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There are two stages in creating a successful catamaran crossbeam solution.

First you must DESIGN the structure, only then can you CALCULATE it. The former is usually the more important and certainly the one most people get wrong.

There are several factors to consider when designing crossbeams: First, you need stiff crossbeams, not just strong ones. Fortunately stiff beams are nearly always over-strong. By stiff I mean one without any obvious deflection. Engineers normally consider that to be 1-2% of length.

Why a stiff beam? Well imagine crossbeams made out of rubber. They would never break, but would be so flexible you could never keep the two hulls in line and the mast would fall down as the rigging flexed.

How can you make a stiff beam? Well, actually it’s not just the beams that you want stiff, rather it’s the boat as a whole. I’ve found that the best way to do this on an open catamaran is to have two crossbeams plus a separate one to take the mast loads. The actual positioning of the beams is also very important.

Although crossbeam size and placement is often complicated by rig and accommodation considerations, the beams must take priority! If they are too near the middle of the boat then the bows can flex up and down and you cannot keep the rig tight. If too close to the ends (especially to the bows) there isn’t enough boat to take the loads and, furthermore the beam cantilever is longer.

Having the first crossbeam almost at midships was the most spectacular mistake the Team Phillips designer made. Pete Goss and I are members of the same sailing club and other members were upset when I expressed concern about its design. After the breakage they realised what I was on about.

Once you’ve designed the structure it’s really a trivial problem to calculate the necessary scantlings. If you use a strain energy analysis you’ll find that the loads will dissipate quite quickly into the hull. Indeed it’s extremely rare for beams to break off the hulls. Usually the problem is the beams themselves breaking.

Fortunately it’s very easy to check the strength of catamaran beams once you’ve built them. You simply jack the boat up with a support under each bow and each stern. Then take one of the chocks away. The boat shouldn’t move appreciably. It looks scary, and is certainly a load that you wouldn’t get at sea. But is very reassuring all the same.

You can see a photo of our Merlin Tucanu surviving this test on the “Review of 2007" page. The Merlin and the similar Strider design use two aluminum tubes with inertias around 500cm4. I usually use ply and timber beams as they are easy to make and to attach to the boat (and of course to attach boat to them), but they are heavy (approx 1.5 x the weight of aluminum tubes).

It is not just open deck boats that need good crossbeams. Unlike most designers my bridgedeck catamaran designs also feature big beams. Apart from a big netting beam there will be one under the mast and one under the aft end of the cockpit. I design the anchor lockers and forward end of the bridgedeck to act like another beam. By adding theses beams I ensure that the boat is extremely stiff and that it is not a problem fitting big deck hatches or large companionway doors. I have seen many production bridge deck cabin catamarans that rely solely on the hull/deck mouldings for strength flex, and even crack, bulkheads because they are simply not stiff enough.

On a large, say 40ft, 7ton boat, beams that are strong enough may weigh 400kgs. Ones that are too weak will still be heavy - they may weigh 300Kgs. It’s NEVER worth trying to save weight in your beams. Carbon beams may weigh 250Kg at a huge cost. Is it worth spending that much to save 150Kgs at best? I’d rather spend the extra money on better sails and deck gear.

Catamaran Beam to Length Ratios Explained: For Beginners

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Starting my sailing career something that struck me was the wast number of weird words and strange terminology, no longer was a rope just a rope, it’s a halyard or a sheet. In this article, I will explain one concept that is important to understand for anyone trying to buy a boat or for someone who wants to better understand the limitations of the vessel they already have.

Catamaran beam-to-length ratios are mathematical representations of the difference between the length of a sailing vessel and its width. There are multiple beam to length ratios, some impacts stability (Bcl/Lwl), and the amount of sail the vessel is able to carry. Others are used to calculate exterior space (B/L). In general, a narrow boat will be less stable but weigh less and cheaper to build.

Most modern catamarans have a beam to length ratio of >50%. You can easily calculate this on your own by following the steps below. But first, let’s check out some more terminology to make sure we really understand this ratio.

Table of Contents

Nautical Terminology

No matter how much you love the ocean, you will have limited success if you are unfamiliar with the words that go with adventuring out on it. I need to clarify some nomenclature before we delve into the ins and outs of ratios and catamarans (and monohulls).

• Beam overall (Boa): is the width of a boat at its widest point. The wider a ship’s beam, the more interior and exterior space. this allows for more gear and and better living accomodations.
• Draft: sometimes spelled “draught,” is the measure of how deep your vessel “sits” in the water. Catamarans have shallower drafts than monohulls, meaning they can sail in shallower waters and some can even be sail all the way up onto the beach, called beaching a cat .
• Catamaran: is a boat with twin hulls positioned parallel to each other. This design lends stability to the craft, and since there are two hulls, each can be narrower than a monohull without giving up stability. 
• Monohulls: boats with one hull. They derive their stability from a heavy keel and a wide hull, in comparison to a catamaran with two thin hulls separated far apart.
• Length over all (Loa): is measured from the aft to the bows including all gear such as bowsprits etc. To be compared with Length on waterline LWL.
• Length on waterline (Lwl) is the boats length measured on the surface of the water.

If you want to better understand catamaran construction and the impact of hull shape on performance and safety I suggest you read the book Catamarans; The complete guide for cruisers . It has helped me to better understand multihull dynamics in a more structured way than just googling. Gabo

Different Beam to Length Ratios

Hull centerline beam to waterline length (bcl/lwl) :.

The distance between the centerlines of the hulls divided by the waterline length on one hull is a good indicator of performance. It measures the points of the boat that interacts with the water. A higher ratio will give a higher resistance to capsizing and a lower ratio will increase drag due to wave interactions under the bridge deck.

