobie Cats and other multihulls can go fast enough to be considered planing by the speed/length definition, but they are achieving practically no lift from dynamic forces and are not even capable of planing on their knife blade hulls.  At the other end of the spectrum, racing monohulls, step bottom hulls and three point hydroplanes go very fast and their speed/length ratio is meaningless.  This last example demonstrates the point that beam of the planing surface is a far greater contributor to dynamic lift than waterline length.  This follows in the same way that the luff length is more important in determining aerodynamic lift of a sail than the chord of the sail.

Although the flow diagrams shown in most books to illustrate planing phenomena may lead to the correct mathematical answer, I think they might obscure the physical events and inhibit understanding of the actual physics.   They always show water flowing under a flat plate fixed at a positive angle of incidence to the water surface.  I am not offering any new or different physics here, only a different way of looking at the same phenomena that may make it easier for the non-professional to understand it.  The usual path shown in the books is to look at the water from the aspect of an observer in the boat.  This makes it appear that the water is flowing under and past the hull. In reality, the water is doing no such thing.  Here, we look at the same actions from the aspect of an observer in the water.  The correct conclusion may be reached from either point of view but I think looking at the actual motion of the water makes it easier to understand the physics.  Here we will go to Issac Newton’s fundamental principles of motion and try to think like a water molecule.  Phil Bolger talks about a “sea of peas” and I think that may be close to what I am referring to.  Of course, the water does not consist of peas but it may help to look at it that way.  In any event, the water is just sitting there at rest until the inclined plate comes along.  The moving plate hits the water and accelerates it in a vector normal (perpendicular) to the plate surface.  This is down and forward.  This motion makes room for the inclined plate to pass and the resulting momentum imparted to the water near the plate surface also makes the nearby water move down and a bit forward also, depending on the angle of incidence or trim of the flat plate.  As the plate passes, the water particles rebound and set up damped oscillations that we see as waves.   It also explains how the forward momentum imparted to the water by the inclination (trim angle) of the plate causes the surface of the water to rise to meet the plate at the forward edge.  Water reacts to a stone dropped into the water by generating waves in much the same way.   Of course, a boat is moving so the wave pattern is different but the forces are the same.

Looking to Newton again, it’s clear that the force imparting momentum to the water particles must have an exactly opposite force vector on the plate.  This is the dynamic force vector of planing, also normal to the plate surface.  This dynamic force vector is exactly equal and opposite to the force imparted to the water and may be resolved into the vertical lifting force and the normally smaller horizontal drag force . Efficient planing therefore becomes the effort to accentuate the former and minimize the latter forces.  Because the water particles hit by the leading edge of the plate are already moving away by the time the next part of the plate arrives, momentum added to the water will therefore be reduced the further aft we go.  To me, this explains the pressure diagrams shown in books although I think they are somewhat in error too.  They usually show the magnitude of pressure asymptotic to, or nearly so, to zero at the trailing edge.  That is not necessarily true although the shape of the hull bottom is all-important in this respect.  The pressure may still be substantial right to the aft edge, beyond which, it must drop to zero.  Addition of trim tabs or hull shape may introduce significant lift at the aft edge or transom.

The downward momentum of the water underneath the plate continues for a short distance aft of the plate and then rebounds with a damped oscillation motion.  These are the standing waves that follow along at the same speed as the boat.  Behind a flat bottom hull, these standing waves follow along directly behind the boat.  Now, in V bottom boats, momentum imparted to the water also has a sideways component vector and the waves move more to the side from directly down in direct proportion to the deadrise angle. The energy consumed in pushing the water sideways by the angle of the V  (deadrise) makes the V bottom boat a less efficient planing hull than the flat bottom one. There are, however, many good reasons for choosing a V bottom hull over a flat one.  Someone experienced in observing these wave patterns might infer the angle of deadrise just from looking at the waves generated by the boat.

As an aside, a look at waves generated by a moving boat is interesting.  It is easy to observe that the faster a planing boat moves, the lower the amplitude of the waves it generates.  How can this be?  The generation of these waves by a planing boat represents work done in supporting the weight of the boat.  Work done has an element of time and is not a static measure.  Provided the boat is planing, the work done on the water in supporting the fixed weight of the boat in a dynamic sense must be approximately the same over the same interval of time no matter what speed the boat is running.  The “approximately” takes care of the fact that there may be a different amount of the boat weight supported by buoyancy at the different speeds.  It takes less time for the boat to move a given distance at higher speed and so the work done in supporting the boat per unit of distance must be less at high speed than at lower speed.  Therefore, it follows that waves made by a boat traveling at high speed will be of lesser height, carrying lesser energy,  than the same boat planing at a lower speed.

That term, “dynamic lift ”, is all-important and, to be called planing, a boat should be getting a significant portion of its lifting force from dynamic sources in addition to the buoyancy force .  Just what portion of lift should be dynamic in order to define a boat as planing, I don’t know, but if the percentage is greater than 50%, we should be safe in saying that the boat is in the planing mode.  I think the planing mode definition should hold well below 50% but don’t know where, or even if, a specific percentage should be chosen.  In any event, such a definition would be far more satisfying than the usual ones of speed/length ratio, etc.  We should not care a fig what it is called anyway, but rather expend our efforts in improving the planing ability of a boat intended for that service.

Now, given two boats of similar size, hull-form and weight, etc., the speed/length ratio can be used to compare their planing efficiencies.  I hope this is not too obtuse a point but it is a bit like the speedometer in a car.  It can measure how fast you are going and can be used to describe whether a car is fast or slow, but has nothing to do with how that speed was achieved. This bit of analog reasoning of the basic physics of planing satisfies me much more than pages of esoteric mathematical formulae and calculations.  Of course, attention must be given to mathematic calculations to arrive at a satisfactory design but such calculations seldom create an understanding of the nature of the phenomena or lead to innovation.

Many readers will have noticed that there has been no mention of “hull speed” in the above analysis.  That is a deliberate omission. I think that dwelling on hull speed formulae as an understanding of boat motion has been a great deterrent to innovative thinking.  Such a formula is only useful in thinking about a narrow range of boat shapes.  In particular, boats that are relatively heavy with curved or compound surfaces on the hull bottom.  In such boats, the value of the “constant”, which is usually given as about 1.34, is a measure of the speed potential of that particular boat.  For other similar boats, the “constant” must be changed, indicating that it is not really a constant at all.  For other than purely displacement or semi-displacement boats, hull speed is only of marginal interest.

To illustrate the limitations of the hull speed formula, many boats are able to actually get on plane before the boat even reaches its “hull speed” as determined by its waterline length.  The modern lightweight “sleds” as found in the around the world races zip right past their “hull speeds” without the slightest nod to the formula. Monohull powerboats can also meet this criteria with careful design and attention to weight.  Given a good design, it is the bottom loading in pounds per square foot that determines the ease with which it will get on to plane.  Lesser values of bottom loading is better in this regard and explains why the great majority of commercially available powerboats expend so much power and fuel in getting onto plane.  They are generally too heavy for their bottom area to allow easy transition from displacement speed to planing speed.  They require a high trim angle to generate adequate lift for planing and thus also need high power to overcome the higher drag generated by the high trim angle.  It is a spiral that has only been overcome by the availability of high power engines and cheap gas.  Such boats are only happy at very low or very high speed and are dinosaurs to many designers, including me.  Length does play an important role in that the longer boat will have a higher hull speed, which may allow it to get on plane before that speed is reached so that the pilot of such a boat will be completely unaware that such a “hull speed” limitation exists.

Most texts describing hull speed will mention that hulls with a length/beam ratio of 10 or more are not limited by the rule. Most, but not all, multihulls fit in this category and are capable of speeds far in excess of theoretical hull speed and never plane while making such speeds. Such explanations evade the wide range of length/beam ratios between the “normal” 3:1 ratio of most monohulls and 10:1 or more of multihulls.  I find the whole hull speed and speed/length thing unsatisfactory when talking about boats capable of planing.  It may be useful in some circumstances but often does more harm than good.

To look at a specific example of how the above reasoning was applied, we will turn to a boat of my own design which is the featured boat of this website and with which I am most familiar.  This is the Bluejacket 24, a small lightweight pilothouse cruiser intended for use on inland waters.  It is primarily a planing boat that will plane with crew and full cruising gear at low power and readily hold plane down to about 10 or 11 mph.  In light trim and ideal conditions with only one aboard, the Bluejacket 24 will actually clear the transom and appear to start to plane at about 9 1/2 mph. I will not venture to say that the boat is planing at this speed, but it is certainly beginning to do so.  Throughout the speed range from 10mph to beyond 20mph, the attitude or trim angle of the Bluejacket 24 remains below 2 degrees and hardly changes at all.  All increase in trim angle is in the bow as the stern does not sink or squat at all.  As any regular powerboater can verify, the lack of stern squat is a desirable, although very rare, feature.  A photo series of  this performance can be seen in the Gallery page under Bluejacket 24.

In designing a boat, you can select any parameter, or set of parameters, that is/are most important and fix those, letting the other parameters be variable.  A small cruising boat is primarily a people carrier, so I chose to fix the physical parameters to suit the people comfort goals first. Therefore, length, beam and interior height are chosen to begin with.  From that, the displacement necessary to make those goals acceptable is calculated.  Higher displacement is detrimental to economy of operation is a planing boat, so that consideration is wedded to all other design decisions.  To satisfy the people comfort goal, the aft chines must be immersed to provide for adequate lateral stability.  This dictates the maximum aft deadrise that can be used in order to keep the chines immersed.  Forward deadrise must be greatly increased to make entry into chop and waves easier — the comfort thing again.  Of course, this is too simplified and several laps around the specifications must be made before they are mutually complementary and individually acceptable.  In a boat intended for other uses, say offshore, one would likely start with deadrise as one of the desirable fixed parameters.

In the above boat the displacement arrived at, with 2 crew and day trip gear, is 2500 lbs on a WL beam of 78” and an aft deadrise of 10 degrees.  The aft buttocks are straight (monohedron) from the transom to about station 6 and rise forward to a sharp entry with deadrise of 26 degrees at a point 25% aft of the waterline entry.  This has proven to satisfy all the design goals and handles a reasonable amount of chop and waves with ease.  The local North Carolina waters are known for their nasty nature and these parameters appear to be at or near an optimum compromise between economy, performance and comfort.

I generally avoid concave sections near the bow as structurally weak and hard riding.  Also avoided is any hint of longitudinally convexity in the aft planing bottom as a performance robber. There are exceptional cases like the Carolina Sports Fisherman boats that do use a bit of rocker near the stern to hold their sharp bow high when running in large waves.   The only place I see concavity acceptable or desirable in a planing hull bottom is in keel, skeg, chine flat fairings or splash rails where the good properties outweigh the negative.  In smooth water lake boats, like classic runabouts, the negative properties of bow section concavity are not always apparent since such boats are for pleasure use only and not required to operate in poor water conditions.

I am a retired electrical engineer and strictly an amateur boat designer.  I work from paper sketches, calculators, testing models, all the good books I have found, discussions with other more and less experienced designers and my own reasoning.  I have never gotten into computer design programs.  As a result, my methods may appear antiquated, but they are the way I like to do it.  The thinking and visualizing parts are what I like most about designing.  Being only semi computer literate, I would like to learn something about that end of the work but have never devoted the necessary to time to do it.

Finally, here is another disclaimer.  There are many established boat design parameters such as prismatic coefficient, displacement/length ratio, power/weight ratio, center of gravity (in all coordinates), center of floatation as well as many others that are important to designers of boats.  I do not intend that this discussion should diminish the importance of any of these or other mathematical operations and my only purpose here is to aid the understanding of the physical forces that allow a boat to plane or allow one boat to plane better than another. Trim angle of planing boats

One of the characteristics of powerboats that often puzzle boat operators is the trim angle of their boat and what causes it to be the value it is. Trim angle is the fore and aft angle in a vertical plane that the boat presents to the water surface. It is dependent on many factors, some of which can be controlled by the operator and some that are built into the design of the boat. The trim angle when the boat is at rest is simply the balance between the weight distribution of the boat and buoyancy distribution of the immersed part of the hull. The center of longitudinal gravity (CG) and the center of longitudinal buoyancy (CB) will always coincide on a boat at rest. When one changes, like shifting the CG by walking forward or aft, the CB will automatically shift to align itself with the CG.

When the boat is underway this changes and the CG may no longer align with the CB. This is the result of dynamic forces resulting from the motion of the boat. As long as nothing moves in the boat, the CG remains fixed while the CB moves (usually aft) as well as being combined with dynamic lift to give a new center of vertical force.

In making the decisions about performance characteristics of the Bluejackets, I wanted to design for a specific range of trim angle for what I think are good reasons. One major goal of my design was to have a boat that planed at low speed so that cruising could be pleasant and economical in the desired speed range of 10 to 20 MPH. Anyone who has had much experience with small powerboats knows that this speed range is generally the worst from both comfort and economic perspectives. From reading my notes of planning theory, you know that I consider weight to be the main killer of the desired performance so I’ll not repeat all that here. Suffice to say that weight, or more accurately bottom loading in weight per unit area of hull bottom, is the bugaboo of far too many planing powerboats and I wanted to avoid that.

While I generally ignore the concept of “hull speed” as being irrelevant to planing boat design, it is still a fact that all boats do start out from rest as displacement craft before accelerating to their operating speed and hull speed may be important in getting onto plane in the first place. “Hull speed” is an empirical way to look at the fact that a displacement vessel has a real limit to the speed that can be achieved because of the waves that it generates. Planing boats are the way to go much faster than that limited speed until speed is limited by other factors. Hull Speed varies proportionally with the length of the boat so, quite simply, holding the bow down when starting out makes the boat longer than if the bow lifted more, thus increasing the “hull speed”.

