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Marine aluminum vs marine steel for ship hull. What to choose?

Discussion in ' Materials ' started by Edem , Apr 21, 2021 .

• #aluminum #marine #hull

Marine aluminum or marine steel ?

Edem Junior Member

Dear forum users! Now I will bring up a topic that may seem absurd, given the stage at which I am. But this is very important to me. My friends and I are building a catamaran in Russia. We are in the midst of designing. But in a high degree of readiness. The design is being developed using marine aluminum AlMg5 (AA5056 (USA), 5056 (Japan)). The catamaran will operate in the tropical zone of the Indian Ocean in the Seychelles region. Dimensions 22 meters long by 7.5 meters wide (the length may be less by 2 meters, we will coordinate this part with the Russian regulations office, but this is a detail). It is important that the catamaran will move at a cruising speed of 12 knots + -2 knots, with a maximum speed of 18 knots + - 2 knots. In the next branch on diesel engines, we are discussing possible engines. Actually, that's why I came to the forum. So this is the problem: we build the catamaran ourselves, and aluminum is a very finicky metal. I started testing welders and submitting welds for examination, and out of 15 welders, none of them was able to make a weld without defects. And we need 3 welding posts, but so far there is only one. And since we do not have the financial ability to order the construction of a boat at a shipyard, the question of welder specialists is very important for us. I am now sitting holding my head and thinking what to do. Maybe move away from naval aluminum and switch to naval steel before aluminum is ordered? The argument is that it is easier to weld the steel hull of the ship, it is easier to control the process and the likelihood of defect (rejects) is less and we do not need an extra high-speed catamaran and we can (possibly) add 20-25 percent to the mass of the ship hull. But this will entail fuel consumption on an ongoing basis. But is it that significant? What material do you think to choose? What are their real performance and durability? How often and to what extent will these enclosures require maintenance in comparison? Is it really worth the question of the durability of the hull, taking into account modern protective case paints? Still interesting tips on how to prime, putty and paint the ship's hull? Are these one materials for steel and aluminum? So that I can compare the costs of the final hull in steel and in aluminum, of approximately the same aesthetic quality (this is also important, we all want to look at beauty, the dream is always beautiful It so happened that when I came to the forum to consult on engines, I thought about the material of the hull- something that I needed to think about from the very beginning. But better late than never!) Thanks to all the forum members, you are creating so much useful content! For me, as a person from Russia, where there is no tradition of private shipbuilding, this is just a fantastic resource! I will be glad and grateful for every opinion! P.S. We cannot work with composite materials for build the ship hull, let's not consider them. Only steel and aluminum.  

TANSL Senior Member

If you are not able to properly weld aluminum the only option you have is steel. Thus, considerations about consumption, maintenance, etc. they go into the background. Why are you going to study an option, aluminum, which is out of your reach?. The boat will be heavier with steel than with aluminum but its quality will be assured and, in addition, a good design of the structure and a good calculation of the scantlings will mean that the boat does not have more weight than it should.  

Mr Efficiency Senior Member

Maybe you can make the hulls from steel, and the superstructure, where less than perfect welds won't be the "end of the world", from Aluminium.  

Will Gilmore Senior Member

Are you docking the boat between two aluminum hulls or between two steel hulls? In either case, I'd rather have steel.  

DogCavalry Senior Member

Welcome to the forum, Edem. Skilled aluminum welders seem to rare everywhere. As TANSL points out, there's no decision to be made. If you can't get a proper aluminum weld, steel it is. Turn your attention that way, and forget Al.  