Compared to the beam overall to length overall (Boa/Loa) that more or less only gives you an understanding of whether or not the boat will fit in a certain slip or what you will pay for a canal passage.

Hull Fineness Ratio (HFR)

Hull Fineness Ratio (HFR) is another name for Hull length-to-beam ratio . This is basically the same as the ratio mentioned above but only measures one of the hulls instead of the entire boat. And “fineness,” essentially, means “thinness.” Most cats have a ratio between 8:8 and 10:1 .

Boat Overall Beam (Boa) to Length Overall (Loa)

These are the exterior measurements of the boat. This ratio will not offer much other information than estimating marina fees and general boat size. To understand catamaran stability the two above ratios are much better since they show how the boat interacts with the water. It is in theory possible to have a very high Boa/Loa ratio but still have a boat that is very unstable due to having a low Bcl/Lwl ratio.

General Rules When Calculating Ratios

Ratios are exercises in long division. Since you remember your rules from math in school, you know that the order of the numbers in the equation makes a difference. 

Make sure you divide Beam by Length (B/L) and not the opposite!

If you mix them up you will get the wrong result and you might assess the stability of the boat incorrectly. And remember to stick to either meter or feet.

The formula looks like this:

B/L = Beam (in ft or meter) to length (in ft or meter) ratio

But how do you measure and from where to where? With those questions in mind, we add even more terminology to all this ciphering.

If, for instance, you have a bowsprit (the railing at the bow that extends past the deck), including this in your length measurement will skew your ratio. The extra length added by the largely cosmetic feature will not contribute to the stability or lack thereof of the craft, mainly because it does not touch the water.

So we look, then, at the measurements at the waterline .

Why Ratios Matters

If your Bcl/Lwl is too low, you will have an unstable craft. Adding a sail to the mix makes it even more so – if you have a ridiculous ratio of something like 1:18, wind in the sails at the correct angle will very likely capsize it. A wave of moderate size could do it, too.

If you want to know why catamarans capsize i suggest you read my other article ! Gabo

But a 1:1 Bcl/Lwl will make for a floating square with the maneuvering ability of a brick. A floating brick, sure, but it’s still a brick. This ratio is something you only see on really fast racing trimarans, since trimarans lift the windward hull the actual ratio when turning is half of that.

The fineness of a hull determines its speed and stability, which means that with every increase to one of those factors comes a decrease in the other. 3:1 seems to be the Goldilocks Zone for most monohulls. But since catamarans have two hulls separated wide apart the cat will be able to have thinner hulls while still maintaining high stability, a ratio around 8-12:1 is common on catamaran cruisers.

Final Thoughts

Casual sailors may never calculate Bcl/Lwl, B/L, or hull fineness ratio. But if you’re looking to buy a boat and want to better understand its sailing capabilities then these numbers will give you the ability to objectively compare different boats.

Speed and stability are the main factors governed by these ratios, and a change in one of them changes the other in the opposite direction. Generally speaking, the wider the beam, the more stable a ship is.

• Boat Building: Catamaran Design Guide – Catamarans Guide
• Marine Link: What Hull Shape Is Best?
• MB Marsh Marine Design: Length-beam ratio
• Multihull Dynamics: Six Kinds of Cats and Two Kinds of Trisi
• Ocean Navigator: Beam and Draft

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High Speed Catamarans and Multihulls pp 535–607 Cite as

Structure Design

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So far in this book we have discussed the different configurations of multihull vessels from the point of view of their form, stability, resistance, and motions in waves. Once we have defined the desirable form, the question is how to create the structure that will support the payload and resist the forces that the environment will apply to it. Our purpose with this chapter is to give a summary of the issues connected with the design of a multihull structure, including how this links to the hydrostatic and dynamic analyses and building from the initial estimates of the synthesis in Chap. 7 .

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Yun, L., Bliault, A., Rong, H.Z. (2019). Structure Design. In: High Speed Catamarans and Multihulls. Springer, New York, NY. https://doi.org/10.1007/978-1-4939-7891-5_12

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Catamaran Hull Design

• Post author By Rick
• Post date June 29, 2010
• 2 Comments on Catamaran Hull Design

Part 1: Notes from Richard Woods

Since the America’s Cup experimented with going multihull, there’s been a lot of interest in catamaran performance and the catamaran hull designs that define performance. Many guys are investigating whether to buy a catamaran or design and build their dream boat. Let it be said here that building a large catamaran is not for the faint of heart. People begin building 100s of boats a year, yet few are ever completed, as life always seems to have a way of interfering with a good boat build. 

Never the less, since the rest of this website is about selecting and buying a boat , it only seems fair to have at least one webpage that covers catamaran design. This page contains notes on boat hull design goals and an accompanying page from Terho Halme has mathematical formulas used in actual catamaran hull design. It has become a popular research stop and an important reference to the catamaran design community.

The content of this page was reproduced from the maestro of Catamaran designs, renown British naval architect, Richard Woods, who not only designs catamarans, he sails them across oceans…. repeatedly. He has a lot to say on the subject of catamaran hull design.

“…When it’ all said and done, the performance of a sailing catamaran is dependent on three primary specs: length, sail area and weight. If the boat is longer it generally means it’ a faster boat. If she has more sail area, it means she’ a faster boat and if she’ light it means she’ a faster boat.  Of course, there are limits: Too much sail area capsizes the boat in brisk winds. If the boat is designed too light, she will not take any kind of punishment. Too slim a hull design and the boat becomes a large Hobie Cat capable of only carrying your lunch. Of course, too long and large and you’d have to be Bill Gates to afford one. Then there are lot of additional and very important factors like underwater hull shape, aspect ratios of boards and sails, wet deck clearance, rotating or fixed rigging and so on….” Richard Woods

All Catamarans are not equal, but all sailboats have two things in common: They travel on water and they’re wind powered, so the Catamaran design equations in the 2nd part should apply to every catamaran from a heavy cruising Cat to a true ocean racer.