In order to achieve planing speed, adequate dynamic lift must be gained to get the boat higher in the water which reduces drag. Dynamic lift is proportional to both speed and trim angle. We can either force higher lift with more speed by adding driving power or reduce the need for lift by reducing weight. Meeting our economic goals requires that we take the lower weight path. At low angles, increasing trim angle increases lift, so we must have some degree of trim to get any lift at all. Trim angle also affects the comfort level in the boat. Running at high trim angle allows the water to first hit the boat bottom where it is flatter and results in uncomfortable pounding. Holding the bow down allows the sharper bow to strike the water first and causes less pounding and more comfort. Pounding is not a trivial factor and makes some boats extremely people unfriendly.

For best efficiency, it is generally accepted that a trim angle of four to five degrees minimizes total resistance from the sum of wave making and frictional drag although deep V hulls have higher optimum trim angle of 6 degrees or more. I am mostly concerned with boats that have low aft deadrise of 10 degrees or so which is also consistent with light weight. Further, it is a fact that high deadrise implies greater weight. Low trim angle reduces wave making resistance while high trim angle reduces skin frictional resistance. The most efficient trim angle for a boat is that which offers the least drag of combined wavemaking and friction. To satisfy my goals, four to five degrees of trim is too high. Boats that run at this angle will usually have a far higher trim while getting onto plane. To repeat, deep V means greater weight which is unacceptable to our goals. Another negative to high trim angle is that forward vision is restricted over the bow. In my opinion some boats have dangerously limited forward vision.

It should be clear that there are many conflicting factors here and choosing the combination that provides the desired result is how a designer moves toward the goal.

Bluejackets are designed and built lightweight and have large bottom surface, both of which contribute to low bottom loading, requiring minimal dynamic lift to initiate planing. This translates to the need for only a low trim angle when getting on plane and less power to do so. Heavier boats always need a higher trim angle to get adequate dynamic lift which requires higher power to overcome the resultant higher wave drag. Anyone who frequents boat building/design forums is familiar with the anguished questions from buyers of such heavy and high powered boats that have been unable to get them to plane satisfactorily. It is a vicious circle that I want to avoid.

In addition to the above factors, Bluejackets have high lift surfaces in the form of wide chine flats added under the stern. These chine flats have a trim angle that is greater than the rest of the aft bottom surface which provides extra lift aft and tends to hold the bow down and the stern up. Lifting the stern up also minimizes the drag associated with powerboats that have a deep transom causing high drag at low speed.

It is not possible to design or build a boat that is optimized in all the desirable characteristics at the same time since many are mutually exclusive. Improving one is almost always detrimental to one or more of the others. The best that can be accomplished is to choose which characteristics are most important for the operating regime and conditions that the boat is intended for. Bluejackets are primarily people carriers intended to be used mainly in inshore waters. Speed is important but some top speed can be sacrificed if it allows better performance in the speed range the boat will be used most in and we deem most important. Low trim angle increases frictional drag at high speed but also aids getting onto plane as well as economic operation at lower planing speed. I consider this a good trade since Bluejackets achieve top speeds that I consider very acceptable for a cruising boat.

If the Bluejacket were intended for regular operation in the open ocean, I would prefer a deeper V, more seakindly, hull but that would demand much more weight, power, fuel use, cost as well as larger towing vehicle and higher fuel use there also. That is not to say that Bluejackets cannot handle rough water but, rather that they are not optimized for that. Bluejackets have been used in some very rough water conditions as well as having made offshore passages and brought their crews home safely. Passages such as the Alaska Archipelago, West coast of Florida and New Jersey Coast are no problem with competent seamanship. The hull shape is a compromise between smooth riding in chop, efficient and economic power requirement, low fuel use and high load carrying ability. One inevitable factor is that many builders have added more weight than the plans dictate but the large foot print of the Bluejackets has allowed this with little loss in performance.

To sum this up:

Trim angle of a planing boat will always be that which is necessary to provide the required lift for the weight of the boat and the speed at which it is traveling. Lower trim angle: more speed; greater planing surface area; CG further forward; lighter weight; lower aft deadrise. High trim angle: lower speed; smaller planing surface; CG further aft; greater weight; higher aft deadrise.

Tom Lathrop Oriental, NC

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My Cruiser Life Magazine

Basics of Sailboat Hull Design – EXPLAINED For Owners

There are a lot of different sailboats in the world. In fact, they’ve been making sailboats for thousands of years. And over that time, mankind and naval architects (okay, mostly the naval architects!) have learned a thing or two.

If you’re wondering what makes one sailboat different from another, consider this article a primer. It certainly doesn’t contain everything you’d need to know to build a sailboat, but it gives the novice boater some ideas of what goes on behind the curtain. It will also provide some tips to help you compare different boats on the water, and hopefully, it will guide you towards the sort of boat you could call home one day.

Table of Contents

Displacement hulls, semi displacement hulls, planing hulls, history of sailboat hull design, greater waterline length, distinctive hull shape and fin keel designs, ratios in hull design, the hull truth and nothing but the truth, sail boat hull design faqs.

white yacht in body of water during daytime

Basics of Hull Design

When you think about a sailboat hull and how it is built, you might start thinking about the shape of a keel. This has certainly spurred a lot of different designs over the years, but the hull of a sailboat today is designed almost independently of the keel. 

In fact, if you look at a particular make and model of sailboat, you’ll notice that the makers often offer it with a variety of keel options. For example, this new Jeanneau Sun Odyssey comes with either a full fin bulb keel, shallow draft bulb fin, or very shallow draft swing keel. Where older long keel designs had the keel included in the hull mold, today’s bolt-on fin keel designs allow the manufacturers more leeway in customizing a yacht to your specifications.

What you’re left with is a hull, and boat hulls take three basic forms.

  • Displacement hull
  • Semi-displacement hulls
  • Planing hulls

Most times, the hull of a sailboat will be a displacement hull. To float, a boat must displace a volume of water equal in weight to that of the yacht. This is Archimedes Principle , and it’s how displacement hulled boats get their name.

The displacement hull sailboat has dominated the Maritimes for thousands of years. It has only been in the last century that other designs have caught on, thanks to advances in engine technologies. In short, sailboats and sail-powered ships are nearly always displacement cruisers because they lack the power to do anything else.

A displacement hull rides low in the water and continuously displaces its weight in water. That means that all of that water must be pushed out of the vessel’s way, and this creates some operating limitations. As it pushes the water, water is built up ahead of the boat in a bow wave. This wave creates a trough along the side of the boat, and the wave goes up again at the stern. The distance between the two waves is a limiting factor because the wave trough between them creates a suction. 

This suction pulls the boat down and creates drag as the vessel moves through the water. So in effect, no matter how much power is applied to a displacement hulled vessel, it cannot go faster than a certain speed. That speed is referred to as the hull speed, and it’s a factor of a boat’s length and width. 

For an average 38 foot sailboat, the hull speed is around 8.3 knots. This is why shipping companies competed to have the fastest ship for many years by building larger and larger ships.

While they might sound old-school and boring, displacement hulls are very efficient because they require very little power—and therefore very little fuel—to get them up to hull speed. This is one reason enormous container ships operate so efficiently. 

white sail boat on sea during daytime

Of course, living in the 21st century, you undoubtedly have seen boats go faster than their hull speed. Going faster is simply a matter of defeating the bow wave in one way or another.

One way is to build the boat so that it can step up onto and ride the bow wave like a surfer. This is basically what a semi-displacement hull does. With enough power, this type of boat can surf its bow wave, break the suction it creates and beat its displacement hull speed.

With even more power, a boat can leave its bow wave in the dust and zoom past it. This requires the boat’s bottom to channel water away and sit on the surface. Once it is out of the water, any speed is achievable with enough power. 

But it takes enormous amounts of power to get a boat on plane, so planing hulls are hardly efficient. But they are fast. Speedboats are planing hulls, so if you require speed, go ahead and research the cost of a speedboat . 

The most stable and forgiving planing hull designs have a deep v hull. A very shallow draft, flat bottomed boat can plane too, but it provides an unforgiving and rough ride in any sort of chop.

white and gray boat on sea during daytime

If you compare the shapes of the sailboats of today with the cruising boat designs of the 1960s and 70s, you’ll notice that quite a lot has changed in the last 50-plus years. Of course, the old designs are still popular among sailors, but it’s not easy to find a boat like that being built today.

Today’s boats are sleeker. They have wide transoms and flat bottoms. They’re more likely to support fin keels and spade rudders. Rigs have also changed, with the fractional sloop being the preferred setup for most modern production boats.

Why have boats changed so much? And why did boats look so different back then?

One reason was the racing standards of the day. Boats in the 1960s were built to the IOR (International Offshore Rule). Since many owners raced their boats, the IOR handicaps standardized things to make fair play between different makes and models on the racecourse.

The IOR rule book was dense and complicated. But as manufacturers started building yachts, or as they looked at the competition and tried to do better, they all took a basic form. The IOR rule wasn’t the only one around . There were also the Universal Rule, International Rule, Yacht Racing Association Rul, Bermuda Rule, and a slew of others. 

Part of this similarity was the rule, and part of it was simply the collective knowledge and tradition of yacht building. But at that time, there was much less distance between the yachts you could buy from the manufacturers and those setting off on long-distance races.

Today, those wishing to compete in serious racing a building boat’s purpose-built for the task. As a result, one-design racing is now more popular. And similarly, pleasure boats designed for leisurely coastal and offshore hops are likewise built for the task at hand. No longer are the lines blurred between the two, and no longer are one set of sailors “making do” with the requirements set by the other set. 

Modern Features of Sailboat Hull Design

So, what exactly sets today’s cruising and liveaboard boats apart from those built-in decades past? 

Today’s designs usually feature plumb bows and the maximum beam carried to the aft end. The broad transom allows for a walk-through swim platform and sometimes even storage for the dinghy in a “garage.”

The other significant advantage of this layout is that it maximizes waterline length, which makes a faster boat. Unfortunately, while the boats of yesteryear might have had lovely graceful overhangs, their waterline lengths are generally no match for newer boats. 

The wide beam carried aft also provides an enormous amount of living space. The surface area of modern cockpits is nothing short of astounding when it comes to living and entertaining.

If you look at the hull lines or can catch a glimpse of these boats out of the water, you’ll notice their underwater profiles are radically different too. It’s hard to find a full keel design boat today. Instead, fin keels dominate, along with high aspect ratio spade rudders. 

The flat bottom boats of today mean a more stable boat that rides flatter. These boats can really move without heeling over like past designs. Additionally, their designs make it possible in some cases for these boats to surf their bow waves, meaning that with enough power, they can easily achieve and sometimes exceed—at least for short bursts—their hull speeds. Many of these features have been found on race boats for decades.

There are downsides to these designs, of course. The flat bottom boats often tend to pound when sailing upwind , but most sailors like the extra speed when heading downwind.

Formentera Ship

How Do You Make a Stable Hull

Ultimately, the job of a sailboat hull is to keep the boat afloat and create stability. These are the fundamentals of a seaworthy vessel. 

There are two types of stability that a design addresses . The first is the initial stability, which is how resistant to heeling the design is. For example, compare a classic, narrow-beamed monohull and a wide catamaran for a moment. The monohull has very little initial stability because it heels over in even light winds. That doesn’t mean it tips over, but it is relatively easy to make heel. 

A catamaran, on the other hand, has very high initial stability. It resists the heel and remains level. Designers call this type of stability form stability.

There is also secondary stability, or ultimate stability. This is how resistant the boat is to a total capsize. Monohull sailboats have an immense amount of ballast low in their keels, which means they have very high ultimate stability. A narrow monohull has low form stability but very high ultimate stability. A sailor would likely describe this boat as “tender,” but they would never doubt its ability to right itself after a knock-down or capsize.

On the other hand, the catamaran has extremely high form stability, but once the boat heels, it has little ultimate stability. In other words, beyond a certain point, there is nothing to prevent it from capsizing. 

Both catamarans and modern monohulls’ hull shapes use their beams to reduce the amount of ballast and weight . A lighter boat can sail fast, but to make it more stable, naval architects increase the beam to increase the form stability.

If you’d like to know more about how stable a hull is, you’ll want to learn about the Gz Curve , which is the mathematical calculation you can make based on a hull’s form and ultimate stabilities. 

How does a lowly sailor make heads or tails out of this? You don’t have to be a naval architect when comparing different designs to understand the basics. Two ratios can help you predict how stable a design will be .

The first is the displacement to length ratio . The formula to calculate it is D / (0.01L)^3 , where D is displacement in tons and L is waterline length in feet. But most sailboat specifications, like those found on , list the D/L Ratio.

This ratio helps understand how heavy a boat is for its length. Heavier boats must move more water to make way, so a heavy boat is more likely to be slower. But, for the ocean-going cruiser, a heavy boat means a stable boat that requires much force to jostle or toss about. A light displacement boat might pound in a seaway, and a heavy one is likely to provide a softer ride.

The second ratio of interest is the sail area to displacement ratio. To calculate, take SA / (D)^0.67 , where SA is the sail area in square feet and D is displacement in cubic feet. Again, many online sites provide the ratio calculated for specific makes and models.

This ratio tells you how much power a boat has. A lower ratio means that the boat doesn’t have much power to move its weight, while a bigger number means it has more “get up and go.” Of course, if you really want to sail fast, you’d want the boat to have a low displacement/length and a high sail area/displacement. 

Multihull Sailboat Hulls

Multihull sailboats are more popular than ever before. While many people quote catamaran speed as their primary interest, the fact is that multihulls have a lot to offer cruising and traveling boaters. These vessels are not limited to coastal cruising, as was once believed. Most sizable cats and trimarans are ocean certified.

Both catamarans and trimaran hull designs allow for fast sailing. Their wide beam allows them to sail flat while having extreme form stability. 

white sailboat on sea near green mountain under blue sky during daytime

Catamarans have two hulls connected by a large bridge deck. The best part for cruisers is that their big surface area is full of living space. The bridge deck usually features large, open cockpits with connecting salons. Wrap around windows let in tons of light and fresh air.

