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

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.

 

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.

 

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.

 

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

 

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.

 

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.

 

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?

 

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.

 

Seems to be your only option, currently.
But before you do...get the welders to do the same class cert test for welding steel. Since they may just be poor welders!

You must have the resistance and weight data.. as such, then only you can calculate the effect and make an informed choice.

 

TANSL, initially, we considered aluminum as a material with better performance than steel. But now I am confused by the results of the seam inspection. At the same time, we are looking only qualified welders. We need 3 people in total, but so far we have found only one.

We are not yet moving away from the idea of welding the boat's hull from aluminum, but I want to objectively evaluate the available alternatives.
Now I admit that perhaps at the very beginning I made the wrong decision on the material of the boat's hull.

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?

Will Gilmore, now, I am considering the option to make both the ship's hull and the steel bridge. Why would you choose steel?

DogCavalry, thank you, very glad to be on the forum!
As it turned out, I have not decided on the material for the ship's body, I want to look at this issue from different points of view, and I already see that the forum helps me with this!

Barry, as I understand it, on average the process looks like this: first tack welding, then the main penetration.
Tactics now:
The inner bead is semi-automatic TIG welding, the outer bead is MIG welding.
Perhaps we need to use other welding equipment, next week we want to get to a large center similar to the ESAB showroom in Krasnogorsk (near Moscow), maybe they will give us feedback on what we did wrong. We will immediately test their equipment and materials.
It may be that the problem will be solved before that, or I will go already prepared with the help of members of the forum.

What equipment would you use to weld an aluminum ship hull?

Bajansailor, welcome to this thread, you are a great help in my thread on choosing a diesel engine! Thank you!
Please tell me what materials did you process aluminum and steel? If possible, describe roughly the process. After all, as you correctly noted, it is necessary to take into account the total labor intensity and subsequent filling.
How often should the interior paint of a ship's steel hull be renewed? Do you think this makes adjustments to the interior decoration strategy, or is there no significant difference between steel and aluminum boats?

comfisherman, thanks for the advice! You give very sensible and correct advice, almost ready-made solutions!
I will consider taking on the training of two of the most qualified welders and discussing with our welding moestro how long it will take to improve their qualifications. Thanks for the idea!
ALMg5 has risen in price very strongly since the beginning of the year, by about 30%, and now the difference in ship steel and aluminum is about 800%! This is also a reason to think))

How did you protect the steel boat from rust?
Which part of the aluminum boat had a galvanic problem? At joints and contacts with other metals? How did you fight them?

For myself, I have no illusion that the aluminum case does not require maintenance. The main thing is to understand how to care for him and where there are subtle nuances. Can you help with this information and share your own experiences? What formula have you come up with for yourself?

 

Bajansailor, I will definitely do this as soon as the project appears in my hands. And I will immediately create a construction branch.
I apologize for my strange traditions, but I'm afraid to present unfinished work, since I believe in omens)

 

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 ..

 

It is an option you might consider, obviously the connection is a mechanical one, but nothing insurmountable as regard dissimilar metals, you can largely isolate them. This idea seems not uncommon with naval vessels, the propensity of alloy to burn fiercely with battle damage may be a deterrent, but presumably you won't come under missile attack !

 

Why not use the most commonly used alloy - 5083?

 

You statement here seems a bit odd to say the least. Over a couple of years, I have had a dozen workboat alu-hulls of different sizes built in Russia (St Petersburg and Yaroslavl). The welder skills have been exceptionally good. I think you simply have not made contact with professional alu-builders, where in Russia are you?