Richard Wood’s comments on catamaran design:

We all know that multihulls can be made faster by making them longer or lighter or by adding more sail. Those factors are the most important and why they are used as the basis of most rating rules. However using just those figures is a bit like determining a cars performance just by its hp and curbside weight. It would also imply that a Tornado would sail as fast forwards as backwards (OK, I know I just wrote that a Catalac went faster backwards than forwards)

So what next?? Weight and length can be combined into the Slenderness Ratio (SLR). But since most multihulls have similar Depth/WL beam ratios you can pretty much say the SLR equates to the LWL/BWL ratio. Typically this will be 8-10:1 for a slow cruising catamaran (or the main hull of most trimarans), 12-14:1 for a performance cruiser and 20:1 for an extreme racer.

So by and large faster boats have finer hulls. But the wetted surface area (WSA) increases proportionately as fineness increases (for a given displacement the half orange shape gives the least WSA) so fine hulls tend to be slower in low wind speeds.

The most important catamaran design hull shape factor, is the Prismatic Coefficient (Cp). This is a measure of the fullness of the ends of the hull. Instinctively you might think that fine ends would be faster as they would “cut through the water better”. But in fact you want a high Cp for high speeds. However everything is interrelated. If you have fine hulls you can use a lower Cp. Most monohulls have a Cp of 0.55- 0.57. And that is about right for displacement speeds.

However the key to Catamaran design is you need a higher Cp if you want to sail fast. So a multihull should be at least 0.61 and a heavy displacement multihull a bit higher still. It is difficult to get much over 0.67 without a very distorted hull shape or one with excessive WSA. So all multihulls should have a Cp between 0.61 and 0.65. None of this is very special or new. It has been well known by naval architects for at least 50 years.

There are various ways of achieving a high Cp. You could fit bulb bows (as Lock Crowther did). Note this bow is a bit different from those seen on ships (which work at very specific hull speeds – which are very low for their LOA). But one problem with them is that these tend to slam in a seaway. 

Another way is to have a very wide planing aft section. But that can increase WSA and leads to other problems I’ll mention in a minute. Finally you can flatten out the hull rocker (the keel shape seen from the side) and add a bustle aft. That is the approach I use, in part because that adds displacement aft, just where it is most needed.

I agree that a high Cp increases drag at low speeds. But at speeds over hull speed drag decreases dramatically on a high Cp boat relative to one with a low Cp. With the correct Cp drag can be reduced by over 10%. In other words you will go 10% faster (and that is a lot!) in the same wind and with the same sails as a boat with a unfavorable Cp. In light winds it is easy to overcome the extra drag because you have lots of stability and so can fly extra light weather sails.

The time you really need a high Cp boat is when beating to windward in a big sea. Then you don’t have the stability and really want to get to your destination fast. At least I do, I don’t mind slowly drifting along in a calm. But I hate “windward bashing”

But when you sail to windward the boat pitches. The sea isn’t like a test tank or a computer program. And here I agree with Evan. Immersed transoms will slow you down (that is why I use a narrower transom than most designers).

I also agree with Evan (and why not, he knows more about Volvo 60 design than nearly anyone else on the planet) in that I don’t think you should compare a catamaran hull to a monohull, even a racing one. Why chose a Volvo 60/Vendee boat with an immersed transom? Why not chose a 60ft Americas Cup boat with a narrow out of the water transom?? 

To be honest I haven’t use Michelet so cannot really comment. But I have tested model catamarans in a big test tank and I know how inaccurate tank test results can be. I cannot believe that a computer program will be better.

It would be easy to prove one way or the other though. A catamaran hull is much like a frigate hull (similar SLR, L/B ratios and Froude numbers) and there is plenty of data available for those. There is also a lot of data for the round bilge narrow non planing motorboats popular in the 1930’-50’s which again are similar to a single multihull hull.

One of the key findings I discovered with my tank test work was just how great the drag was due to wave interference between the hulls. Even a catamaran with a modern wide hull spacing had a drag increase of up to 20 % when compared to hulls at infinite spacing. One reason why just flying a hull is fast (the Cp increases when you do as well, which also helps). So you cannot just double the drag of a single hull and expect to get accurate results. And any speed prediction formula must include a windage factor if it is to give meaningful results.About 25 years ago we sailed two identical 24ft Striders next to each other. They were the same speed. Then we moved the crew of one boat to the bow. That boat IMMEDIATELY went ½ knot faster. That is why I now arrange the deck layout of my racing boats so that the crew can stay in front of the mast at all times, even when tacking or using the spinnaker.

I once raced against a bridge deck cabin catamaran whose skipper kept the 5 crew on the forward netting beam the whole race. He won.

Richard Woods of Woods Designs www.sailingcatamarans.com

• Tags Buying Advice , Catamaran Designers

Owner of a Catalac 8M and Catamaransite webmaster.

2 replies on “Catamaran Hull Design”

I totally agree with what you say. But Uli only talk sailing catamarans.

If only solar power. You need the very best. As limited watts. Hp.

The closer to 1-20 the better.

Closing the hulls to fit in cheaper marina berth. ?

You say not too close. But is that for sailing only.

Any comment is greatly appreciated

Kind regards Jeppe

Superb article

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STRUCTURAL ANALYSIS AND DESIGN OF A CATAMARAN CROSS-STRUCTURE BY THE FINITE ELEMENT METHOD

This problem is examined using the Finite Element Method. The behavior of a multicell box girder representing a catamaran cross-structure is studied. Because of the relatively low length-to-breadth ratio of a typical catamaran cross-structure, the deck plating effectiveness is found to be rather small. Parametric studies are performed, and curves of the effectiveness as a function of the relevant geometric and loading parameters are presented. These curves may be helpful in the early stages of new designs.