Trimarans are basically monohulls with an outrigger hull on each side. Their designs are generally less spacious than catamarans, but they sail even faster. In addition, the outer hulls eliminate the need for heavy ballast, significantly reducing the wetted area of the hulls. 

Boaters and cruising sailors don’t need to be experts in yacht design, but having a rough understanding of the basics can help you pick the right boat. Boat design is a series of compromises, and knowing the ones that designers and builders take will help you understand what the boat is for and how it should be used. 

What is the most efficient boat hull design?

The most efficient hull design is the displacement hull. This type of boat sits low in the water and pushes the water out of its way. It is limited to its designed hull speed, a factor of its length. But cruising at hull speed or less requires very little energy and can be done very efficiently. 

By way of example, most sailboats have very small engines. A typical 40-foot sailboat has a 50 horsepower motor that burns around one gallon of diesel every hour. In contrast, a 40-foot planing speedboat may have 1,000 horsepower (or more). Its multiple motors would likely be consuming more than 100 gallons per hour (or more). Using these rough numbers, the sailboat achieves about 8 miles per gallon, while the speedboat gets around 2 mpg.

What are sail boat hulls made of?

Nearly all modern sailboats are made of fiberglass. 

Traditionally, boats were made of wood, and many traditional vessels still are today. There are also metal boats made of steel or aluminum, but these designs are less common. Metal boats are more common in expedition yachts or those used in high-latitude sailing.

planing monohull sailboat

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.

M.B. Marsh Design

Understanding monohull sailboat stability curves.

One of the first questions people ask when they discover I mess around with boat designs is: "How do you know it will float?"

Well, making it float is just Archimedes' principle of buoyancy, which we all know about from elementary school: A floating boat displaces water equal to its own weight, and the water pushes upward on the boat with a force equal to its weight. What people usually mean when they ask "How do you know it will float" is really "How do you know it will float upright?"

That's a little bit more complicated, but it's something every skipper and potential boat buyer should understand, at least conceptually. (Warning: High school mathematics is necessary for today's article.)

A yacht at an angle of heel

Let's consider a boat at rest, sitting level in calm water. The boat's mass is centred on a point G, the centre of gravity, and we can think of the force of gravity as acting straight down through this point. The centroid of the boat's underwater volume is called B, the centre of buoyancy. The force of buoyancy is directed straight up through this point.

We now heel the boat over by an angle "phi". Point G doesn't move, but point B does: by heeling the boat, we've lifted her windward side out of the water and immersed her leeward side. The centre of buoyancy, B, therefore shifts to leeward.

The force of buoyancy, acting upward through B, is now offset from the force of gravity, acting downward through G. The perpendicular distance between these two forces, which by convention we call GZ, can be thought of as the length of the lever that the buoyancy force is using to try to bring the boat upright. GZ is the "righting arm".

If we draw a line straight upward from B, it will intersect the ship's centreline at a point called M, known as the "metacentre". (Strictly speaking, the term "metacentre" applies only when phi is very tiny, but a pseudo-metacentre exists at any given angle of heel.) The metacentric height is a useful quantity to know when calculating changes in trim and heel.

(Can't see the images? Click here for now , then go update your web browser.)

We can easily draw a few conclusions simply by looking at the geometry:

  • The boat will be harder to heel, i.e. more stable, if GZ is increased.
  • Lowering the centre of gravity, G, will increase GZ.
  • Moving the heeled centre of buoyancy to leeward will increase GZ.
  • If GZ changes direction- i.e. if Z is to the left of G- the lever arm will help to capsize the boat instead of righting it.

Stability Curves: GZ at all angles of heel

To prepare a stability curve, the designer must find GZ for each angle of heel. To do this, she must compute the location of B at each angle of heel, and determine the height of G above the base of the keel (the distance KG).

In the early 20th century, finding B at each angle of heel was an extremely tedious process involving a lot of trial-and-error, a lot of calculus, and days or weeks of an engineer's time. Today, this can be computerized, and takes only a few seconds once the hull is modelled in a CAD program. Finding KG, though, is still a tedious process: it can either be measured by moving weights around on an existing boat and measuring the resulting angle of heel, or it can be calculated by tallying up every piece of structure, ballast, equipment and cargo on the boat.

Once that math is done, the designer can plot GZ (or righting moment, i.e. displacement times GZ) over all possible angles of heel. This produces the familar stability curve:

All yacht skippers should be at least somewhat familiar with their own boat's stability curve, and it's a useful thing to study when buying a boat. To read the curve, we look at the following features:

  • The slope of the curve at low angles of heel tells us whether the boat is tender (shallow slope) or stiff (steep slope).
  • The righting moment at 15 to 30 degrees of heel tells us about the boat's sail-carrying power. A large righting moment indicates a boat that can fly a lot of sail; a boat with a lower righting moment will need her sails reefed down earlier.
  • The maximum righting arm (or righting moment), and the heel angle at that point, tells us where the boat will be fighting her hardest to get back upright. If this is at a low angle of heel, we have a boat with high initial stability- she'll feel very stable under normal conditions, but a bit touchy at her limits, and relies on her skipper's skill to avoid knock-downs. If the maximum righting arm occurs at a very large angle of heel, the designer chose to emphasize ultimate stability- she'll be hard to capsize, but will heel more than you might expect in normal sailing.
  • The angle of vanishing stability is the point where the boat says "Meh, I'm done" and stops trying to right herself. Looking at the diagram above, this means that Z is now at the same point as G. A larger AVS indicates a boat that's harder to capsize.
  • The region of positive stability is the region in which the boat will try to right herself. The integral of the righting moment curve (i.e. the area of the green region) is an indicator of how much energy is needed to capsize her.
  • In the region of negative stability , the boat will give up and roll on her back, her keel pointing skyward. The integral of this region (i.e. the blue area) tells us how much energy it'll take to right her from a capsize; if this area is relatively small, the waves that helped capsize her might have enough energy to bring her back upright.

Try it on a real boat

How does this apply to some real boats? Let's consider a 10 metre, 8 tonne double-ender yacht of fairly typical layout and proportions. The parent hull looks something like this:

Keeping her draught (1.5 m), displacement (8 tonnes), length (10 m), freeboard, deckhouse shape, etc. the same, we'll adjust the shape of the midship section to yield four boats that are directly comparable in all respects except beam and section shape. Hull A is a deep "plank on edge" style , hulls B and C are moderate cruising yacht shapes, and the wide, shallow-bilged hull D resembles an old sandbagger - or a modern racing sloop.

Now, assuming that G lies on the waterline (so KG = 1.5 m), we can compute the righting arm GZ as a function of the heel angle. If we multiply the righting arm GZ by the displacement, we get the righting moment.

Some immediate observations from this graph:

  • The narrow hull "A" has relatively little sail-carrying power at low angles of heel, but will self-right from any capsize. Her good "ultimate stability" comes from using ballast to get G as low as possible.
  • The wide hull "D" can fly a lot more sail, but if she goes over, she ain't coming back up. She gets her high "initial stability" from her wide beam, which moves the heeled centre of buoyancy farther to leeward.

There's a problem, though: We've assumed an identical centre of gravity for all four boats. That's not realistic. The deep, narrow hull will have her engine and tanks low in the bilge; the wide hull must mount these heavy components higher up. Let's reduce hull A's KG measurement to 1.35 m, and increase hull D's KG measurement to 1.65 m, a more realistic value. We'll scale KG for the other two accordingly.

The overall conclusions don't change much, but we now have some realistic numbers to play with.

  • Hull A, the narrow one, will have a hard time flying much sail. She'll heel way over in a breeze. But she can't get stuck upside down.
  • Hull B, a moderately slender cruising shape, also can't get stuck upside down- her AVS is 170 degrees. Her extra beam causes the centre of buoyancy to move farther to leeward when she heels, so she has more initial / form stability than hull A and can carry more sail.
  • Hull C, which is typical of modern cruising yachts, has over twice the sail-carrying power of the slender hull A. She'll heel less, and since her midship section is much larger, she'll have more space for accommodations. The penalty is an AVS of 130 degrees. That's high enough that she can't be knocked down by wind alone, but wind plus a breaking wave- such as in a broach situation - could leave the boat upside down until a sufficiently large wave comes along.
  • Hull D, the broad-beamed flyer, can hoist more than three times the sail of hull A at the same angle of heel. She'll be quite a sight on the race course with all that canvas flying. Her maximum righting moment, though, is only 37% more than hull A's, which leaves less of a margin for error- hull D is more likely to get caught with too much sail up, and will reach zero stability at a lower angle of heel. If she does go over, she has considerable negative stability, making it unlikely that she'll get back upright.

Work to capsize

If you're one of that slim percentage who paid attention in high school physics, you're probably looking at those curves and thinking: "Force (or moment) as a function of distance (or angle).... hey, if you integrate that, you get the work done !

And so you do, with the caveat that we're using a static approximation to a dynamic situation. The results are valid for comparison, but the actual numbers may not mean very much.

Let's do that for each of our hulls. We'll integrate the righting moment curve as a function of heel angle, up to the angle of vanishing stability, to get the work done to capsize the boat. We'll also integrate from the AVS to 180 degrees to get the work done to right the boat from a capsize.

Our four boats require roughly the same work to capsize! Changing the shape of the midsection affected the shape of the stability curve- a wider boat had more initial stability and less ultimate stability. In this case, though, our vessels are all about the same size and require about the same amount of work to capsize.

Righting from a capsize is another matter. The narrow, deep hulls A and B will self-right without any outside influence- a nice confidence-booster if you're heading into the open ocean, although the reduced sail-carrying power and limited interior space of these vessels will probably be more important to most skippers.

The moderate cruising hull, C, needs a bit of help to self-right, but any combination of wind and waves that can do 95 kN.m.rad of work on the boat is likely to produce a wave that can do 10 kN.m.rad of work on that same boat.

Our broad-beamed racer, hull D, is not so fortunate. Righting her from a capsize takes one-third the work that capsizing her in the first place did, and her acres of canvas were probably a major factor in the initial capsize- they're now underwater, damping her roll motion instead of catching the wind. The odds are that this boat will stay upside-down until someone comes along with a tugboat or crane.

Lessons Learned

What's the take-home message from all this?

If you're buying a new boat: Look at her stability curve, and compare it to other boats.

  • Good: Large region of positive stability, small region of negative stability, high angle of vanishing stability, steep slope at low heel angles.
  • Iffy: Shallow slope at low heel angles (makes it hard to fly lots of sail, excessive heeling when underway).
  • Risky: Low angle of vanishing stability, large region of negative stability.

If you already have a boat:

  • If you know her point of maximum stability, you can be sure to reef the sails well before  that point.
  • If you know her AVS and the shape of the curve in that region, then when a broach or knockdown happens, you already know how hard she'll fight to come back upright.
  • If you know how much area is covered by the negative stability region of the curve, you'll have some idea of whether she'll come back from a capsize on her own or else have to wait for help.
  • Determine if anything you've changed- a dinghy added on the deck, perhaps- has moved the centre of gravity.
  • If G has moved, adjust your mental model of the stability curve accordingly: just shift the curve up or down by (change in height KG) * sin(heel angle).

Confounding Factors

What we've discussed here is just about how to read the stability curve- it's not a complete picture.

There are many other factors that must be considered to get a complete understanding of a boat's stability. Among them:

  • Dynamic effects. Everything discussed so far is for the static case, and is good for comparison purposes. But in practice, boats move.
  • Waves. Stability curves are calculated for flat water, ignoring the effect of waves.
  • Differences in rigging. Weight aloft has a much larger effect on the boat than weight down low- particularly where the roll moment of inertia, an important property for dynamic stability, is concerned.
  • Keel shape. Keels tend to damp rolling motion; this behaviour is quite different with a long keel than with a fin keel, or with a fin keel underway versus a fin keel at rest.
  • Downflooding. Everything we've discussed here assumes that the boat is watertight in any position. If she takes on water when rolled, everything changes.
  • Cockpits. Our demonstration boat doesn't have a cockpit. A large cockpit could hold several tonnes of water- and with a free surface, no less. That means that G will move all over the place, usually in the wrong direction.

Further Reading

Steve Dashew's article " Evaluating Stability and Capsize Risks For Yachts ", and others on his site, discuss stability-related risks as they relate to cruising yachts.

Technically-minded readers should refer to a naval architecture textbook, of which my present favourite is Larsson & Eliasson "Principles of Yacht Design" (McGraw-Hill).

Don't even think about buying a cruising yacht without first reading John Harries' extensive series of articles on boat and gear selection .


  • Boat Design


Great stuff.

A really great piece, thank you. You have the very unusual gift of being able to make complex issues easy to understand.

Other confounding factors

One major confounding factor which most English-speaking designers still seem to routinely dismiss, or overlook, is to do with the nature of knockdown lever moments in a 'survival storm' situation:

You specifically state you're not taking waves into account, so this is addressed at those who do, in the conventional way -- generally led by the insights of academics and researchers tracing their conceptual methodology back to the likes of Marchaj.

The lever moments I'm thinking of arise from the vertical offset between: Where the wave force vector acts, and Where the hull resistance vector is located.

It has long been contended by the school of expedition yacht designers, going back to around the days of Damien II, from France in the 70s, that the greatest risk ... and arguably the only one worth worrying about for such vessels ... was due to the tripping moment caused by the vertical offset between the centre of effort of a true breaking wave, and the centre of resistance of the hull AND UNDERWATER APPENDAGES

When a large ocean wave breaks entirely forwards, the part which was formerly the crest avalanches down the front of the wave. Admittedly this behaviour is VERY rare offshore - where almost all 'breakers' actually spill most of the water down the back, but it's these events which present a real survival threat, and which define the limits to a vessel's capability.

Unlike the water particles in the body of the wave, which are circulating in the well known way of text book diagrams, and effectively not going anywhere over time, this "former crest" water has escaped from the wave system and is travelling rapidly under the influence of gravity down a steep ramp whose geometry (as opposed to constituent particles), in the case of a Southern Ocean wave of truly heroic proportions, might itself be advancing as fast as 30 to 40 knots.

