Aluminium vs Steel

Discussion in 'Boat Design' started by Wynand N, Dec 3, 2004.

  1. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hi everyone, :)

    Which is the better building material for cruising sail boats - Steel or aluminium :?:

    I am on record for my faith in steel but in some previous posts and threads aluminium can up quite often. This got me thinking.

    It is one thing to design and build a boat out of aluminium, preach its pros as presented to us on spec sheets compared to steel. But, how does aluminium delivers to its promises in the real world, meaning owning one, cruising the four corners of the globe, maintaing it etc.

    Are their any case studies out their regarding alu boats in peril such as striking reefs, collisions etc. ?

    After all, when you sink your money in a yacht, (pun intended) on average the hull only represents about 15% of capital outlay.
    Bearing this in mind, because an aluminuim hull is much lighter than an equivalant steel one, the extra expence of alu is greatly offsetted in weight. Looks promising if alu is realy great.

    Please convince me gentleman.

    Fair winds to all

    Wynand Nortje
  2. CDBarry
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    CDBarry Senior Member

    Depends on everything - that's why there are both aluminum and steel boats built. If one was better, you wouldn't see the other.
  3. MDV
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    MDV Junior Member

    One of the big differences in that in most places you will find someone with the skills and equipment to peform repairs of steel structues. It may be harder to perform repairs of aluminium in remote places.

  4. Sean Herron
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    Sean Herron Senior Member



    Aluminum is bloody expensive and thus not imported by many countries you might end up at - and doubly expensive for same reason if it is...

    This cost can be offset by not needing to blast - prep - and paint it like steel - if staying local...

    You can torch any piece of steel out of an old Pontiac if need be... :)

    Tricky to weld too without good equipment...

    However - I prefer fabricating and machining aluminum to steel - common tooling will cut the stuff - annealing can be fun - spinning too...

    If I was going anywhere third world though - I would build in steel...

    But I never will - why would I...

  5. Portager
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    Portager Senior Member

    It is true that per pound Aluminum is more expensive than Steel, however on a per boat basis, it is about the same and if you don’t paint it, except below the water line, aluminum is actually cheaper.

    In terms of damage tolerance, steel has a higher ultimate strength. However, since Aluminum plating is 50% thicker than steel (per most construction standards) the aluminum hull will survive 12% higher ultimate loads. In addition, since aluminum has a 17% higher yield strength than steel, when combined with the 50% higher thickness, it will tolerate 75% higher loads before deforming (i.e. denting).

    A more subtle advantage is that aluminum has a lower modulus of elasticity than steel (10.5 versus 29 Mpsi), which means aluminum is springier. The advantage of this is that when you hit something, the aluminum will deflect more than the steel. In most cases, this deflection brings more surface area into contact effectively distributing the load over a larger area, further reducing the potential damage.

    From a corrosion resistance stand point, aluminum is intrinsically better than steel, however you need to be careful of galvanic or dissimilar metals corrosion. It is vital to never leave anything made of steel laying on the aluminum.

    One of the main advantages of aluminum is it typically produces about a 45% lighter hull (the advantage is even higher below 35 feet due to minimum plate thicknesses). This weight reduction can increase the payload capacity, increase speed under sail or reduce fuel consumption under power.

    One down side of aluminum for a hull material is it transmits noise much better that GRP or wood and ~2.25 times better than steel laterally (primarily due to the 50% greater plating thickness). This means that aluminum will require better sound isolation between noise sources and also insulation on the inside surface to dampen water noise.

    Aluminum is also a better thermal conductor so there will be more heat loss or gain and higher potential for condensation in the bilge. Once again the solution is good insulation.

    Here are a couple of informative articles on the subject.

    Mike Schooley
  6. mattotoole
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    mattotoole Senior Member

    Aluminum was popular for one-off racing sailboats in the 70s. I sailed on several of these boats. They were all very damp below from condensation, and cold too. Of course this can probably be overcome with insulation.