• Find a library where document is available. Order URL: http://worldcat.org/issn/00281425

American Society of Naval Engineers

• Mansour, A E
• Fenton, P H
• Publication Date: 1973-2
• Features: References;
• Pagination: p. 33-42
• Naval Engineers Journal
• Issue Number: 1
• ISSN: 0028-1425

Subject/Index Terms

• TRT Terms: Catamarans ; Finite element method
• Subject Areas: Marine Transportation; Materials; Vehicles and Equipment;

Filing Info

• Accession Number: 00046644
• Record Type: Publication
• Source Agency: Engineering Index
• Files: TRIS
• Created Date: Sep 27 1974 12:00AM

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CROSS BEAMS FOR CATAMARANS Our cross beams for catamarans are designed to be more than just a structural connection between the hulls. Integrated cable conduits, fittings for navigational lights and a clever attachment for the trampoline are good examples of details appreciated by the boat builders. Non-Slip area on top of the cross beam. The bridle wire is secured in a slot on top of the bridle support. All prepared for navigational lights. Hull brackets can articulate to absorb movements between the hulls and the beam. The cable from the navigational light is fed into the cross beam... The forestay load is absorbed by a stainless support underneath the cross beam. SELDEN is a registered trademark of Selden Mast AB Selden Mast AB, Sweden e-mail [email protected] www.seldenmast.com

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Catamaran Design Guide

Spectacular sunsets in the Pacific turn the horizon into a brilliant spectrum of gold and orange colors.

Copyright © 2006, 2008 by Gregor Tarjan. Click here for terms of use.

performance, yet desire high daily averages and passage times, which should be as short as possible. When choosing a large multihull, sailors look, above all else, for safety and comfort, long before the consideration for flat-out speed comes into the discussion. Nevertheless, performance is a highly important design consideration. No catamaran sailor wants to sail slower than a same length ballasted keelboat. Below are some EVALUATION & COEFFICIENTS useful coefficients, which will help compare monohulls and multihulls objectively.

Bruce Number (BN)

below "Indigo," a magnificent Wormwood 70, sailing in sparkling Caribbean waters.

Various multihull characteristics and design features can be expressed in mathematical formulas. Their results are crucial and will give prospective owners a basis of comparison between different types of catamarans. These numbers are important, as they eliminate ambiguity and clearly display various advantages or concessions of a design, which would be hard to quantify any other way. Mathematical coefficients not only will provide insight into a boat's performance in varying conditions, they also reflect concerns about loads to be carried safely, speed and stability.

We have already mentioned the Displacement/Length and Sail Area/ Displacement ratio in our chapter on Multihull Advantages, illustrating the point of a multihull's efficiency. Let's look at some other coefficients that give us an indication of a boat's performance.

What is performance and how do we really measure it? Most people who buy a cruising catamaran are not really interested in racing

The Bruce Number is very similar to the Sail Area to Displacement ratio although the formula is slightly different. It is the square root of the sail area in feet, divided by the cube root of the boat's displacement in pounds:

SA = upwind sail area (mainsail and 100% jib)

Displ = weight of the boat in pounds

Similar to the Sail Area to Displacement ratio, the higher the coefficient the faster the boat and better is its performance in light air. Typically a BN of 1.1 will be the threshold between fast and more sluggish multihulls. A heavy displacement monohull might have a BN of .7, whereas a modern cruising catamaran shows a BN of 1.3. Offshore multihull racers can have BNs of 2.0 and higher. The BN will also tell us about a catamaran's ability to withstand stronger winds before reefing. A boat with a higher BN is usually overcanvassed in strong conditions and will have to be reefed earlier than one with a lower coefficient.

On the other hand, they will be able to produce more "power" than their counterparts in lighter winds and perform better.

Sail Area to Wetted Surface (SAWS)

SA/WS = Sail Area Wetted Surface Coefficient

SA = upwind sail area

WS = total underwater surface area (hull and appendages)

This formula simply divides the upwind sail area of the boat (mainsail and 100% jib) by the wetted surface. This coefficient will give us a statistical indication of the multihull's lightair performance since in low wind conditions skin friction becomes an important factor. Monohulls can have coefficients of at least 7% more than multihulls.

Hull Fineness Ratio (HFR)

The Hull Fineness Ratio, known as the hull's beam-to-length ratio, is an interesting number. It is derived by simply dividing the waterline length of the hull by the waterline beam of the hull.

Max. WL/Max. Beam WL = Hull Fineness Ratio Max. WL = length of the hull at waterline in ft. Max. Beam WL = beam of the hull at the waterline in feet.

Monohulls, when compared to multihulls, have low hull/fineness ratios. In Part 1 of this

book, discussing "Efficiency," we saw that ballasted keelboats are limited to Archimedes' principle of hull speed (1.34 x VWL). Multihulls do not have these theoretical barriers, because their hulls are narrower.

The thinner the hull the faster it will be able to travel through the water. But, attention! It will also carry less unless you are on a mega cat. Typically, a 40' cruising catamaran's HFR will range from 8:1 to 10:1. Dennis Conner's above While sailing under spinnaker and experiencing virtually no roll at all, guests will always find a comfortable spot to relax on the foredeck, an impossibility on a monohull.

There are various methods of calculating the transverse stability of a catamaran. One of the simplest and most utilized techniques is establishing a relationship between the height of the Center of Effort (CE), displacement, beam and sail area. Multihull designer, James Wharram added safety factors of 20% to compensate for gusts and the dynamic environment of the ocean. Another method is described in the text below.