So we have an aerated but still rather massive entity tumbling down above this already very fast moving ramp, hitting the topsides and cabin coamings, in the worst case, perpendicularly.

The contention of the French school was that, in this situation, while a high freeboard is clearly undesirable, the absolute last thing you want, which trumps everything else, is deep appendages providing lots of lateral grip, situated down in green water. This would provide a lever arm converting the sideways impulse (which is at a height not very far from the centre of mass, and hence not inherently an insuperable problem) into a very dangerous overturning moment.

The insight was based on simple empirical observations, such as of a flat wooden plank, or a surfboard with no appendages, floating side on to breaking waves at a surf beach. Despite having no ballast whatsoever, and a zero GZ in the plank case, this will sideslip down those waves and stay happily the same way up, in conditions where (say) a windsurf board with a deep centreboard (whether ballasted or not) will be tumbled repeatedly.

They reasoned that the thing to avoid at all costs, for a well found expedition yacht, was a knockdown with an angular acceleration sufficient to snap the rig.

This turned everything on its head with regard to the conventions of stability calculations: the relative positions of the centre of mass and the centre of buoyancy become largely irrelevant: the former should if anything ideally be high, so the vector from the striking crest passes through or near it, (to minimise the inertial overturning moment) while the latter is almost irrelevant because on the face of such a steep wave, the hull is in virtual freefall, and the hull is largely disengaged from green water. Aerated water offers little buoyancy.

This is so divorced from statics (which are arguably most useful for calculating how to prevent ships capsizing at a dock) that it is a shame to see so much reliance on static measures persisting to this day, in educating sailors, defining ultimate seaworthiness, and framing regulations and recommendations.

Be that as it may: this insight led to a completely different school of storm management by the adventurous people who sailed off to places like the subAntarctic and Antarctic in the new generation of beamy, generally low-freeboard # hulls, equipped with swing (or even dagger) ballasted keels capable of retracting - in many cases - right within the canoe body.

# ideally, no cabin trunk - which on the face of it is bad for self-righting...

In survival conditions, these sailors began retracting these keels, even though on the face of static stability calcs, this is entirely wrong. And (AFAIK*) not one of these yachts has yet been lost in the deep south, despite them making up the majority of the fleet, and I'm not even aware of a single 180deg knockdown to such a vessel configured in this way.

There have been, and continue to be, numerous knockdowns and dismastings of fixed-keel yachts designed to the other, older paradigm.

*(The first two losses of private expedition yachts in Antarctic waters both occurred within the last two years, and neither was a vessel of this type)

So even if these sailors are not right, they're clearly not VERY wrong.

Re: Other confounding factors

You are quite correct that when you are facing breaking waves, static stability analysis is not going to show the whole picture. Being caught in large breakers is certainly one of the highest-risk situations a yacht can face.

The "let it slide sideways" approach can have considerable merit in such a situation, if the boat is designed with this in mind. On a monohull sailing vessel, this calls for a retractable keel and a canoe body with relatively little lateral resistance of its own. If you do this, of course, you also have to ensure that the vessel won't trip over the leeward gunwale when she's surfing sideways with the keel retracted. There are plenty of good, seaworthy vessels out there with such a configuration.

The price you pay for doing it that way is that it's harder to right the boat if she does capsize. Frankly, though, I would rather not capsize in a non-self-righting boat than be upside-down in one that will eventually get herself back up. There are tens of thousands of catamaran sailors out there who would seem to agree.

This is not to say that static stability traits are not important: they certainly are. Given two vessels of generally similar configuration, the stability curves will tell you quite a lot about what kind of behaviour can be expected from each.

Static stability curves are certainly not the whole picture. There are several important dynamic aspects- the lateral resistance effects and the roll moment of inertia, among other features- that can have a huge effect in extreme situations. I'll discuss these in more detail in future posts.

I am thinking about. Buying a

I am thinking about. Buying a 38 foot guimond lobster boat. I am thinking Of widening the stern to 10 feet from 8 ft 8 in. Also I want to add some fiberglass to the keel to make her a little deeper maybe 36 in from present 32 inches. Should I make the new hull water line 90 degrees? Will this be better than a round traditional edge? Should I add bilge keel fins for more stability?

Modifying a design

The kind of modifications you're describing are fairly extensive. You would be wise to arrange a meeting with a naval architect, or with a builder who has extensive experience with that type of boat. With the boat's drawings and a good description of what performance characteristics you want, the professional will be able to assess what modifications (if any) would be appropriate- or if you'd be better off choosing a different design from the start.

Stabilty of Twin Keel Monohulls (Bilge Keel)

Wondering about the stability of bilge keeled sailboats, specifically the Snapdragon 26. How does a second keel affect relative stability of this kind of vessel? Any thoughts appreciated.

Static stability is determined by the hull shape and by the distribution of mass, i.e. the centre of gravity. Two identical hulls, one with a single fin and one with twin keels, will have approximately the same stability curve if they have the same centre of gravity. The twin keel configuration is usually chosen to allow shallower draught, though, so the centre of gravity will often be higher than for a single-fin boat.

There is a significant performance sacrifice with this configuration. A higher centre of gravity reduces the sail-carrying ability, the lower aspect ratio foils are not as efficient to windward, and the extra wetted surface increases drag. The flip side is that you can safely dry out at low tide in places where most monohulls would never be able to go.

Ultimately, though, the keel configuration is a fundamental part of a design, and there's no real answer to "How does a second keel affect stability". It's the performance of the entire boat that matters, and unless you have two boats that are identical except for keel configuration, it doesn't make much sense to separate out this one aspect of the design. The class's performance record and the experiences of skippers who have sailed that class in bad weather are better ways to assess the relative seaworthiness of an existing design.

Stability Curves for Hunter 34

I'm french and it's not that easy for me to understand all of this but here is my question:

Do you know who I can contact to know the stability curves of my sailboat. It's a Hunter Sloop 34' 1985

I asked directly at Marlow-Hunter, they said they don't have this information.

Someone told me that Hunter Manufacturer has it and that I can have it for some dollars but it seems that this is not the case.

Can you help me?

Tracking down data for old boats

Danielle, if I'm not mistaken, that Hunter would be one of Cortland Steck's designs. There's a chance that he might have the data you're looking for.

Stability curves are incredibly tedious to calculate without a computer, though, so many- if not most- boats designed prior to the advent of modern 3D CAD never had one calculated at all. It's possible to build a computer model of an existing boat and calculate the required data, but for most practical purposes you can find the important information through an inclining experiment. This essentially consists of moving known weights around the boat and measuring how she heels in various load conditions, and it's one of the more common ways of measuring stability data for an existing vessel in commercial service where all of these details must, by law, be properly measured and documented.

Righting a Capsized Vanguard Nomad 17

I read on the web that it takes 420 lbs of crew weight to right a capsized Nomad. Is that true? I weigh 135 lbs and I sail single-handed. It's now November and the water is getting too cold to find out.

Re: Righting a Capsized Vanguard Nomad 17

Gerardo, A 625 pound boat with a beam of 8 feet is not going to be an easy thing to right. You might find Sailing World's article on the boat interesting. They were advised by the manufacturer's rep that the boat can't be righted by one person in the way that you'd right something small like a Laser. But if you flood the tank (through the spinnaker well) on one side, you'll be able to roll her far enough to pull her back up like a dinghy, and then drain the tank again. I agree that you would NOT want to test this in November!

37 Foot Sailboat

I am from the Maldives in the Indian Ocean. I am building a fiberglass sailing yacht using local boat builders. Its 37 feet and 11 feet with a long keel of 3 foot deep. And will use concrete in the keel. They will be putting 9 fiberglass mats. Interior and the bulkheads will be done using marine plywood. The hull is going to look more like a Fisher 37. And the cabins like a Nauticat. I am intending to use ketch style two masts. I was surfing the internet and am trying to understand what are the issues that I need to take into consideration. Your explanations is very helpful. I am just wondering whether you will comfortable if I communicate on this topic. Thanking you.

Re: 37 Foot Sailboat

Ahmed, it's good to have you here and feel free to chime in on relevant threads, or to contact me directly. It's always neat to see what everyone else is building.

Help with stability estimate

Matt, I found your article very informative, good stuff! Where might you think my vessel Crusoe might fit A thru D.
 57' O.L. 13' beam-25 tons-4.5 ton ballast lifting keel. Here is the vessel: 

To summarize, in very general terms: Category A is an offshore-capable yacht. Category B is a coastal cruising vessel, able to handle weather at sea but not recommended for extended offshore use. Category C is a short-range inshore vessel that is expected to take shelter rather than facing a storm out in the open. Category D is a small, fair-weather vessel such as a skiff or dinghy. The static stability properties are the main factor that determine which category a particular boat design is intended to fall in. But, in addition, the builder must comply with dozens of requirements for structural integrity, watertightness, emergency equipment, etc. for the boat to actually fall in that category. It's quite possible for a boat designed for Category A to end up being a Category B vessel because of corner-cutting during the build.

Assessing Southerlies and Tayanas

Would you care to give an opinion on the Southerly Yachts with retractible keels and twin rudders, also on Tayanas as to seaworthiness and construction. Thank you

Southerly & Tayana

I don't have first-hand experience with either of these marques, so I'm afraid I can't offer much that's meaningful.

Southerly tends to have a fairly good reputation. You do pay a fairly substantial premium for the complicated retracting keel, but there are some cruising grounds where the only options are a retractable keel or a multihull.

The Tayana line has produced a mix of models from several different designers, some very traditional, rugged and slow, others relatively modern. I'd have to know exactly which one you have in mind to say much more than that.

Your best bet for meaningful data on either line would be to prowl some forums looking for the owner's club for each marque. Yacht owners generally love to talk about their yachts, and if you're patient, you can usually find most or all of a particular model's weak spots by asking owners how they handle rough weather and what they've had to fix or replace so far.

I really enjoyed your article

I really enjoyed your article. I'm trying to make a stability model myself and I was interesting in the equations you used to find GZ as a function of heel angle and then how you found the displacement. I'm also interested in how you calculated the different curves for the different hull designs. Any pointers would be greatly appreciated. Thanks!

I'm not sure if I mentioned

I'm not sure if I mentioned it in my last comment, but I'd also like the equations for getting the displacement you multiplied GZ by. Thanks!

Sources for calculations

Hi Cole, Finding the displacement from the lines is pretty easy. If it's a CAD model, just find the volume; if it's a 2D drawing, find the area of each of the stations and use Simpson's rule to integrate over the waterline length. Finding G is just a matter of adding up the weights and moments for every component of the ship - each frame, the hull planking, the engine, each piece of hardware, and so on. Finding GZ for a given heel angle is relatively tedious, but it's essentially the same procedure (find the station areas, integrate over the waterline length, find the station centroids, weight the centroid offsets by station area to find the CB). There is an iterative step here as you must adjust the waterline position to make the displacement the same as in the at-rest case. For practical purposes, though, virtually everyone computes their stability curves using a proven software tool like Orca3D or ArchimedesMB. The actual calculations are described in detail in most good yacht design textbooks, eg. Larsson & Eliasson's "Principles of Yacht Design".

Stability of Chinese Junk Hull

Hi Matt, Your article is very informative. I am studying the feasibility of building a wooden ocean going Chinese Junk. History recorded that there were huge junks sailing 600 years ago in Zhenghe's days. The latest record for a large junk sailing across oceans is the Keying which sailed from Hong Kong to New York and London in 1848. She is 160ft LOA, 33ft BEAM and 13ft (rudder up) 23ft (rudder down) DRAFT, 700-800 ton DISPLACEMENT. As it is too difficult to re-build a wooden junk of such size, I am studying the record of fishing junks built about 30 years ago. A junk capable of sailing in force 8 wind. She is 23m(75.4ft)LOA, 5.66m BEAM, 1.69m(DRAFT), 1.2m(FREEBOARD), 138000kg (DISPLACEMENT). There is a dagger board extending 2.5m from the bottom, located about 1/3 waterline from the bow in front of the main mast. The rudder can be raised in shallow water. It is perforated with an area of 6.7sq.meter. The bottom is almost flat. The design of junks were evolved from generations of experience without scientific verification. I am surprised that the length and beam is so close to Volvo 65, but the displacement is 10 times those of Volvo. I am wondering if a flat bottomed boat is stable in rough ocean condition until I read the comment by Andrew Troup in 2012 about a boat without appendages can surf safely on the steep slope of the waves. I am glad if you can shine some light on the stability of traditional Chinese junks. John Kwong

Chinese Junk

A hundred and thirty-eight tonnes on 23m LOA? Yowzah, that's quite the boat. There's nothing fundamentally wrong with a relatively flat bottomed shape, or with retractable appendages. The risk of a flat bottom is more to do with slamming and pounding, which is much less of a problem in a heavy boat. Before investing hundreds of thousands of dollars in such a boat today, it would certainly be prudent to have the design drawn up and analyzed with modern software tools. There are certainly improvements from the last 50 years that could be applied to a much older design. A six-century pedigree is nothing to sneer at, though, and the fundamental design - updated with some modern construction techniques and with the added confidence of a full stability analysis - might still be a good one.

Relative locations of G and B

Hi Matthew. Thanks for such an interesting and informative article. Most diagrams show B below G so I guess this must be the most usual arrangement. However, I wondered if there might be a class of yacht (lightweight but with deep bulb keel) where G moved below B. I guess this would give a very good static G-Z curve (but I note also the comments made by Andrew (above) re dynamic stability that this might not be the best design to go winter sailing in the Southern Ocean!)