Barry Senior Member

I find it hard to believe that a certified welder cannot make up an aluminum joint. While welding aluminum requires perhaps a bit more care, especially in thickness less than 5 mm ( I realize that many smaller boats are welded using 3 mm) when you begin to get into thicker pieces the welding gets easier to make a sound joint with Mig. Perhaps what is more important is the sequences in welds and some other processes. The advantages of aluminum is that the inside does not have to be maintained as steel has to, it is lighter than steel though more expensive. With aluminum you will need more attention to corrosion, dissimilar metals issues but aluminum can be cut with normal woodworking equipment. You say that you are building the boat yourself. You could learn how to tack weld up plates and extrusions in about 6 hours of instruction and do the tacking yourself then have a competent welder do the final welding. The welder would be able to properly tack adjacent to yours if necessary, zip cut out yours, and do the welding. More time is spent fitting up a hull than the actual welding itself.  

bajansailor Marine Surveyor

I have been involved in the past with the design and construction of a 15 metre aluminium power cat (the one in my avatar), and a 12 metre steel power cat. Both were built 20 years ago. The aluminium one is still working well, and is in good condition generally, and has not needed any costly re-fits yet. The owner of the steel cat decided to go with steel rather than aluminium on the basis of initial cost of construction. Yet when we were doing the outfitting he was amazed by the cost of all the paint that was required to protect the steel - everything has to be painted, inside and out, otherwise it rusts away very quickly. Especially in comparison to the ally cat, where only the hull exterior below the waterline and the deck was / is painted. And the poor steel cat is now sitting ashore, rusting away happily, despite the 3rd owner starting a major re-fit about 10 years ago, and then giving up after having spent a small fortune on her. I would strongly recommend to any potential future owner of a catamaran that is less than say 24 metres in length here to go for an aluminium cat rather than steel. Especially so if you want the hull shapes to be efficient, and fairly slender. We do have good aluminium welders here which is a very strong positive factor. But Barry has suggested some good ways of achieving 'proper' welds in his post above. Edem, can you post any sketches that you have created so far of your new design please?  

comfisherman Senior Member

Its 2021, I can get aluminum specific welding equipment of high quality for a very small percentage of what this project will cost. To extreme remote Alaska, certainly its possible in Russia. The average semi handy individual can come up to speed on an acceptable weld in several weeks of instruction. Right now enough 5086 marine plate is going to be a weeks to months lead time, would seem like plenty of time to get some weld practice in. A miller 255 pulse mig with an aluma pro push pull is under 5500 usd. Dunno about global availability but there has to be a global equivalent machine. With pulse mig and and a mid tier push pull, most folks learn fast. Guessing the cost of even a few machines will look small compared to that much alloy. I've not bought 5086 since sept of 2020, but even then it was getting expensive. I sold a commercial aluminum boat in 2014, it had some galvanic issues over the years but was a good boat. Still kinda miss that boat. I sold a steel boat of similar size in 2020, and sold the ingersoll rand compressor and blast pot the next day..... I've not missed either the steel or the blaster needed to keep the steel from rusting.  

Ad Hoc Naval Architect

Edem said: ↑ ....Maybe move away from naval aluminum and switch to naval steel before aluminum is ordered?.... Click to expand...

TANSL said: ↑ Why are you going to study an option, aluminum, which is out of your reach? Click to expand...

bajansailor said: ↑ Edem, can you post any sketches that you have created so far of your new design please? Click to expand...

Ad Hoc said: ↑ Since they may just be poor welders! Click to expand...

Edem said: ↑ Mr Efficiency, I am considering this option. Make the critical part of the boat from steel, and the superstructure from aluminum. The hulls of the sideboards and the bridge are made of steel, as they are subject to a high load on twisting and slamming, and the superstructures are made of aluminum. But the question of combining different metals is also very complicated. Could you tell me about this? Click to expand...

Edem said: ↑ ...The design is being developed using marine aluminum AlMg5 (AA5056 (USA), 5056 (Japan)).... Click to expand...

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baeckmo Hydrodynamics

Edem said: ↑ Ad Hoc, they explained to me that this is due to the fact that we have very few production facilities in Russia where the skills of aluminum welding are required in the conditions of welding aluminum hulls of ships. There is also a question about the welding mode and the welding current. It may indeed be easier to train welders .. Here you have to think .. Click to expand...

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