Multihull Stability & Capsizing Moment d - Displacement (kg) x half beam (m) max ~ Sail Area (sq m) x Height of Center of Effort (m)

P max = maximum pressure exerted onto sails

Multihull Stability & Capsizing Moment

height of sailplan CE

half overall beam (half hull beam)

racing cat "Stars and Stripes" had a 16:1 HFR. Of course, the larger the boat, the narrower the hulls will become in comparison to its length. For example, the HFR of a 100' luxury catamaran may be 12:1, providing it with a high speed potential. However, monohulls can show HFRs of 3:1, though the comparison is complicated as their angle of heel affects the measurement.

One has to be very careful when analyzing the Hull Fineness Ratio of a cruising catamaran, because other factors such as the actual shape of the hull cross sections (Prismatic Coefficient, PC) can throw the analysis off balance. Go-fast sailors like to think that fine hulls are always fast. That is not necessarily true because a slim hull could have a large underwater volume, thus slowing it down. Consequently, a wide waterline-beam hull could have less drag than a narrower one. It could have a shallow underbody (low PC), which would be beneficial to load carrying (Pounds Per Inch Immersion Number, PPI) and early surfing characteristics at speed.

Stability Coefficient (SC)

This mathematical formula has been devised by the distinguished catamaran designer and sailor James Wharram and his team. This coefficient analyzes a multihull's ability (in a static environment) to resist capsizing due to wind.

( 0.682 VW x (.5 Boa) ) x .555 = CW .00178 x SA x h

W = Wind speed, apparent, in mph CW = Critical Wind Speed to capsize in mph SA = upwind sail area in sq ft. h = height of Center of Effort (CE) of total sail area

Boa = Beam overall

This formula will tell us how much wind it will take to overturn our multihull. By instinct we will know that a catamaran with a wide stance and a conservative sail plan will be very stable offshore. The SC formula will inevitably illustrate that a wider beamed catamaran with a tall sail plan will be as resistant to wind induced capsize as a short-rigged, narrower boat. This is not so if one considers the chaotic environment of waves and the real world of heavy weather sailing. It is interesting to note that a wide beamed boat (regardless of the SC) is more resistant to capsize in seas due to the effects of a higher moment of inertia. In an open-ocean environment, which is everything but static, the SC formula has little meaning. Nevertheless, it serves as a good basis to evaluate stability as a factor of wind force.

below When the wind suddenly comes up, all that is needed is a couple of turns on the jib furler to quickly reduce the headsail size. The catamaran will hardly sail any slower, but feel more comfortable.

Wide hulls and a large overall beam will increase the overall righting moment of a catamaran. A word of caution: Excessive beam will reduce the fore and aft stability. Designers strive to compromise hull fineness ratios, place heavy weights towards the CG (Center of Gravity), and engineer hull and overall beam to achieve a seaworthy balance, which is safe, yet provides ample liveaboard accommodations.

Catamaran Stability Considerations

Diagonal Stability & Beam-to-Length Ratio (BLR)

Stability of a multihull, or the resistance to capsize, should be seen as three components. Athwartship Stability is one well-publicized type and the one often talked about. The other much more important types are Fore and Aft and Diagonal Stability. Fore and aft stability is established by the relationship between the boat's waterline length and the distance between the hull centerlines. It will reflect the catamaran's resistance to tripping. This relationship should be in the vicinity of 39% to 42%. For a seaworthy cruising multihull it is important maintain the proper ratio between length and beam, which, in turn, balances equal amounts of athwartship with diagonal stability. The goal should be to prevent the possibility of a sudden discrepancy of powers between fore and aft and sideways resistance. Most of today's multihulls keep these two component forces in equilibrium, making them extremely seakindly and safe.

Some early design multihulls were very narrow, partly due to the material limitations of that time. But things have changed. Contemporary composite construction allows designers to build wider boats without compromising stiffness. Production catamarans of today have a wide stance and have the benefit of greater safety margins in gusty wind conditions than their older cousins. Multihulls are sophisticated structures and true modern miracles. They provide a more comfortable ride and more interior room. Thanks to modern materials they weigh less and perform better than catamarans built only 10 years ago.

Some catamarans, especially production boats, which are very popular in the charter fleets, are growing wider by the year. The businesses who rent these beamy monsters adore them. Lots of room plus open decks are ideal for clients and the bigger (wider) the boat, the more paying guests can share the fees. But there certainly is a limit as to how wide is too wide. Extreme beam can be dangerous. It can lead to instability fore and aft and to excessive bridgedeck slamming, as the relative distance from the bridge deck to the water will decrease with an increase in width. A vessel with excessive beam might seem stable athwartships, but it will compromise overall stability.

We know that multihulls can, in extreme cases of seamanship error in wild storms, be thrown over from any side - front, back and beam-on. The best examples of this phenomenon are racing multihulls, especially Formula 1 trimarans, which have fine hulls for speed and huge sailplans to provide driving power. They are initially extremely stable athwartships (High Beam-to-Length Ratio), but have a tendency to become unstable fore and aft. They will surf down waves and reach a point where the power of the sails, and speed, will exceed the ability to keep the bows out of the water and the boat will pitchpole. This is the reason why catamaran designers usually draw their multihulls with a Beam-to-Length relationship of between 50% and 55%. The longer the vessel the lower that percentage becomes.

I am currently involved in the "Gemini" project, which presents an example. It very well might become the world's largest sailing catamaran. She will have an overall length of 145 feet, yet her beam will "only" be 54.4'.

Please, don't worry. "Gemini" will not be tender and tip over in the slightest breeze. On the contrary, this monster will be one of the most stable craft afloat, although the beam-to-length relationship is only 37%. The relatively low beam-to-length ratio also involves the fact that the boat would be too heavy and building costs would be prohibitive if she were to have a standard 52% BL relationship. Most importantly, could you imagine turning a 75-foot-wide boat?

above Asymmetric spinnakers on furlers are great inventions. They add instant sail area, yet can be doused in a matter of seconds when the wind picks up strength.

above Although this Edel 35 was a good-looking and popular catamaran, it suffered from excessive bridgedeck pounding, which was caused by only several inches of clearance between the saloon's underwing and the sea.