Monocat Hull

Matt what would you think this Monocat 50 Hull Form (see link)? Its a very different design- Monohull at the Bow, Catamaran at the Stern, 2x Lift Keels, One Ballasted, the other Forward non ballasted dagger board. I just cannot find information on it anywhere? I'd assume it would have similar characteristics to a very beamy monohull and thus would not self-right from a knockdown!? This is what im wanting to find out, will it self-right & is it safe offshore? Mashford Monocat 50 15.24m LOA 5m Beam 3Ton Ballested Lift Keel 0.8m - 2.1m

(there is a cad drawing of its underwater hull design in this advert) NB: Unfortunately your Spam Filter will not let me paste the link, but if you search the internet for MASHFORD MONOCAT it comes up for sale everywhere.

Ive been trying to locate the Designer Chris Mashford with no luck? feel free to email me too any info, cheers. Mal

Mashford Monocat

I'm not too familiar with the Monocat. My educated guess would be that stability-wise, it'll be much like a "skimming dish" racer - very stiff and powerful at first, hairy at the edge, and not self-righting. I'd have to sail one to be sure, but I have a suspicion that it could have the worst of both worlds - the relatively high drag and the ballast burden of a mono, with the complexity and high sailing loads of a cat. The main appeal seems to be the huge living space in a relatively modest beam, suggesting it's meant for short-term coastal cruises and charter work. Reliable reports on them seem to be very hard to come by, I suspect they weren't built in large numbers.

Great article! Thanks. My question is on actual statistics of vessels that have actually capsized. Understanding that this would likely be under reported, it would seem fruitful ground to examine questions of which static or dynamic factors pan out and are predictive for hulls that ended up upside down, and the stories behind them?

Does such a database exist?

reason for knowing the departure gm

Sorry I am bringing in a different topic entirely . pls I have read most of your articles and I have found them to be very useful . Pls I really want to know the importance of knowing your departure gm before commencing on a voyage... thank you


Hi Matthew - I was reading your blog just now on Aug 23. I wanted to know how intake of 450l water affected the stability of a 9000kg / 41ft sailing yacht that I was skippering in a force 9 storm around Dover on Aug 3rd 2017. We encountered rather high waves of estimated 7m and had 52 kts apparent wind, which may have been the beginning of a force 10, because we did only 4kts through the water under storm jib and 3x reefed main. Once safely parked in Dover, we pumped 450l water out of the boat. Floorboards were floating... Any idea how that amount of water may have affected stability?

Kind regards

Martin Lossie

Calculating a stability curve

You mentioned calculating stability curves is tedious, and mostly done with CAD these days. I'm a new owner of a 1969 Columbia 26 Mk II and would love to understand the stability curve for my boat. A few enterprising owners have rescued the blueprints of this boat and placed them online, so I have the measurements available. Are there folks out there willing to do the CAD work to create the curve? Otherwise, what would be the easiest way for me to get one created for my boat?

Thanks for a GREAT article explaining this concept!

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Nomadic Sailing

The Definitive Guide to Sailboat Hull Types


If you’ve ever been on a sailboat or any kind of boat, one of the first parts of the boat you saw was its hull and you might not have even known it.

Simply put, the hull is the bottom part of a boat that rides in and on top of the water. When a sailboat is underwater, it’s accompanied by the keel and the rudder.

Just like knowing the different types of sails , knowing the hull type on your sailboat means you’ll have a better understanding of how your boat operates while it’s out on the water.

All in all, the hull of any boat is meant to keep the boat afloat and to ensure minimum resistance against the water while being propelled forward. Now let’s dive into the different sailboat hull types and even some other types of hulls in boats in general!

Main Sailboat Hull Types

There are two main hull types that we’ll be looking at that encompass the many other types of hulls that vary from these two main types.

Depending on the type of boat you have, you’ll be floating around with one or the other. We’ll take a look at what you can expect if your boat has either of these hull types.

Displacement Hulls

The most common sailboat hull type you’ll find out there is the displacement hull, which is very effective at pushing the water aside and powering through it during forward propulsion.

A displacement hull is often found not only on sailboats, but also fishing, freight, cruise, and other larger boats.

All boats that have a displacement hull will be limited in their speed based on the waterline length of the hull. Regardless of how much power you use, whether it’s from the wind or motor, the maximum speed can’t be increased.

This is why you’ll see people mention the waterline length of a boat’s hull when putting them on the market to sell.

The big advantage of having a displacement hull is that they require far less power to get moving across the water compared to the other main hull type; the planing hull.

What this means is that your boat will be able to cruise for a long time with the same amount of energy, which also allows you to carry more items on board.

Planing Hulls

It’s almost guaranteed that your sailboat won’t have a planing hull since they’re most commonly found on powerboats and personal watercrafts (PWCs), like jet skis.

Planing hulls allow the boat to lift itself out of the water, reducing drag and increasing the speed of the boat.

Almost any boat that’s equipped with a planing hull will be able to attain a speed much greater than a boat with a displacement boat.

The main reason for this is the lift that’s produced when traveling at high speeds which reduces drag on the water.

The maximum speed of a boat with a planing hull is dependent on the horsepower of the engine and how much of the hull can be removed from the water while still cruising.

The biggest advantage of having a planing hull is that your boat will be able to pick up speed quickly and reach a greater maximum speed.

This allows for shorter journey times. However, there needs to be a source of all that energy, which comes directly from a combustion engine. The faster a boat with a planing hull goes, the larger the cost of fuel will be.

How Planing Works

The way planing works is actually pretty interesting, so I thought I’d dive into it a bit. Even though a sailboat is virtually guaranteed not to have one, it’s always nice to know how other boats operate while out on the water.

1. Displacement

Before a boat with a planing hull actually planes, it starts out acting like a displacement hull.

As a matter of fact, a boat with a planing hull needs to reach a certain speed before it starts to produce lift. Before that happens, it’s essentially a displacement hull.

While a boat with a planing hull is picking up speed and lifting itself out of the water, it’s in a plowing mode.

You’ll know when a boat is in plowing mode when the bow of the boat is elevated and the boat is throwing a relatively large wake. The goal, however, is to move from plowing mode to planing mode, which requires further acceleration.

Once the boat with a planing hull reaches a certain speed, it’ll leave plowing mode and enter planing mode.

As I already described, planing is when the hull is gliding across the water with a smaller amount of the hull dragging in the water when compared to the previous modes. Different boats will start planing when reaching different speeds.

Common Sailboat Hull Styles

Now that we’ve gone over the two main types of hulls you’ll find in sailboats and other types of boats, we have a good foundation for the hull styles you’ll commonly see when out on the water.

There are three main hull styles that you’ll see quite often, so let’s take a look at those.

planing monohull sailboat

By far the most common hull style you’ll see on sailboats is the monohull, which is simply a single hull.

Traditionally, a sailboat will have a monohull and they can be found all over the place. It’s probably the style of hull that comes to most peoples’ mind when imagining a sailboat.

Monohulls on sailboats are virtually all displacement hulls. As we went over previously, this allows your sailboat to cruise for long stretches and has a greater efficiency compared to planing hulls.

However, most boats that exist on planet earth are monohulls, including powerboats, which can also be of the planing hull type.

When it comes to a monohull on a sailboat, the only way it can keep its stability is to have a proper keel attached to it.

A keel is a wing-like object that sticks out of the bottom of the hull in the water and provides a sailboat with ballast for stability. It’s important to understand how a keel works when operating a sailboat with a monohull since it’s one of the main reasons a sailboat can move forward without tipping.

planing monohull sailboat

There are certainly a lot of monohull sailboats out there, but there’s no doubt that you’ll also see your fair share of catamarans.

Catamarans are sailboats with two hulls and operate quite differently than their monohull cousin. Catamarans are known to be fast and are likely to outrun most monohull sailboats.

Unlike monohull sailboats, catamarans can be fitted with displacement hulls as well as planing hulls. However, even if they have a planing hull they can still produce a relatively good amount of cruising time and do so rather efficiently.

Catamarans are a bit different than monohulls in the sense that they can reach greater speeds. There are several reasons for this. For one, a catamaran doesn’t need a ballast for stability since the broad stance between the two hulls provides enough stability.

This means there’s no need for a large, heavy keel. Second, they’re often built out of lightweight materials that allow the boat to reach a higher maximum speed compared to heavier sailboats.

Also, if a catamaran has a planing hull, it’ll have the ability to produce lift resulting in reduced drag on the water and even greater speeds.

Unfortunately, catamarans do have the disadvantage of being more likely to capsize in unwanted high-wind situations.

Also, it’s very difficult for a catamaran to recover from capsizing as opposed to a monohull sailboat that has a good ballast from its keel.

planing monohull sailboat

You might have already guessed from the name, but I’ll state the obvious anyway. A trimaran is exactly like a catamaran but with three hulls instead of two.

Often times you’ll see a trimaran look like a monohull sailboat with a pair of hulls attached to its side.

Similar to a catamaran, trimarans can hit speeds much greater than your average monohull sailboat. As a matter of fact, they’re known to be “unsinkable” under the situation that the hulls on the port and starboard side of the central hull are completely filled up with water.

One of the coolest aspects of having a trimaran is that when it has a planing hull and/or a hydrofoil, the trimaran’s central hull will lift completely out of the water.

This gives it the effect that it’s floating across the air, which is the result of lift produced from the planing hull or a hydrofoil. It’s very cool to see this!

Sailboat Hull Bottoms

Apart from the main boat hull styles, like the monohull, catamaran, and trimaran, there are hull bottoms that pop up in the world of boating that can differ in style and function.

These hull bottoms are more of a deeper look at the hulls of a monohull, catamaran, or trimaran, so you can think of them more as a feature of any of the previously mentioned styles of hull.

Flat Bottom

planing monohull sailboat

A very common hull bottom for boats that are derived from the planing hull type is a flat bottom hull.

The flat bottom hull is considered to be one of the less stable styles of hulls, especially when confronted with rough waters.

However, you’ll often find them on boats that don’t necessarily ride in these situations, including fishing or taxi areas.

  • Good for small lakes and rivers due to having a shallow draft.
  • Able to hit relatively high speeds once entering planing mode.


  • Not good at handling choppy waters resulting in a rough ride.

Round Bottom

planing monohull sailboat

When it comes to sailboats, you’re most likely going to run into monohull sailboats that have a displacement style hull with a round bottom.

While these are the most common hull bottom for sailboats, they can also be found on smaller boats that are used for fishing, canoeing, and other similar kinds of boats.

  • Easily moves through the water due to being a displacement hull type.
  • When accompanied by a keel, it produces a great amount of stability from the ballast.
  • Without a keel, it can roll when entering and exiting the boat as well as when waves are present.
  • Less maneuverable compared to other hull styles.

Deep ‘V’ Bottom

planing monohull sailboat

If you’re operating a powerboat, then in all likeliness your boat has a planing hull with a deep ‘V’ bottom.

Since deep ‘V’ bottoms are found on planing hulls, these types of boats will be able to pick up speed quickly and at high maximums. This is the most common setup for powerboats out on the water.

This is the most common type of powerboat hull. This hull type allows boats to move through rough water at higher speeds and they provide a smoother ride than other hull types.

  • Provides a smooth ride compared to its flat bottom rival.
  • Good at handling rough water.
  • Requires more power to plane compared to its flat bottom rival.
  • Cannot handle sharp turns very well resulting in potential rolling or banking.

Multi-Chine Bottom

We took a good look at multi-hull styles like the catamaran and the trimaran earlier, which are the exact style of hulls that have a multi-chine bottom.

A multi-chine bottom is a great example of a displacement hull on either a catamaran or trimaran as it’s the most common bottom you’ll find.

  • In a multi-hull boat, it has a great amount of stability due to its wide beam.
  • In a multi-hull boat, it needs a large area when either tacking or jibing.

Main Parts of a Sailboat Hull

There’s some terminology I threw around while describing the many types of hulls a sailboat and other types of boats have.

As is the case with a lot of activities, learning the terminology is just something you have to do.

Thankfully, the terminology will eventually sink in overtime and eventually you’ll be able to ring off any hull terminology that comes up.

The bow is simply the most forward part of a sailboat and, thus, the very front of the hull.

The stern, conversely to the bow, is the most backward part of a sailboat and, thus, the very end of the hull.

The port side of a hull is the left side. I always remember this with the phrase “I left my port on the table”, with the port being wine.

This just so happens to also be the side where boats will have a red light turned on at night, which is the color of port wine.

The starboard side of a hull is on the right side.

Opposite the port side, in the evening boats will have a green light turned on and will be located on the starboard side of the boat.

Fore is a sailor’s way of saying “forward”.

Aft is a sailor’s way of saying “back”.

A transom is the aft-most (see what I did there?) section of the boat that connects the port and starboard sections of the boat.

The flare of a hull is where the hull starts to form a large angle the closer the hull gets to the deck.

The waterline is the line around the hull where the water touches when under a normal load.

Waterline Length

The waterline length, once referred to as the Load Waterline Length (LWL), is the length of the hull where the waterline is located.

This is not the entire length of the boat.

Length Overall (LOA)

The length overall (LOA) is, you guessed it, the overall length of the boat. This is measured from the tip of the bow to the end of the stern.

The freeboard is the space on the hull of a boat above the waterline and below the deck.

The draft is the length from the bottom-most part of a boat (the tip of the keel on a sailboat) and the waterline.

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planing monohull sailboat

Hydrodynamics of Planing Monohull Watercraft

  • © 2017
  • William S. Vorus 0

School of Naval Arch. and Marine Eng., University of New Orleans School of Naval Arch. and Marine Eng., NEW ORLEANS, USA

You can also search for this author in PubMed   Google Scholar

  • Emphasizes the correct understanding of, and ability to implement, the driving physical principles of planing
  • Addresses three important areas: boat resistance, seaway response, and propulsion
  • Identifies problem areas in need of better understanding in monohull planing, leading to better design and engineering
  • Includes supplementary material:

Part of the book series: SpringerBriefs in Applied Sciences and Technology (BRIEFSAPPLSCIENCES)

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Table of contents (9 chapters)

Front matter, boat hull hydrodynamics, conceptual monohull planing in calm water.

