3/8 or 1/2 inch ply?

Discussion in 'Wooden Boat Building and Restoration' started by robertgrandbois, Apr 1, 2010.

  1. robertgrandbois
    Joined: Mar 2010
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    robertgrandbois Junior Member

    I am restoring an old plywood boat. The frame is all red oak and in very decent shape. There was 3/8 plywood on it before, but I can get 1/2 inch on sale at half price but I am scared it will not bend at the bow. Can I do the bow with two layers of 1/4 and the rest with 1/2 inch?
     

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  2. alan white
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    alan white Senior Member

    You're right to be worried. The thicker plywood isn't just 50% stiffer. It's probably about twice as stiff as the 3/8. Unfortunately, short of running the finished piece through a drum sander to make it thinner, you'll have to bite the bullet and buy the more expensive plywood.
    You could, as you mentioned, use two layers of 1/4" plywood, but you will use a lot of epoxy becuae the whole piece ought to be 1/4", or else you'll have to scarf it in, which is a lot of bother, to say the least.
     
  3. robertgrandbois
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    robertgrandbois Junior Member

    that is what i tought but was hoping to save a few bucks. i will go with the 3/8 plywood. Now could i use the 1/2 inch for the top? it does not seem to have much curves to it.
     
  4. alan white
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    alan white Senior Member

    Yes, of course, at the cost of some extra weight.
     
  5. tom28571
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    tom28571 Senior Member

    You can scarf a piece of 3/8 to the 1/2 for the bow section. Works fine and this method is used by many.
     
  6. robertgrandbois
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    robertgrandbois Junior Member

    ok tom i but i was thinking of doing a but joint would that work?
     
  7. lewisboats
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    lewisboats Obsessed Member

    no...the transition needs to be as strong as possible. To use a butt block you need to have the same thickness to butt together. That option really needs a 12:1 scarf.
     
  8. alan white
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    alan white Senior Member

    That's true. It would work fine and would be practical if a scarf was required anyway to lengthen the panel. If not, buy the 3/8" plywood and save the hassle.
     
  9. robertgrandbois
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    robertgrandbois Junior Member

    I got the ok to get all 3/8 from upper management(wife). There originally was a scarf but I was thinking of a butt joint instead be faster.
     
  10. alan white
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    alan white Senior Member

    A butt located where there's not much curve or twist, with a backing plate, is not a problem. Especially if the exterior will be glassed.
     

  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Material stiffness varies as the cube of thickness. 4/8 thick ply compared to 3/8 ply would be stiffer by a factor of (4/3)^3 = 2.37

    Thats assuming uniform material. However if the ply has relatively few, thick veneers, it would be more flexible in one direction, so perhaps you could use it "cross-grained" and still get around the bends.
     
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