Obviously there is a sweet spot in the beam vs. stability question. Designing too beamy a boat will also necessitate more freeboard to preserve bridgedeck clearance which, in turn, will increase windage and complicate maneuvering. Unless sophisticated aramid construction methods are utilized, more beam will also add more weight and stress to the structure. Adding more mass will, to a certain point, help make the boat more stable, but where do we stop? Is it better to add weight or width to make a boat stiffer? Of course, both characteristics are interrelated as a beamier boat normally is also heavier. Just adding weight to a catamaran simply to make her more stable will not pay off. Consequently, making a boat too wide might increase living space yet it will also burden the structure, require a beefier manufacture, and yield an even heavier boat. Needless to say, a boat which is too wide will also create practical restrictions such as maneuvering, the ability to haul the vessel and much higher building costs.

Beam has a great effect on bridgedeck clearance, which is one of the most vital characteristics of a good cruising catamaran. As standard practice, the well-known rule of 1" of bridgedeck clearance for each foot of beam was a safe way to prevent excessive wave slap. The wider the beam the more the relationship changes and the necessary height of 1" per foot of beam needs to be increased to 1.3" or more. In the extreme case of overly square boats, that number will have to be closer to 1.8" per foot of beam. This will have a negative effect on any seaworthy multihull that has a bridgedeck saloon. The wide beam will necessitate a high cabin sole to remain a safe distance from the waterline. In order to provide standing headroom, the coachroof might be higher than practical, which could result in a boxy, high-windage multihull. Not only will this be unattractive, but also raise the Center of Gravity (CG) which really should be kept as low as possible.

More overall beam on the other hand (given that there is still sufficient bridgedeck height) has a less known benefit, as it reduces the possibility of hull-wave interference, which is particularly important for fast designs. The wave interaction between the hulls can lead to additional resistance, and especially in an agitated sea state, the formation of wave crests can pound the bridge deck. Most early narrow-beamed catamarans suffered from this phenomenon,

Ultimately, a boat's design has a major influence on its ability to stand against the forces of nature, and to keep occupants safe. Manufacturing excessively wide catamarans is like trying to market monohulls with super deep-draft keels. Both are totally impractical. We designers have to make sensible compromises and learn from past experiences of what has worked at sea by balancing the benefits of a wide boat with its disadvantages.

below This narrow-hulled Outremer 64 Light has completed her third circumnavigation with the same owners. Note the smooth underwing clearance, lacking any protrusions or steps.

"A great cape, for us, can't be expressed in latitude and longitude alone. A great cape has a soul, with very soft, very violent shadows and colors. A soul as smooth as a child's, and as hard as a criminal's. And that is why we go!"

~ Bernard Moitessier

Dinghies, windsurfers and every imaginable type of water toy can be stored conveniently on large catamarans and easily launched from the wide transom steps for shore-side pleasures. Note the twin life rafts located in special compartments on the massive aft crossbeam.

Continue reading here: Hull

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• Design Dynamics - Catamarans Guide
• Hull Construction - Ship Design
• Geometry - Ship Design
• Heavy Weather Tactics - Catamarans Guide
• Configuration Types - Catamarans Guide

Readers' Questions

What length should a stub keel be to waterline length on a catamaran?

There is no set rule for the length of a stub keel on a catamaran in relation to its waterline length. The length of the stub keel will depend on various factors, such as the size and design of the catamaran, intended use, and specific requirements of the boat builder. Generally, the stub keel on a catamaran is designed to provide stability and improve sailing performance, so it is important to consult with a naval architect or boat designer to determine the appropriate length for a specific catamaran.

What is a 16 passenger catarmarn like?

A 16-passenger catamaran is a type of boat or vessel specifically designed to carry 16 people comfortably. Catamarans are multihull boats with two parallel hulls, which are connected by a deck or a structure. They offer stability, speed, and efficiency in the water. A 16-passenger catamaran can vary in size and design, but generally, it will have enough seating or lounge areas for all passengers. It may have indoor cabins with beds or seating areas, as well as outdoor spaces for relaxation or socializing. These boats often come equipped with amenities such as bathrooms, kitchens or galleys for meals, and sometimes even entertainment systems. The catamaran's size can influence its specific features. Some catamarans are designed for day trips or shorter excursions, while others are built for longer journeys or overnight accommodations. Additionally, they can be used for various purposes, such as whale watching, diving trips, ferry services, or private charters. Overall, a 16-passenger catamaran provides a comfortable and stable platform for small groups or gatherings, allowing passengers to enjoy the beauty of the water while ensuring safety and comfort.

Is the catamaran hull floor always on the waterline?

No, the hull floor of a catamaran is not always on the waterline. The design of a catamaran allows for the hulls to be elevated above the waterline, reducing drag and increasing speed. The position of the hulls in relation to the waterline can vary depending on factors such as the weight distribution, load, and sailing conditions.

How close to a catamarans design reefing points should you go?

You should always be careful when approaching reefing points on a catamaran and stay as far away as possible. Generally, you should aim to stay at least 10 meters away.

What keel to length ratio for catamarans?

The keel-to-length ratio for catamarans typically ranges from 0.1 to 0.25.

Is 70% length to beam ok for a catAMARAN?

Yes, it is generally accepted that a catamaran should have a length to beam ratio of between approximately 6:1 and 8:1. Therefore, a 70% length to beam ratio would be within an acceptable range.

What is the waterline length to baem ratio of a typical cruising catamarans?