William S. Vorus

Slender-Body Hydrodynamics

Time domain analysis, calm water mechanics, planing in sea waves, boat propulsion hydrodynamics, data requirements for design, engineering design, blade strength, open-water curves, back matter.

  • Boat Hydrostatics
  • Froude number
  • Hydrostatic solution
  • Shallow water planing
  • Calm water planing
  • Boat steady performance
  • Boat resistance
  • Seaway response
  • DOF of rigid-body-dynamics
  • fluid- and aerodynamics

About this book

Authors and affiliations, about the author, bibliographic information.

Book Title : Hydrodynamics of Planing Monohull Watercraft

Authors : William S. Vorus

Series Title : SpringerBriefs in Applied Sciences and Technology


Publisher : Springer Cham

eBook Packages : Engineering , Engineering (R0)

Copyright Information : Springer International Publishing Switzerland 2017

Softcover ISBN : 978-3-319-39218-9 Published: 02 September 2016

eBook ISBN : 978-3-319-39219-6 Published: 26 August 2016

Series ISSN : 2191-530X

Series E-ISSN : 2191-5318

Edition Number : 1

Number of Pages : X, 105

Number of Illustrations : 15 b/w illustrations, 35 illustrations in colour

Topics : Engineering Design , Fluid- and Aerodynamics , Theoretical and Applied Mechanics

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All About Sailing Logo

All you need to know about Monohull Sailboats

The definition of monohull is a sailboat with a single hull.

Monohull sailboats are often categorised by the type of rig (mast and sails. Monohull sailboats are also called a sloop, cutter, ketch, yawl, or schooner.

Modern sailboats are most commonly the sloop. It has one mast and two sails. Typically, a Bermuda-rigged main and a headsail. This configuration is extremely efficient for sailing into wind.

A cutter is similar to an sloop, with one mast and mainsail. However, the mast is generally carried further aft to permit a staysail and jib to be attached to the inner forestay and head stay.

The ketches has 2 masts, and a shorter mast at the end of the mainmast and forward of the rudder posts. The shorter mast is called the mizzen mast. You can also Cutter-rig a ketch with two head sails.

A yawl is similar to a ketch, with a shorter mizzen mast carried astern the rudderpost more for balancing the helm than propulsion.

A schooner’s mainmast is taller than its foremast. This distinguishes it from a ketch. A schooner may have more than one mast, but the foremast is always lower than its foremost main. Topsail schooners of the past had topmasts that allowed triangular topsails sails above their gaff sails. Many modern schooners are Bermuda-rigged.

A monohull sailboat is a type of boat that has only one hull, or main body. The monohull design is the most common type of sailboat, and it is also one of the oldest types of boats in existence.

The monohull design is simple and efficient, and it is able to provide good stability and speed. Monohull sailboats are typically used for racing, cruising, and other types of sailing. Some monohull sailboats are also used for fishing or other commercial purposes.

Monohull sailboats come in a variety of sizes, from small boats that can be sailed by one person to large boats that require a crew of several people.

Sailboats have been around for centuries, and their origins can be traced back to the early days of human history. The first sailboats were probably simple rafts or canoes that were propelled by the wind. These early boats were likely used for fishing or transportation, and they gradually became more sophisticated as humans learned to build better vessels.

Around 4,000 years ago, the Egyptians began using sailboats to transport goods along the Nile River. The sailboat soon became an essential part of life in ancient Egypt, and it continued to be used for trade and travel throughout the Mediterranean region.

Today, sailboats are still used for transportation and leisure, and they continue to evoke a sense of adventure and exploration.

Monohull Sailboat

The monohull sailboat is a sailboat with a single hull. A monohull sailing boat has the following characteristics – The monohull, is propelled by wind and kept on course by the fins in the water beneath it. – The rudder controls the trajectory. It consists of the “wing” at the rear of hull, which is submerged in water and the helm. – Monohulls can be mono- or twin-rudder. A monohull’s speed will determine how thrilling it is. Monohulls are a great choice for speed enthusiasts. – The monohull can be broken down into two parts: the rig or the hull. This is what makes the sailboat float. Ballast is a mixture of tanks filled with water that stabilizes the monohull. This counterbalances the list. (This is an inclination that’s a little under wind pressure or for some other reason). The monohull’s daggerboards are visible from the hull. They will keep the monohull on its course.

A keel is a device that ensures stability and prevents the boat from capsizing. The rigging refers to all parts that help a boat propel itself and manoeuvre.

Just a random website for/by Sailors

planing monohull sailboat


Yacht design.



We provide stock boat plans for both monohull and multihull sailing vessels, including sailing skiffs and sharpies. Our designs mainly feature timber construction, in plywood or cedar strip plank composite construction, using the W.E.S.T. system (wood epoxy saturation technique). Our designs are intended mainly as cruising boats, although several have done well in racing. All designs are suitable for amateur boat builders.

Monohull designs by Mike Waller Yacht Design.png


 multihulls  , photos from our builders.

Waller TS 540 by Mike Waller Yacht Design -  under sail

Photo galleries are provided on each design page where available


Types of Sailboat Hulls

Types of Sailboat Hulls | Life of Sailing

Last Updated by

Daniel Wade

June 15, 2022

Sailboats come in numerous hull shapes. These include single-hull monohulls, along with double and triple-hull multihulls.

There are two main categories of sailboat hulls: monohulls and multihulls. Common monohull types include flat-bottom vessels, fin-keel racers, bulb and bilge keel cruisers, heavy semi-displacement sailboats, and dense full-keel displacement cruisers. Multihull designs include catamarans and trimarans.

In this article, we'll cover the most common types of sailboat hulls along with their best uses. We'll explain the difference between monohulls and multihulls, along with how keel shape influences sailboat performance.

We sourced the information for this article from sailing experts, hull shape guides, and the written wisdom of famous sailboat designers. Additionally, we researched sailboat sales figures to determine the most popular vessel configurations available today.

Table of contents

‍ Importance of Sailboat Hull Design

A sailboat is defined by its rig and hull shape. Sailboat hull shape is one of the deciding factors on how it will handle. Additionally, the shape (and displacement) of a sailboat hull can be used to determine its strengths and weaknesses. Learning about sailboat hull shape can help you understand what kind of boat you need and what your vessel is capable of.

You can easily categorize sailboats based on their hull shape. For example, a heavy deep-draft displacement hull is likely a slow, steady, and comfortable cruiser. In contrast, a sleek flat-bottomed sailboat or catamaran is likely built for speed and could easily outpace even the most nimble displacement cruisers.

The most common kind of sailboat is the monohull. When you think of a sailboat, probably think of a monohull. The term simply means that the vessel has one single hull and nothing more. This is in contrast to multihulls such as catamarans, which are easy to spot and differentiate from traditional designs.

Monohulls are popular because they work. They're easy to build and narrow enough to fit in most marina dock spaces. Monohulls are also generally easy to handle in a variety of conditions, both fair and foul.

One drawback of monohull designs is that they are not quite as stable as most multihulls, though monohulls can recover more easily from a serious roll or capsize. They also cost a lot less, as the vast majority of production sailboats ever constructed were of the same basic single-hull configuration.

Centerboards and Swing Keels

The windward performance of sailboats is greatly improved by the use of a long keel or centerboard. The centerboard is the most simple type of stabilizing device used on sailboats. Usually, the centerboard is simply a long fin that protrudes from the bottom of the hull.

The centerboard keeps the boat on track when the wind is not moving in the boat's direction of travel. This is why sailboats can sail at different angles to the wind without being pushed to the side. A key characteristic of centerboards is that they can be raised and lowered, which is convenient on small boats that need to be trailered or beached.

Swing keels are similar to centerboards in that they can be raised and lowered, though they pivot on a hinge instead of sliding up and down in a truck. Swing keels are either recessed into the hull or held in a housing just below it, which usually also contains much of the boat's ballast. Swing keel designs free up cabin space that would normally be occupied by a bulky centerboard trunk.

Centerboards and most swing keels are an alternative to a permanently affixed keel. They're generally not considered to be as seaworthy as other hull designs, so their use is confined primarily to inland and coastal cruising.

Monohull Sailboat Hull Shapes

When in the water, it's difficult to distinguish between the different types of monohull shapes. In most cases, you have to pull the boat out of the water to figure out what hull shape you're dealing with. Next, we'll go over the most common monohull sailboat shapes and their uses.

Flat-Bottom Sailboats

Flat bottom sailboats are the easiest to build and often the fastest. These vessels have a very shallow draft and are often lightweight, so they slide easily and quickly across the water. Flat bottom sailboats make excellent racing boats and 'gunkholers,' which are primarily used for camping and hopping between shallow Islands.

Flat bottom sailboats usually have centerboards or swing keels, which makes them great for shallow water, beaching, and towing on a trailer. The use of flat bottom sailboats is confined primarily to inland and coastal waters, as a flat bottom does not handle well in swells and rough weather. Flat bottom sailboats pound hard on chop, and they lack the low center of gravity that's necessary for good stability.

Fin Keel Sailboat Hulls

The fin keel is a popular alternative to centerboards, and vessels utilizing this low-profile hull shape have proven to be quite seaworthy. Fin keels are popular on fast racing boats and lightweight cruisers. A fin keel resembles a centerboard, but it usually extends much further from the base of the hull.

The majority of a sailboat's draft comes from the fin keel, as the hulls of these sailboats tend to be rounded and shallow. They resemble flat-bottom designs, but slight rounding significantly increases comfort. Fin keel sailboats are ideal for racing and coastal cruising, and some models can be used for extended offshore passages.

Bulb Keel Sailboat Hulls

A bulb keel sailboat hull usually resembles most fin keel varieties. The hulls of these vessels tend to be shallow and rounded, with a long and thin fin extending from the base of the hull. A bulb keel is essentially just a thin blade with a bulb on the bottom.

Bulb keels are different from fin keels as they usually contain additional ballast weight for stability. The hydrodynamic properties of bulb keels are proven to be efficient. As a result, these boats can also be quite fast. In a direct comparison, a vessel with a bulb keel will likely be more seaworthy than the same sailboat with only a fin keel or a centerboard.

Bilge Keel Sailboat Hulls

The hull shape of a bilge keel sailboat usually resembles that of a bulb or fin keel sailboat, with one major distinction. Instead of one long and thin keel descending from the center of the hull, a bilge keel sailboat has two lengthier fins offset on the port and starboard side.

The idea behind the bilge keel design is that when the vessel heels to one side, one of the two keels will be straightened out. This, in theory, provides better tracking and improves stability. It also distributes ballast evenly on both sides. Bilge keels can also improve motion comfort, and they can reduce the vessel's draft by a small margin.

Bilge keel sailboats offer a balance between seaworthiness and speed. These vessels can be used as bluewater cruisers and coastal cruisers. They can also hold their own in any yacht club regatta.

While a bilge keel sailboat may not be ideal for cruising the North Atlantic during the winter, it can certainly make a safe and comfortable passage maker that can gain a knot or two of speed above its heavier counterparts.

Semi-Displacement Sailboat Hulls

Now, we'll look at some true bluewater cruising designs. The semi-displacement hull features a long and deep keel that runs from about the center of the hull all the way back to the rudder. Semi-displacement hulls get deeper the further back you go, reaching their longest point at the very aft end of the boat.

The offshore benefits of a long and deep keel are numerous, as this hull shape provides an enormous amount of stability and a very low center of gravity. The design itself it's quite old, and it's featured on many classic cruising sailboats and workboats.

Though less common in the modern era than more contemporary fin keel designs, a traditional semi-displacement sailboat offers easy handling and enhanced motion comfort. Semi-displacement hulls tend to have a deep draft and therefore are not ideal for shallow water. They handle confidently in all conditions, though they usually aren't as fast as newer designs.

Displacement Sailboat Hulls

Displacement hulls, also known as full keel hulls, are the bulldozers of the sailboat world. These traditional vessels are deep, heavy, relatively slow, and capable of plowing through the roughest weather conditions.

Displacement hulls have a long keel that begins at the bow and extends all the way after the rudder. Like semi-displacement hulls, full keel sailboats offer excellent motion comfort and confident handling.

Displacement hulls have the best directional stability and downwind maneuvering abilities. Their handling is more forgiving, and they're less jumpy at the helm. Many of these boats heel gently and give the crew more time to respond to changing conditions.

The primary downside to displacement hulls is their high cost and sheer mass. Displacement boats are large and take up a lot of space. They're usually too tall and heavy for trailering, so they tend to remain in the water most of the time.

Displacement hulls aren't made to just sit at the dock or jump around the lake; they're designed for real-deal offshore sailing. They also have the roomiest cabins, as the hull extends further down and longer than any other hull shape.

Now, let's examine multihull sailboat designs and why you may want to consider one. Some of the earliest seagoing vessels had multiple hulls, usually featuring one long hull (occupied by the crew) and a small stabilizing hull off to one side.

Multihulls have only recently become popular, and they make up a decent portion of the modern production boat market. This is because of their numerous design benefits and spacious cabins. Multihulls are almost guaranteed to be more expensive than monohulls (both new and used), and the used market is still saturated with expensive luxury cruising sailboats.

Modern multihull sailboats feature a large pilothouse in the center and plenty of cabin space in each full-size hull. They offer excellent motion comfort and achieve very high speeds. Due to their wide beam, they provide spacious living spaces and excellent stability. Here are the two main types of multihull sailboats.

From above, a catamaran looks like two thin monohull sailboats lashed together and spaced apart. Fundamentally, that's exactly what they are. Except catamarans have a very shallow draft and the capability to reach very high speeds.

Catamarans have two hulls instead of one, and each hull is typically a mirror of the other. They achieve their space using width rather than length, so a 30-foot catamaran has significantly more interior room than a 30-foot monohull.

Their primary drawback is that, due to their width, catamarans usually require two standard dock spaces instead of one. But at sea, they don't heel over dramatically like monohulls, which makes them much more comfortable to eat, sleep, and cook inside of.