    Other than that, aluminum boats are very appealing. The bare metal can be very attractive, and of course low-maintenance. I like that you can just weld hardware on wherever you want -- no leaky decks!
  7. alloyed2sea
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    alloyed2sea Junior Member

  8. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hi :)

    Portager & Alloyed2sea

    Thanks for the links.
  9. dincerd
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    dincerd Junior Member

    Aluminium hulls are more useful if you want much performance. because it is lighter than steel construction.
    but unfortunately as MDV told, aluminium is hard to repair and also much more expensive. however you must see an aluminium hull, shining in the sun and look GREAT!

  10. MikeJohns
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    MikeJohns Senior Member

    It is not a good comparison to compare the materials on their rule scantling equivalents. The scantlings also have to account for fatigue strength weld zone weakening, corrosion rates. Thats why the Al ends up looking stronger. Consider also that aluminium has a habit of failing along weld lines, fatiguing to failure with flexure and erroding and corroding with rapidity.

    Using steel plate a couple of grades thicker on the underwater areas is much easier to work with than doing the same with the already thick aluminium plate. Plasma cutters have made steel working so easy now.

    8mm steel bottom plate is good insurnace for a 35 tonne vessel. This sort of strength is hard to work into an Al hull as the plating gets too thick to be easily worked.

    More deformable but not springier! Since we are talking energy absorbtion you are better off with steels stress strain curve. Steel gets out of the elastic range too and has a very large deformation prior to rupture, far better than aluminiums in terms of energy absorbed for a given section modulus.

    I'm not convinced that the lower modulus of elasticity is an advantage! your theory sounds good, in practice with fishing boats I have found that damage to steel hulls is more localised and much cheaper to reapair. Aluminium vessels don't survive grounding on rocks very well, and due to the deformation can be very hard to get off without considerably more damage.

    Steel can be repeatedly welded, ground out, re welded, and still acheive full original strength, the welding is easy and tolerant of poor operators.

    With modern coatings and proper attention to detail, steel vessels can be made completely inert in a marine environment too.

    Steel has a huge advantage in its fatigue stength limits, fatigue cycle stresses on a boat are numerous, consider a yacht on a mooring loading opposing chainplates, every 3 seconds for 20 years!

    Vibration fatigue is also an issue with aluminium, particularly due to prop and engine vibration leading to cracking and failure, this is another thing factored into the scantling rules.

    Aluminium has its place and can be a good material, but I really think steel is the better one.

    However I do have a penchant for designing heavy displacement boats.
    1 person likes this.
  11. Dutch Peter
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    Dutch Peter Senior Member

  12. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hi :)

    About 18 years or so ago I read an article (I think it was Cruising World) of a steelyacht that landed on a submerged reef during high tide in a storm. The real peril was that she laid about 100 + meters from deep water.
    Low tide she sat high and dry on the reef and with every high tide she was subjected to bumping, slamming and grinding on the coral. There were some nice photos of the said yacht on the reef.
    If I remembered correctly, she took this battering for about 10 days or more. Eventually help came in the way of a freightship, a cable was attached to the yacht, and then she was dragged the 100 meters or so across the reef into deep water!!

    The yacht floated well without leaks and the only damage sustained except for pride and paint job, was a damaged rudder and shaft. She made it back to port under own power.
    This is the stuff that makes me excited about the virtues of steel as a building material for cruising yachts. Imagine a fibreglass boat ending up in the same situation....

    My question: would aluminuim survived the above incident with the same or less damage?

    Mike and Dutch Peter, your views and sound advice are always welcome. :)
  13. MikeJohns
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    MikeJohns Senior Member


    No it wouldn't,
    It it has too poor an abrasion resistance. (The same attribute that make it easy to work with woodworking tools).

    But note Wynand as I have said before a lightly plated steel boat will not be so good either.

    Happy Christmas
  14. B. Hamm
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    B. Hamm Junior Member

    Actually there was an advertisement a number of years back of I think a Pacific Seacraft built of fiberglass that did basically the same thing, sat on a reef for a fairly long time and survived and was sailed away.

    You can make a design for any material that's strong enough to do that if you add enough material.

    Bill H.

  15. Skippy
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    Skippy Senior Member

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