This ratio will vary depending on the type and size of the catamaran. Generally, the ratio should be between 1:1.5 and 1:2.5, with 1:2 being the most common.

Calculating crossbeam size without waterstays

Some basic engineering.

This is one of many similar questions I have received over the last 3 years. As with all 'engineering formula', I get rather nervous trying to explain this to someone not familiar with all the intricacies, as anything I spell out, can easily be wrongly applied or taken out of context. So my best advice would be to go to a library and pull out a book on basic Mechanics of Materials, or sit down and discuss your needs with a qualified engineer/designer with practical experience.

But, having said that and hoping this does not come back to haunt me, here's a simple introduction.

First of all, you need to identify certain factors and it's best to use one of the industry standards for naming them. Here are 6 relatively easy, basic factors that you'll need to understand to get started:

Now these will relate to each other in various ways (S = I/y for example) and an important one for this application is Stress (f) = M × y/I (or f = M/S).

Next you need to decide what UNITS to work in, as they have to match for the results to make ANY sense. Let's say we use Imperial Units here—inches, ft, lbs etc. (I'll add them in […bkts…] ). Then you'd calculate the 'Mt of Inertia' of say your crossbeam at the hull (a purely 2-dimensional geometric calculation, that is often available from suppliers for say a round pipe, square tube or other structural section [ in 4 ]. The 'y' distance will typically be ½ the depth in this case [ ins ].

Another common option is adding a waterstay as this will generally create the lightest solution by removing most of the bending moment from the beam, but still adds a high compression load to it, of a value close to the tension in the waterstay. One can then get into things like wall buckling or buckling by an excessive slenderness ratio, a factor requiring special study for long, slim pillars or struts, such as for a stayed masts. See Calculating the strength of a waterstay on a trimaran .

The slenderness ratio is typically defined as the ratio of the unsupported length to the geometric radius of gyration—equal to about ⅓ the outside diameter of a standard pipe. Even without side sail loads, anything over 120 is considered vulnerable to collapse from compression and therefore the allowable stress must be reduced significantly. For example, a slenderness ratio of say 200 for a pillar, could mean the allowable stress must be reduced to only ⅓ of that acceptable at 120. Also see Article under the MAST design section .

But for this simple cantilever example, the resulting Stress (f), would be (M × y) divided by Inertia (I). Check out your Units… (in · lbs) × (in) divided by (in 4 ) — this gives lbs/in² which is correct and viable for a stress value.

You then compare your result with the allowable stress of the material you use, (steel, aluminum, fiberglass, wood etc) Combining materials, like glass and wood, get's complicated—but doable with larger safety factors incorporated. If you use carbon fiber, best to forget the wood shell and only count the carbon fiber, as CF will take most of the load due to its much lower flex.

You will need to establish an FS—or Factor of Safety. Each material and each application will likely justify a different one depending on the risks and knowledge of the material consistency. Naturally, wood is the weakest and most unknown—especially when compared to say steel. Fiberglass quality and lay up is still not 100% consistent, but more consistent than wood.

So you'll need to take the stress for the material you're considering at its Elastic Limit—the point where it starts to fail and from which cannot recuperate. (The ultimate stress may indeed be higher, but the product would have been too permanently weakened to use that figure.) Divide the Stress at Elastic limit by the FS (anywhere from 1.5 to 6) and that's the stress you should not exceed. To get there, you adjust your section Inertia or Modulus with a larger or smaller section.

==========================================================================================

Added Dec 2017:

Let's looks at a specific example:     Using the above sketch as a guide, this beam is unsupported by a waterstay or tie rod, so is therefore loaded as what we call a cantilever … one of the tougher loads to resist.    So let's take the Ama Lever at 6' (1.83m) and the Ama buoyancy volume at 12 cu,ft  (0.34 cu.m or 340 litres).     As 1 cu.ft of fresh water displacement  = 62.4 lbs, the buoyancy will be 12 x 62.4 lbs in fresh water.   The bending moment (in imperial units) on the beam will therefore be:  12 x 62.4# x 6', or  4493 ft.lbs.    (Salt water is heavier at 64 lbs per cu/ft, so the buoyancy from the same volume would then be 2.6% greater).

We now need to set an ‘allowable stress’ (f) for the material we plan to use.    Let’s assume an alum. tube and a Factor of Safety of say 4, and a yield stress of the material at say 28,000 lbs/sq in.    This would give an allowable stress (f) of just 7,000 lbs/sq in. 

So to now calculate what the physical size of an aluminum pipe would be that would satisfy this installation, the formula S = M/f will apply, as noted in the text above.   In this example, S = 4493 x 12 / 7000 = 7.7 in 3 (as the stress is in lbs/inch 2 , you will see that we’ve converted the bending moment into lb.inches, by multiplying it by 12).     So the section modulus (S or I/y) of the pipe will require to be 7.7 in 3 in this example.   As a 5” dia standard pipe has a listed I of 15.16 in 4 and a y (D/2) of 2.78”, its S value calculates out to 15.16/2.78 or 5.45 in 3 .     As we need 7.7 in 3 with the present load assumptions, we will have to look at a thicker pipe.   The ‘extra strong’ 5” pipe has an I of 20.7 in 4  and this would have a section modulus (S) of 7.45 in 3 ... which in practice would be close enough for our needs (ie: within 10%).      Note that in this example, we assumed the FULL buoyancy is acting on just ONE cross beam, but that’s what I normally recommend for the forward beam. The thinner 5” pipe would work fine for the less loaded aft beam.