Trimarans follow the same basic design principles as catamarans, except they have a third hull in the center. From above, a trimaran looks like a monohull with two smaller hulls lashed to the sides. Unlike a catamaran, the primary living space of a trimaran is in the large center hull. Trimarans are essentially just monohulls with stabilizers on the side, resembling ancient sailing canoes.

Trimarans have the same spatial and stability benefits as catamarans, though they can achieve higher speeds and better sea keeping. This is because of the additional stability provided by the center hall. Trimarans tend to be costlier than catamarans, though many sailors believe that the benefits outweigh the cost.

Best Sailboat Hull Shape for Speed

If we take wave height and weather conditions out of the equation, the fastest sailboats are usually the longest. Sailboats are limited by hull speed and sail plan size regardless of their hull shape. That said, the fastest sailboats tend to be flat bottom monohulls, fin keel monohulls, and trimarans.

Best Sailboat Hull Shape for Motion Comfort

The best sailboat for motion comfort is the catamaran. These wide and seaworthy vessels 'stance up' and minimize rolling. They also come close to completely eliminating heeling.

Wide and stable multihulls are popular because they alleviate some of the most common complaints of sailors. Trimarans are also an excellent choice for comfort, as their stabilizers minimize the effect of rolling in heavy seas.

Most Seaworthy Sailboat Hull Shape

Today, many people consider multihulls to be the most seaworthy design on the market. However, seaworthiness is more than just average stability in rough weather. Many Sailors argue that traditional displacement sailboat hull designs are the most seaworthy.

Displacement hulls have a low center of gravity which improves their knockdown survivability. In other words, in the (rare) event of a displacement boat knockdown, the weight of the keel is more likely to swing the boat back up and out of trouble. Multihulls cannot recover from a knockdown, as they like the pendulum-like recoil ability.

Most Spacious Sailboat Hull Type

The most spacious hull sailboat type is the catamaran. Catamarans have two nearly full-size hulls (one on each side) plus a large central pilothouse that resembles the main cabin of a large powerboat.

Many typical catamarans fit an entire kitchen into the Pilot House along with four private births and two full-sized heads in its hulls. Some mid-size catamarans even come with a bathtub, which is essentially unheard of on equivalent monohulls.

Spaciousness varies on small monohulls. Larger cabins are usually found on bulb and bilge keel designs, as swing keel and centerboard boats need somewhere to hide their skegs. Centerboard boats are the least spacious, as the centerboard trunk must occupy the middle of the cabin space.

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The Illustrated Guide To Boat Hull Types (11 Examples)

I didn't understand anything about boat hull types. So I've researched what hulls I need for different conditions. Here's a complete list of the most common hulls.

What are the different boat hull types? There are three boat hull categories: displacement hulls, which displace water when moving; planing hulls, which create lift at high speeds; and semi-displacement hulls, which displace water and generate lift at low speeds. The most common hull types are round-bottomed, flat-bottomed, multi, V-shaped, and pontoon hulls.

But that's all pretty abstract if you ask me, so below I'll give a simple overview of what it all means. After that, I'll give a list with pictures of all the different designs.

A Simple Overview of Boat Hull Types

Your boat hull will be the biggest factor in how your boat handles or sails, how wet it is, how bumpy - absolutely everything is determined by the hull shape. So it's important to understand what different hulls will do for you, and what each hull is best for. First, let's slice it up into rough categories.

Roughly, you can divide boat hulls into three categories:

  • Displacement hulls - Lie inside the water and push it away when they move
  • Planing hulls - Lie on top of the water and don't push it away
  • Semi-displacement hulls - Lie inside the water and push it away, but can generate lift

Everything I'll be mentioning below is one of those three, or something in between.

There are five common boat hull types:

  • Round-bottomed hulls - handle well in rough water: sailboats
  • Flat-bottomed hulls - very stable for calm inland waters: fishing boats
  • Multihulls - very stable and buoyant: catamarans
  • V-Shaped Hulls - fast and comfortable in chop: powerboats
  • Pontoon hulls - fast and stable: pontoon boats

And then there's everything in-between.

Here's a quick and handy overview of the different hull types

planing monohull sailboat

In each category, we find different designs and styles that have different characteristics. There isn't a real clear distinction between categories and styles: there are semi-displacement hulls and so on. So I thought the best way to learn you the different hull types is by simply creating a list with lots of pictures, instead of getting all theoretical about it.

So below I've listed all the different hull styles I could possibly think of, mention what category and type it is, the pros and cons of each one, and give you examples and illustrations for each one.

planing monohull sailboat

On this page:

Displacement hulls, round-bottom hull, catamaran hull, trimaran hull, planing hulls, flat-bottom hull, deep v-hull, modified-v hull, stepped hull, pontoon hull, semi-displacement hulls.

Examples: Sailboats, trawlers, fishing boats

planing monohull sailboat

Displacement hulls displace water when moving. These hulls lie in the water, instead of on top of it. The amount of water they displace is equal to the boat's weight. Displacement hulls handle way better in rough waters than flat-bottom hulls. That's why most cruisers have some sort of displacement hulls. There are actually all kinds, shapes, and forms of the displacement hull design, which we'll go over later.

The most important thing to understand about the displacement hull, is that it operates on buoyancy. This means that most of the boat's weight is supported by its capacity to float . Planing hulls, on the other hand, operate on lift instead, but we'll dive into that later.

Sailboats typically have displacement hulls, but also fishing boats, trawlers and crabbers. All in all, it's used for each boat that needs to handle well in rough conditions.

Learn everything there is to know about displacement hulls in this article . It lists all the pros and cons and really goes into detail on the nitty-gritty about how displacement hulls actually work .

planing monohull sailboat

But they are also slower than flat and planing hulls because the boat creates more resistance when moving. It has to push the water aside. In fact, this type of hull has a built-in upper-speed limit.

This upper-speed limit is called maximum hull speed . It means that the length of a displacement hull directly determines the maximum speed. It can't go faster, because the water-resistance increases with the boat's speed. To learn everything about calculating maximum hull speed , please check out my previous article here.

planing monohull sailboat

A round-bottomed hull is a type of displacement hull - it lies in the water and has to power through it. But since it's rounded, it creates little resistance and is effortless to move through the water. It's a very smooth ride and typical for any sailboat that sort of glides through the waves. In contrast, powerboats really have to eat their way through the water.

Examples: Canoes, sailboats

They are also one of the least stable. Since the bottom is rounded, your boat or canoe will rock plenty when boarding or moving around. They are also easy to capsize. That's why pro canoers learn to do a 360 in their canoes. I've never did a roll myself but came close enough a couple of times.

planing monohull sailboat

Almost all sailboats use a round bilge as well. This provides it its buoyancy and makes sure it handles well in waves. But since a rounded bilge is easy to capsize, a lot of sailboats have some sort of keel, which stabilizes the roll.

Nearly all ocean-going vessels use some sort of displacement hull, and the round bottom is the most common one. But our next guest is very popular as well.

The catamaran is similar to the pontoon hull (read on to learn more on that one), but it is a displacement multihull instead of a planing one. So it has two hulls, that lie inside the water and displace it. Like the pontoon, you will have to try really hard to capsize this design (and it won't work).

Examples: well, catamaran sailboats. But also this cool catamaran trawler:

planing monohull sailboat

Catamarans are extremely popular ocean cruisers. Their biggest pro is their extreme stability and buoyancy. And they have a very shallow draft for a displacement hull, making them very popular for sailing reefs and shallow waters, like the Caribbean.

Some cons for the catamaran are less agile than monohulls. They have a large turning radius, making them less maneuverable. Also, expect to pay high marina fees with this one.

Speaking of marina fees, our next one can go either way.

I think trimarans are incredibly cool, and especially the second type.

There are two types of trimarans:

  • a catamaran with three hulls instead of two,
  • or a displacement monohull with two floaters.

The first has the same characteristics as the catamaran: it's a displacement multihull, but now with three hulls:

planing monohull sailboat

The second can be a regular displacement monohull, with two pontoon-type floaters that provide extra buoyancy, making the total thing a hybrid between pontoon and displacement:

planing monohull sailboat

This last one has all the pros of a catamaran in terms of stability, but: you can simply wheel in those floaters whenever you head for port. That saves you a lot of money. And you can trailer her! Imagine that, a towing a trimaran home.

So those were the most common displacement hulls, aka what lives in the water. Let's move on to the planing hulls, aka what lives on the water.

planing monohull sailboat

Planing hulls are a hybrid between the flat-bottom and displacement hulls. Planing hulls displace water at low speeds , but create lift at higher speeds . The shape of their hull + speed lifts them out of the water, making them glide on top of the water. Most powerboats look like flat-bottom boats but use a shallow V-shape that helps the boat to handle better at higher speeds.

Examples: Water sports boat, powerboats

The most important thing to understand about planing hulls is that they operate mainly on lift instead of buoyancy. This means the weight of the boat is mainly supported by dynamic forces 1 . With the right amount of power, this design generates lift, which results in less resistance. This is why they are a lot faster than boats with displacement hulls, but also a lot rougher, even with mild chop.

A lot of powerboats use some sort of planing hull. Again, there are many designs and variations on the planing hull, and I'll try to mention as many as I can below.

Because the wedge of the hull runs into the water, it is much easier to handle at high speeds. At lower speeds, it is able to keep its course, even with a bit of wind. However, whenever the boat starts planing, it is prone to wind gusts, since the wedge shape no longer stabilizes the boat.

The flatter the hull, the faster it will go, but also the more poorly it will handle. Other powerboats use deep V-hulls, which I'll discuss below. But first, let's take a look at the flattest hulls you'll ever see.

A flat-bottom hull lies on top of the water and doesn't displace water (okay, very little) as it moves. Since there is no displacement, there is also little to no friction when moving. This makes it potentially fast, but it handles pretty poorly. It is one of the most stable hull design.

Examples: rowboats, (old) high-performance powerboats, small skiffs, small fishing boats, tug boats

planing monohull sailboat

They aren't just incredibly stable, they're also very practical. Because the bottom is practically flat, they maximize boat surface. But they are also extremely choppy in rough weather and waves. They will handle very poorly with stiff winds, as the wind can simply catch them and blow them across the water surface. That's why this design is almost exclusively used for calm, small, inland waters.

This type of hull operates mainly on buoyancy , like the displacement hull, but it doesn't require the same amount of power to propel, which is why it's faster.

Because of the uncomfortable ride, not a lot of boats use a perfectly flat bottom. Most boats nowadays use some sort of v-hull or hybrid design, like a semi-displacement hull; especially larger boats. So not a lot of boats have a real flat bottom. However, we do call a lot of boats flat-bottomed. How come?

planing monohull sailboat

There are two types of hulls we call flat-bottoms:

  • Of course boats with an actual flat bottom
  • Boats with almost no deadrise
What is the hull's deadrise? The deadrise is the angle of the front of the hull to the horizontal waterline.

As you can see, the green sailing dinghy in the picture above has a deadrise that's barely noticeable.

Let's move on to other variations of the planing hull. One of the most popular hull design for modern-day powerboats is the Deep Vee hull. And that's as cool as it sounds.

planing monohull sailboat

This is a type of planing hull that combines the best of both worlds.

These types of hulls are very popular on modern-day powerboats, and no wonder. With a V-shape that runs from bow to stern, deep into the water, you can handle this boat even in offshore conditions. It handles a lot better than flat-bottomed hulls, while it's at the same time extremely fast.

Examples: Most modern powerboats.

The Deep V-shape acts as a tiny keel of sorts, stabilizing the boat and making it more reliable and maneuverable. The rest of the hull acts as a planing hull, giving the boat its fast edge. Even at high speeds, the Deep V will cut into the water, making it more handleable.

The deep-V design is just one of many variants on the V-hull. Below we'll talk over another, the modified V hull.

planing monohull sailboat

The modified V hull is the ultimate crossover of all planing hull types. It's a mix of the flat-bottom and Deep V hull. It is one of the most popular hull designs for small motorboats. It's flat in the back and then runs into a narrow V-shape to the front. The flat back makes it more stable, and adds a little speed, while the V-shape front ensures good handling.

It is, in short, kind of the compromise-family-sedan of boat hulls. It's the fastest design that's also stable, that's also safe, and that also handles well. But it's not the best in any of those things.

Most powerboats you've seen will have some sort of Vee or Modified-V hull.

Stepped hulls are used on high-performance powerboats. It's a type of planing hull that reduces the hull surface by adding steps, or indents in the hull below the waterline. It looks something like this:

It is said to work extremely well at high speed (60 knots and up) and adds up to 10 knots to your top speed.

On to our next design. There are also planing multihulls, and they might even look like catamarans to you. Meet the pontoon hull.

planing monohull sailboat

Pontoon hulls float on top of the water using pontoons or floaters that create lift. It's a type of planing multihull that doesn't lie in the water, so it doesn't displace a lot of water. They don't really handle well. As with any multihull, they aren't agile - they're not great at maneuvering. They also have a very large turning radius. But they are extremely stable: there's no chance you'll capsize this.

Examples: Cruisers, modern trawlers, motor yachts, Maine lobster boats

planing monohull sailboat

Semi-displacement hulls are smack bang in the center of planning and displacement hulls. They are a bit better for speed than displacement hulls are. They are a bit better for handling rough waters than planing hulls are. This makes them very versatile.

planing monohull sailboat

You can see these a bit like being 'half-planing' hulls. These hulls are designed to plane at lower speeds than normal planing hulls - somewhere in the range of 15 - 20 knots, depending on the length of the boat. It also requires less power. When the hull lifts, it reduces drag (water resistance), making it faster and more efficient.

Semi-displacement hulls are perfect for boats that need to be steady and seaworthy but fast at the same time.

For more information about semi-displacement hulls, please check out my in-depth guide to semi-displacement hulls here . It has a diagram and lists all the pros and cons.

