plywood thickness to size craft ratio

Discussion in 'Boatbuilding' started by stu large, Sep 16, 2009.

  1. stu large
    Joined: Sep 2009
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    Location: philippines

    stu large Junior Member

    Plywood to size craft ratio

    Sorry Guys, Perhaps I did'nt phrase my question properly, but I would use thin ply with lots of layers, I was just wondering how many layers.
    You see if you double the length of a boat, the volume increases by a factor of 8, and probably the weight too, does that mean that you need to increase thickness by eight ?, seems a lot to me.
    Thanks Stu
  2. boat fan
    Joined: Sep 2008
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    Location: Australia

    boat fan Senior Member

    No , no need to go that far .Would be too heavy too.

    Depends on how much structure is inside the boat too .
    Not that easy to answer that one stu.....cold moulded is strong , but your best guide would be to base your skin thickness on similar boats .

    There maybe some designer here that could help out , but without knowing exactly what the boat is , is hard to determine.

    If it were me , I would make the hull the same or slightly thicker than (or) equivalent thickness of the same boat built the stingers on frame way.

    If you add stringers inside to support the flatter areas of your skin you should be able to reduce your hull thickness.

    This might begin to sound a little repetitive , but if you can get hold of the Geougeon book there is some great stuff in there ....:)

  3. stu large
    Joined: Sep 2009
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    Location: philippines

    stu large Junior Member

    Tortured plywood

    Yes I know tortured ply is a hit and miss thing, it is also difficult to measure, so you need a good eye, to check it's shape,
    What I do is check first, how much the ply will bend, and then reduce that by a big factor so as not to stress it too much, also use jigs where ever possible, to hold the boat in the right shape, and continually check for twist.
    Not only does different types of ply bend differently, but no two sheets of ply are exactly the same, so letting the plywood assume it's own natural shape is not always a good idea.
    You need a design that is not too hard for the ply, that means long and thin, so this method is only really suitable for multihulls and canoes etc., I put length to beam ratio of 15 to 1 as a minimum, allthough I have heard of some people being able to get down to 10 to 1.
    The really good thing is when finally you get the shape you want, and complete the boat, it is incredibly stiff, and will hold it's shape allways, you can jack up my 20ft canoe just by it's stem and stern, with no sag at all.
    Thanks Guys
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