Plywood Vs. Veneer in Cold Molding of boats with Compound Curvature

Discussion in 'Wooden Boat Building and Restoration' started by fpjeepy05, Jan 18, 2013.

  1. fpjeepy05
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    fpjeepy05 Senior Member

    Agreed the engineering is a little cude, but you get the general idea. Testing is really the only way to prove anything.

    I would be curious to see at what width the plywood cross grain begins to help with cross grain strength. My personal intuition says it is much wider than you are suggesting. Like 24 inches. Just MHO. I can see it getting a little extra stiffness, but in the end I think it is going to come down to the weak link, the glass skins bridging the gap between the boards edge. I think one or two layers of glass is going to add enough cross-grain strength that the additional cross-grain stiffness of the plywood, is going to be negligible. Again just IMHO.
     
  2. cavalier mk2
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    cavalier mk2 Senior Member

    Better cross grain strength allows lighter glass covering, saving weight etc.... I put in the ply lapstrake angle because in 4-6" planks there is significant cross grained strength added with only a small planking overlap. This was proven out years ago by builders of planing runabouts like Thompson, Reinell and others....
     
  3. fpjeepy05
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    fpjeepy05 Senior Member

    In lapstrake... yes. In double diagonal cold molding... agree to disagree. Cross grain stiffness, yes. Cross grain strength, very small amount at best. IMHO.

    On a different note, Does anyone have any veneer suppliers that stock, or are willing to produce relatively small volumes of 1/8" slice veneers?
     
  4. michael pierzga
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    michael pierzga Senior Member

  5. cavalier mk2
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    cavalier mk2 Senior Member

    Definitely agree to disagree. You could test this one for your self with overlapping veneer. Cut 10" lengths, lay them end to end for a few feet and stagger the second layer with the joints in the centers when you glue. Bend in all directions, use weights to compare. For data compare with a single whole veneer and one made of 2 full length laminations. In a actual boat of course the planks aren't free to flop around but it will give you an idea of the material and you won't have to engineer based on opinion.
     
  6. fpjeepy05
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    fpjeepy05 Senior Member

  7. PAR
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    PAR Yacht Designer/Builder

    I get veneers out of California and shipping isn't a problem, if willing to accept 9' lengths. Longer lengths can usually be "rolled", if the veneer is thin enough.
     
  8. fpjeepy05
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    fpjeepy05 Senior Member

    Can we get a name? Do you get just WRC? Do they do up to 4mm or just 1/8?
     
  9. michael pierzga
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    michael pierzga Senior Member

  10. PAR
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    PAR Yacht Designer/Builder

    Most veneer suppliers sell cloth or paper backed stock. You'll want naked veneers, preferably with live edges, which is sold in flitches, if you have a sizable project. I use Certainly Wood (certainlywood.com), though there are others.

    It appears that M.L. Condon produces "backed" veneers, which isn't what you want. I'm not sure about Global.
     
  11. fpjeepy05
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    fpjeepy05 Senior Member

    It looks like Certainly Wood definitely has some nice products. Some of the prices might be a little higher than I would expect, but I guess that's the way it goes these days. $3.25 sq ft of 1/8" Western Red Cedar, thats $26 a bdft Seems high. I see WRC for $3.25 / bdft some places.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Web site pricing seems at retail, but you can get deals, re-sale, volume and commercial account pricing.
     
  13. fpjeepy05
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    fpjeepy05 Senior Member

    With up to 85% off retail?
     

  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Modified Ashcroft Method

    From a strength point of view, the problem with plywood is that only 50% of the grain runs along the length of the strips, and the grain in the other plies has butt joints that prevent those plies from contributing more than a fraction of their strength to the strength of the hull. So it seems obvious that for a plywood hull to have the same strength its weight must be greater than - as much as double - that of a similar hull built using veneer strips.

    This is true even in the case of the Ashcroft method since 50% of the grain in one driection is interrupted by butted end-grain joints. The problem is the plywood material is being used inefficiently. In the Ashcroft method the inner layer mostly acts to bond the strips of the outer layer together.

    Between the joints this layer could be removed with little impact on strength and less discontinuity of stiffness. In effect, this would reduce the inner strips to butt blocks. An efficient way to build using plywood would be to reduce the width of the inner strips so they are sufficient to bond the outer strips together. Using maximum strip width compatible with a smooth hull would be most efficient. The spacing of the inner strips would have to be quite precise, but accumulation of errors can be corrected by the occasional use of a wider or narrower outer strip.
     

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