So those were my 11 examples, and my step by step explanation of the different types of boat hulls and functions. You now have a solid basic understanding of boat hulls, and can recognize the most common ones. I hope it was helpful, and if you want more good sailing information, be sure to check out my other articles below. .  ↩

I was wondering what your opinion would be on the ship uss Texas as far as hull type and bow type. I think it has a plumb bow and it looks to have a displacement or flat bottom hull. Im doing some research and a better trained eye would be of great help. I used images “bb-35 dry dock” to help see the hull shape. Thank you

Shawn Buckles

Hi Kirk, I don’t know about trained but here we go. I’ve checked the picture, it’s definitely a displacement hull I’d also say it’s a plumb bow.

Hahahahaa imagine liking boats hehehehehe Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water Extremely stable & faster Handles well in rough water

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You may also like, a complete guide to displacement hulls (illustrated).

The displacement hull is the classic go-to hull design for sailboats and one of the most recognizable ones out there. In this guide, I explain all there is to know …

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How do hydrodynamics relate to planing of a boat? a planing hull uses hydrodynamic lift to rise up and out that all true planning monohulls share a number of. A sailboat which uses hydrofoils either immersed or planning on the water's surface to resist heeling. the boat uses two hydrofoil assemblies, one for the. A sailboat or sailing boat is a boat propelled partly or entirely by sails smaller number and configuration of masts, and sail plan. popular monohull designs. This book addresses the principles involved in the design and engineering of planing monohull power boats, with an emphasis on the theoretical fundamentals that.

Foiling on a Laser??? - Dinghy Anarchy - Sailing Anarchy Forums

I have always wanted to sail a boat like a 2.4 meter but with much higher performance. i suggested a concept years ago and wonder if anyone else has.... This book addresses the principles involved in the design and engineering of planing monohull power boats, with an emphasis on the theoretical. Planing in its simplest useful form starts with a prismatic box of rectangular cross-section, with length l larger than the cross-sectional dimensions, b and d, in. A sailboat which uses hydrofoils either immersed or planning on the water's surface to resist heeling. the boat uses two hydrofoil assemblies, one for the.

Real Small boat launch design | Tals

“planing” is bunkum – myths about planing, displacement and equal the weight of the boat. planing lift planing, displacement and semi planing.

505 (dinghy) - Wikipedia

How do hydrodynamics relate to planing of a boat? a planing hull uses hydrodynamic lift to rise up and out that all true planning monohulls share a number of. A monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another.. A sailboat or sailing boat is a boat propelled partly or entirely by sails smaller number and configuration of masts, and sail plan. popular monohull designs. “planing” is bunkum – myths about planing, displacement and equal the weight of the boat. planing lift planing, displacement and semi planing.

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It's pretty obvious from my website that sailboats are my favorite choice for life afloat.  

Even though a sailboat doesn't have as much interior room as a trawler, houseboat or typical power boat, I like to sail.

I like being able to use the wind to get from Point A to Point B and I like the sound of halyards slapping against the mast when I'm in an anchorage or mooring field or marina on a windy night.

Sometimes in marinas a neighbor or dockmaster will complain about the clanging halyards and I'll have to tie them down.  Otherwise the sound is as appealing to me as church bells might be for others.

Most sailboats can be lived aboard with enough comfort that they can be considered to be liveaboard sailboats. Some are better than others, and here are a few of my favorites.


I have loved sailing since my boyhood days and would live on a sailboat no matter its disadvantages.  I just feel right aboard a sailing vessel.  If you like sailboats too, you have a lot of choices.

As the name implies, these boats have a single hull.  They usually have a fairly deep keel or centerboard that limits the places you can liveaboard.  You will usually have to forego shallow marinas or moorings. 

Space is also a problem.  Sailboats have pointy ends, the forward one known as the bow, and the sides of the hull are curved.  This presents a problem for headroom and storage space.

The mast height also limits you to marinas that are not constrained by low overhead bridges.

Cutter Rigged Sailboat, Pier 66, Fort Lauderdale

The cutter rigged monohull sailboat above was in Pier 66 Marina in Fort Lauderdale when this photo was taken.  It would be a comfy and spacious liveaboard.  The large pilot house with abundant glass means plenty of light below and good headroom.  

CSY 33 Cutter Under Sail

Another example of this type of sailboat is the CSY-33.  Although it's no longer in production, a good used one can be an ideal liveaboard.  These types of boats - pilothouse sloops or cutters - are very comfortable homes afloat.

Don't let budget or size discourage you from living aboard.  Many people have lived aboard the Flicka, a twenty foot long sailboat formerly manufactured by Pacific Seacraft.


1.  The Pacific Seacraft Flicka


A Flicka may be one of the smallest boats that I could consider a liveaboard. Plenty of people would agree.

The Pacific Seacraft Flicka is only 20 feet long on deck, and is almost the same length on the waterline.   It has tremendous room below for such a small boat and has served as a liveaboard home for many people.

The Flicka has even made several Atlantic and Pacific crossings and circumnavigated the globe.  This small boat proves you don't need a big boat to live aboard.

Pacific Seacraft Flicka Under Sail Downwind Wing and Wing

The designer of the Flicka, Bruce Bingham , lived aboard with his then companion, naval architect Katy Burke for many years.  They made it their full time home and cruised extensively.

Pacific Seacraft Flicka at Anchor

2.  The Island Packet 27

Island Packet 27 at Anchor

An Island Packet of any size makes a wonderful liveaboard sailboat.  They are extremely well built and beamy and offer a lot of room below.

The entry level Island Packet for many years was the IP26, and it was superseded in following years by the IP27.  Island Packet no longer makes these boats;  they have moved into larger and larger sailboats over the years.

Good used IP26s and IP27s are available on the used market for prices in the range of $35K to $45K.  These are among the few Cadillacs of American sailboats.

3.  Nonsuch 30 Ultra

The Nonsuch 30 has an amazing amount of room below because it is rigged as a catboat with the mast stepped far forward in the bow.

planing monohull sailboat

The boat is no longer manufactured.  It was made in Canada in the years 1978 to 1995. More than 500 of the 30 foot model were manufactured.

I first saw one at the Miami International Boat Show in 1980 and fell in instant love.  Not only is the boat solid and well made, it has a very liveable interior and even includes a separate shower.  The Ultra model, shown in this floor plan, has a pullman berth on the port side forward and an amazing amount of hanging locker space.

planing monohull sailboat

The simple sailing rig means that even an old sailor can single hand this beautiful craft.

The very active International Nonsuch Association has a website where you can learn a lot about the boat.

planing monohull sailboat


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  6. Timelapse of Boats from a Pier in Lido


  1. 14 Best Monohull Sailboats

    7. Bénéteau Oceanis 45. cmhogarth. Named yacht of the year in 2012, the Bénéteau Oceanis 45 remains one of the most popular monohull sailboats in the world and for a good reason. This is a sailboat that redefines the important themes that made the Oceanis 50 so popular in a much better way.

  2. Planing boat theory

    It is primarily a planing boat that will plane with crew and full cruising gear at low power and readily hold plane down to about 10 or 11 mph. In light trim and ideal conditions with only one aboard, the Bluejacket 24 will actually clear the transom and appear to start to plane at about 9 1/2 mph.

  3. Basics of Sailboat Hull Design

    Planing Hulls. With even more power, a boat can leave its bow wave in the dust and zoom past it. This requires the boat's bottom to channel water away and sit on the surface. ... Monohull sailboats have an immense amount of ballast low in their keels, which means they have very high ultimate stability. A narrow monohull has low form stability ...

  4. Foiling Monohull Design

    Mar 4, 2024. Original: Apr 13, 2017. Although still in development, the Beneteau Figaro 3 represents the cutting edge of production foiing monohull design. Image Courtesy of Beneteau. Examining Vendée Globe boats on the dock in Les Sables d'Olonne prior to the start of the race last November, one thing was perfectly clear: the genie is out ...

  5. Understanding monohull sailboat stability curves

    A larger AVS indicates a boat that's harder to capsize. The region of positive stability is the region in which the boat will try to right herself. The integral of the righting moment curve (i.e. the area of the green region) is an indicator of how much energy is needed to capsize her.

  6. Planing (boat)

    Planing (boat) A Contender dinghy planing on a broad reach. Note the typical way the bow lifts up while the stern skims over the water. Planing ( / ˈpleɪnɪŋ / PLAY-ning) is the mode of operation for a waterborne craft in which its weight is predominantly supported by hydrodynamic lift, rather than hydrostatic lift ( buoyancy ).

  7. Experimental and statistical analysis of the ...

    These vessels are monohull planing boats propelled with fossil fuel outboard engines with an output power of 200 HP that travel at high speeds, concretely at around 55 km/h on average, and are able to convey between 20 and 25 passengers. Fig. 3 shows a reference image of the monohull boat.

  8. The Definitive Guide to Sailboat Hull Types

    Before that happens, it's essentially a displacement hull. 2. Plowing. While a boat with a planing hull is picking up speed and lifting itself out of the water, it's in a plowing mode. You'll know when a boat is in plowing mode when the bow of the boat is elevated and the boat is throwing a relatively large wake.

  9. Monohull

    A monohull is a type of boat having only one hull, unlike multihulled boats which can have two or more individual hulls connected to one another. ... Planing hulls - Hulls that ride on top of the water are called planing hulls, because when they reach speed, the hulls are substantially lifted above the water; this is known as planing (to plane).

  10. William˜S. Vorus Hydrodynamics of Planing Monohull Watercraft

    boat settles-in to steady planing on the calm water surface. Decisions required in the rational design of planing craft, particularly with regard to two aspects, are particularly difficult to conclude with precision: these aspects are planing speed and seaway motions (slamming). High speed and low

  11. Crouch's Calculator

    This useful calculator computes an estimate of boat speed for a modern planing monohull using inputs of the power at the propeller shaft, the total boat weight, and a coefficient called the hull factor. The calculator can also compute any one parameter from the other three parameters—a most useful feature. The hull factor for predicting speed ...

  12. Hydrodynamics of Planing Monohull Watercraft

    This book addresses the principles involved in the design and engineering of planing monohull power boats, with an emphasis on the theoretical fundamentals that readers need in order to be fully functional in marine design and engineering. ... Author William Vorus focuses on three topics: boat resistance, seaway response, and propulsion and ...

  13. All you need to know about Monohull Sailboats

    The definition of monohull is a sailboat with a single hull. Monohull sailboats are often categorised by the type of rig (mast and sails. Monohull sailboats are also called a sloop, cutter, ketch, yawl, or schooner. Sloop. Modern sailboats are most commonly the sloop. It has one mast and two sails. Typically, a Bermuda-rigged main and a headsail.

  14. Plans For Catamarans And Monohulls

    We provide stock boat plans for both monohull and multihull sailing vessels, including sailing skiffs and sharpies. Our designs mainly feature timber construction, in plywood or cedar strip plank composite construction, using the W.E.S.T. system (wood epoxy saturation technique). Our designs are intended mainly as cruising boats, although ...

  15. Types of Sailboat Hulls

    June 15, 2022. Sailboats come in numerous hull shapes. These include single-hull monohulls, along with double and triple-hull multihulls. There are two main categories of sailboat hulls: monohulls and multihulls. Common monohull types include flat-bottom vessels, fin-keel racers, bulb and bilge keel cruisers, heavy semi-displacement sailboats ...

  16. The Illustrated Guide To Boat Hull Types (11 Examples)

    The second can be a regular displacement monohull, with two pontoon-type floaters that provide extra buoyancy, making the total thing a hybrid between pontoon and displacement: ... The rest of the hull acts as a planing hull, giving the boat its fast edge. Even at high speeds, the Deep V will cut into the water, making it more handleable. The ...

  17. Monohull Sailboats & Charter

    Ultimate sailing performance. With its classic, single hulled template, the monohull is the original sailboat. Our monohulls are designed by renowned manufacturers Jeanneau, Beneteau, and Dufour. With performance in mind, our monohulls give you optimal control whether the conditions are flat and calm or rough and wild.

  18. Monohull Sailboat Charter

    Monohull Sailboat. Classically refined and contemporarily equipped, The Moorings' monohulls set the standard for unforgettable performance and comfort. Backed by the boat-building heritage of leading manufacturers Beneteau and Jeanneau, these vessels are the premier choice for some of the most sought-after yacht charters available today.

  19. Moorings 45.4

    Take to the ocean confidently aboard The Moorings 45.4, a beautiful yacht in which comfort reigns supreme and exceptional maneuverability delivers remarkable performance on the water. Designed by Beneteau, The Moorings 45.4 allows you to voyage with ease. The mainsail arch and moving back the mast offer new balanced and high-performing sailing ...

  20. Sunsail 47

    The Sunsail 47 is a sleek and spacious monohull that preforms well under sail. Find out more about the 3-cabin configuration. ... Graceful offshore cruiser with exceptional interior and deck plan designed with ease of handling as a priority. To find out what's included on a bareboat and flotilla sailing vacation, please click below. BAREBOAt ...

  21. Planing Monohull Sailboat

    A sailboat or sailing boat is a boat propelled partly or entirely by sails smaller number and configuration of masts, and sail plan. popular monohull designs. This book addresses the principles involved in the design and engineering of planing monohull power boats, with an emphasis on the theoretical fundamentals that.

  22. Liveaboard Monohull Sailboats are the Choice of Traditionalist

    The cutter rigged monohull sailboat above was in Pier 66 Marina in Fort Lauderdale when this photo was taken. It would be a comfy and spacious liveaboard. The large pilot house with abundant glass means plenty of light below and good headroom. ... The Ultra model, shown in this floor plan, has a pullman berth on the port side forward and an ...

  23. Sunsail 38

    Cabins 3. Length Overall 37'2". Beams 12'4". Sail Area 754 ft². Water Capacity 87 gal. Fuel Capacity 34 gal. Air Conditioning This yacht has shore powered A/C. Skipper/Cook A hired skipper will require an interior cabin. Convertible Saloon: Yes, this yacht has a convertible saloon which is included as a double berth in the maximum capacity.