If the beam were made as a wood box, the various parts (solid corners and/or plywood etc) would need to be calculated separately, with the load each part can carry, added together.  Composite beams are even more complex to calculate and it’s important to think about the relative elasticity of each material within such a loaded beam, as a material with low elasticity (like carbon fiber) will likely take ALL the load before another more elastic material (like fiberglass)  can take any useful load.   Rather than such a mix, it will generally be better to stay with one material or the other, but if using two together, carefully consider the job and loading of each.   Of course, an all fiberglass beam would be heavier and more flexible, but that’s not all bad.    I for one, prefer to use fiberglass for certain things, such as for hinges, when a small amount of added flexibility can help distribute high loads more evenly throughout the hinge knuckles and mounting bolts.    Choosing the right material for the job is very much part of good overall design.

Adding a tie rod under such a beam, can shorten the lever, lower the bending moment and permit a smaller section beam (aka).  Or adding a waterstay can remove ALL the bending moment and change the beam load to one primarily of compression, typically resulting in a smaller, lighter beam section.   Bending on such a beam can still come from any weight added on it though, such as a 200lb person’s weight.

Hope this helps.

==================================================================================================================

But as I said at the beginning, each case is different and an experienced designer would typically recognize those differences and how the stresses will apply and where the maximum ones are likely to be. Analyzing composite structures is a whole new science—often requiring special FEA (finite element analysis) software to take more of the variables into account (though typically, still not all) — so as noted above, although still valid and hopefully useful, this is just a very small introduction.

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carbon spar cross beam augmentation

Discussion in ' Multihulls ' started by barrymac , Nov 28, 2019 .

Is adding a carbon fibre cross beam in the bridge under the mast of a catamaran design crazy?

Can't work, too expensive, would add a lot of value and increase performance.

barrymac Junior Member

I'm considering a grainger Raku project, but would like to push up the stiffness and strength of the bridge under the mast as it's the most hard working part of the structure. Any views on adding a carbon cross beam to the design?  

fastsailing Senior Member

barrymac said: ↑ I'm considering a grainger Raku project, but would like to push up the stiffness and strength of the bridge under the mast as it's the most hard working part of the structure. Any views on adding a carbon cross beam to the design? Click to expand...

gonzo Senior Member

In principle it is a bad idea. Stiffening a small part of a structure creates concentrated stresses and leads to failure. Also, your survey if biased to get the answer you want: people to agree with you. What do you mean by "can't work"? The other two are definitely wrong, so I am choosing the first as the best  

TANSL Senior Member

gonzo said: ↑ Stiffening a small part of a structure creates concentrated stresses and leads to failure. Click to expand...

redreuben redreuben

What is the evidence to say that the structure as designed needs reinforcement ?  

By giving you an easy answer, I will tell you that without knowing the design loads used by the designer, nothing can be said about it. It could also happen, of course, that the design loads used are incorrect. So, the evidence you ask for does not exist.  

Ad Hoc Naval Architect

barrymac said: ↑ I'm considering a grainger Raku project, but would like to push up the stiffness and strength of the bridge under the mast as it's the most hard working part of the structure. .. Click to expand...

Tansl, So your first assumption is the designer doesn’t know what he’s doing ?  

@redreuben , No, at all, I have no idea what the designer has designed, the assumptions he has made or the calculations made. How am I going to question that work? I did not work that way.  

My question was based on 5 years ownership of a Fontaine Pajot Bahia 46, now 20 years old, which shows signs of stress in this area on the surface. I've seen other Bahia's with similar marks. I presume this model was modelled well, from a well resourced company. As such I figured if there was one area I would overbuild a little it would be here, but not being qualified to evaluate the idea, thought I'd see what people here thought. The mast is supported by a pillar, and I guess this would be bearing only compression loads. It is the twisting of the bridge by the hulls combined with shock loads of wave slamming that I am most concerned about. I don't want to feel delicate about beating upwind in inclement conditions, as I do now in the Bahia. This results in increasing the tacking angle to something a bit depressing.  

The structure needs to have the stresses spread out rather than concentrated it is failing. However, the cracks may simply be on the gelcoat and not structural. Hav e you removed the gelcoat to inspect the damage?  

barrymac said: ↑ The mast is supported by a pillar, and I guess this would be bearing only compression loads. Click to expand...

catsketcher Senior Member

I don't know about Raku's but Schionning Wildernesses use large layups of unis in their beams. Old Multi has the layup of a Schionning like cat of about 13 metres in his structure thread. I have seen a friend lay up a heap of unis into the top of a routed out balsa duflex panel. The unis can be put on top of the bulkhead (which is a shear web) and laminated on the underside of the deck. There are more unis on the top of the bulkhead than the bottom - because the glass does not like compression as much as tension. If you were looking to stiffen a Raku, then adding more glass or switching to a similar amount of carbon instead of this uni could be good. It would be best to ask Grainger. He would probably say don't change it and ask if you have sailed on a Raku and found it too flexible. I have a reasonably light main beam on my cat and have no stress cracks there. However it is made very differently to other similar cats - so there seems to be a wide variation in build technique of the main beams of cats. The only way I can notice deflection is by looking at the rigging going slack in heavy air, just before reefing, but that is probably mostly wire stretch rather than deformation. Designers are often happy for someone to take a punt and do something only a little out of the ordinary. The can then learn from it and incorporate it in the next design. It could be fine with Grainger for you to replace the standard E glass unis with carbon. I can't see a problem right off the top of my head but Grainger would know the structure best. cheers Phil  

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catsketcher said: ↑ I don't know about Raku's but Schionning Wildernesses use large layups of unis in their beams. Old Multi has the layup of a Schionning like cat of about 13 metres in his structure thread. I have seen a friend lay up a heap of unis into the top of a routed out balsa duflex panel. The unis can be put on top of the bulkhead (which is a shear web) and laminated on the underside of the deck. There are more unis on the top of the bulkhead than the bottom - because the glass does not like compression as much as tension. Phil Click to expand...

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