Aluminium vs ply strength to weight ratio

Discussion in 'Boat Design' started by Quidnic, Jul 13, 2022.

  1. Quidnic
    Joined: Apr 2020
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    Quidnic Junior Member

    If you had to choose between wood and aluminum where both would cost the same and have the boat ready within the same period of time, which would you choose and why?

    Please don’t vote for either and just say “because it looks cooler”, I’d really like some technical answers to this to give the team all the information they need to make a good decision.
     
  2. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Too little information to give an informed opinion. In this case size really does matter. So.....
    Do not confuse stiffness to weight with strength to weight and why either is chosen for a hull design. "It all depends..." Rather ask yourself for a 3-10 m boat would you rather spends 2X hundreds of pounds to build a 50 year boat, or X hundreds of pounds to build a 20 year boat. Above ~10 m, ask yourself do you need the strength over 50+ years.
     
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  3. Quidnic
    Joined: Apr 2020
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    Quidnic Junior Member

    For a small boat where weight really does matter

    which is the lightest for the strength ali or ply with epoxy?
     
  4. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    You could opt for one or the other material depending on whether or not you have the right tools to work with each of them. If this is not the problem, consider whether losing weight could be an advantage. If this doesn't matter either, think about the maintenance costs of the boat. If that doesn't help either, think about the resale value. If that doesn't influence your decision either..., then I don't know what else to say.
     
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  5. ExileMoon
    Joined: Oct 2009
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    ExileMoon Junior Member

    Most people who build their own boats often use plywood for their boats, not because plywood boats are stronger. Compared with aluminum boats, it is relatively fragile, which is mainly manifested in poor toughness.

    The biggest advantage of using plywood to make a boat is that it does not require special tools and equipment and special skills, and ordinary people can do it. Building aluminum boats requires specialized knowledge and equipment, and because of these constraints, there are actually some underdeveloped and remote areas where aluminum boats cannot even be repaired, let alone built.

    In addition, plywood is relatively low density, which results in a much thicker wall thickness than aluminum for the same weight, which is an advantage. This is more pronounced in smaller boats. But as the size of the boat increases, the advantages of aluminum will gradually manifest.
     
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  6. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Jehardiman is right. More info is needed. Where will the boat operate, how will it be used, will it have an electrical system and plug into the marina? ExileMoon's point is excellent too. Which material will be easiest to find repair services for. If the build cost, time, tools and effort are factored out. You are left with the considerations of maintenance and repair plus corrosion and rot mitigation.
     
    Last edited: Jul 13, 2022
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  7. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    @Quidnic what is the size / type of boat that you would like to have built?
     
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  8. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    I had a friend do some aluminum repairs on my boat for beer and gas.
    The price of argon is astronomical!
     
  9. jehardiman
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    jehardiman Senior Member

    How small is small? And what do you want to do with it?
    Material choices are all about the requirements. You could build a kayak out of ply to come in under 5-6 kilos. A 3 m Al production jon boat will come in about 50 kilos while a 5 m 3 sheet ply dory that is much more rugged, faster, and capable would be about the same. A fully rigged 6 m Tornado tortured ply cat is 133 Kg all up while a 6 m Al skiff for Alaska fishing is about 300 kg less motor.
    As I said, it all depends on what you want to use it for.
     
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  10. comfisherman
    Joined: Apr 2009
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    comfisherman Senior Member

    With all the extensive information given... aluminum.
     
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  11. rnlock
    Joined: Aug 2016
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    rnlock Senior Member

    There are so many variables that it's hard to say. In a lot of cases, plywood's lighter weight allows the use of greater thickness, making it stronger and stiffer in bending for the weight. There are, though, dodges you can take with aluminum that will compensate for this. Aluminum may actually dent a bit easier, but I bet it takes a bigger, faster swordfish to stab a hole in it. ;-) Toughness and strength are NOT the same. When I was a kid, we had a riveted aluminum rowboat that took lots of abuse that a plywood boat might not have survived. It sunk several times, banged into a concrete dock a bunch of times, got washed up an a beach consisting of little flat rocks. And so on. But, it was still functional when my folks sold it, even if it looked a bit rough.
    BTW, unless you can heat treat the whole structure, welding dramatically weakens heat treated aluminum alloys.
     
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