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Thread: Plywood query

  1. #1
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    Default Plywood query

    Hi guys, I'm new to all this, both boats AND forums, but keen to learn. I'm not really up with the lingo so I won't mind if you steer me in the right direction.

    Do you have a convenient "rule of thumb" to determine the amount of "cupping"(?) a lofted(?) sheet of ply naturally assumes when it's twisted over the front frames (from stem to amidships) on a hull bottom? (or maybe sides?) I think I recall something like 4% of the span (keel to chine) though I guess it would depend on the thickness of the ply. Any takers?

    Hope you can help
    Thanks
    Norm

  2. #2
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    Default Re: Plywood query

    Norm, I use plywood frequently but have no idea about the "cupping" to which you are referring. What I do know is that plywood sheets perform in accordance with the concepts of conic projections; they cannot bend in two planes at once. Careful use of conic projection in design can often create the shape you want. For long slender shapes and using thin plywood, I understand that a small amount of distortion (tortured ply) can be tolerated in sheathing, but it is not a factor that I ever rely on.

  3. #3
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    Default Re: Plywood query

    Quote Originally Posted by W Grabow View Post
    Norm, I use plywood frequently but have no idea about the "cupping" to which you are referring. What I do know is that plywood sheets perform in accordance with the concepts of conic projections; they cannot bend in two planes at once. Careful use of conic projection in design can often create the shape you want. For long slender shapes and using thin plywood, I understand that a small amount of distortion (tortured ply) can be tolerated in sheathing, but it is not a factor that I ever rely on.
    WG
    Thanks for your prompt reply. I may have the terminology wrong; maybe it's more of a "bowing" than a "cupping" depending on which side you're looking from. Most plans I've seen show the frame profiles(?) looking at the front are shown as straight lines, but I understand it's a bit of a mission to get the ply to lay flat on said frames, hence the slight bowing outward tending toward a roundish profile(?) It's the amount of that bowing/cupping thing that concerned me so that one could work that into the initial design. I may have been misled somewhere in the 40+ years I've been curious about these things, and may have recalled a snippet of something completely unrelated. Worse things have happened at sea.
    Thanks
    Norm

  4. #4
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    Default Re: Plywood query

    Aha! Found it. Lindsay Lords' "Naval Architecture of Planing Hulls" (itself apparently a profanity in some circles, but until I discovered Dave Gerr it was the only source I could find on powerboat design).

    Page 253, last paragraph, relating to a "very rugged little 35-footer..." ..."Of particular interest is the sectional convexity of the bottom. This is the natural transverse curve taken by a single plywood sheet when wrapped on to the longitudinal curve of the chine." Lord however was not impressed by this, claiming it's scientifically incorrect for high speed.

  5. #5
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    Default Re: Plywood query

    You might try the boatbuilders forum at Glen L (LINK).

    Their book is a classic -- I helped my Dad build boats with the first edition, almost 60 years ago.


    Boatbuilding With Plywood (LINK)



    This is the "modified" third edition of the well-known text covering plywood and its use in boatbuilding, especially from the standpoint of the amateur builder. This "modified" version is the same as our third edition but a few photos have been replaced, some typos were corrected, and the cover was updated.

    This book has become the recognized standard on a subject usually covered in otherbooks in a chapter at best.

    Widely used as a reference text in college level boatbuilding classes, this practical book explains the simple ways to build your own boat, taking you step-by-step from lofting to finish. You can almost build a boat by "reading" the pictures and drawings. All aspects of plywood boat construction are covered including lofting, materials to use, tools required, framing methods, covering with fiberglass using either epoxy or polyester resin, cold-molded planking, cabin joinery work, Stitch-N-Glue construction, and much more.

    This is a "must" book for both the beginner and accomplished boatbuilder who plans to build a boat using plywood. Numerous drawings and photos help put this building method in perspective and illustrate how almost anyone can build a plywood boat. The revised text also covers the use of epoxies for adhesives, encapsulation, and fiberglass over plywood use.
    #include [std-disclaimer]

    Good luck and Have Fun!

    .
    .

    “What use is a house if you haven’t got a tolerable planet to put it on?”


    ~~~ Henry David Thoreau

  6. #6
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    Default Re: Plywood query

    Good one, thank you very much. I'll get on to it

    Cheers

    Norm

  7. #7
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    Default Re: Plywood query

    The other classic on wood construction (with epoxy) is Gougeon Brothers on Woodenboat Building .
    Available as a free download.
    http://www.westsystem.com/wp-content...k-061205-1.pdf

  8. #8
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    Default Re: Plywood query

    Quote Originally Posted by normil View Post
    WG
    Thanks for your prompt reply. I may have the terminology wrong; maybe it's more of a "bowing" than a "cupping" depending on which side you're looking from. Most plans I've seen show the frame profiles(?) looking at the front are shown as straight lines, but I understand it's a bit of a mission to get the ply to lay flat on said frames, hence the slight bowing outward tending toward a roundish profile(?) It's the amount of that bowing/cupping thing that concerned me so that one could work that into the initial design. I may have been misled somewhere in the 40+ years I've been curious about these things, and may have recalled a snippet of something completely unrelated. Worse things have happened at sea.
    Thanks
    Norm
    A conical projection around that bow section generally leads to a somewhat bulbous entry, it can be modified by shaping the stem to avoid that but that has to happen in the design stage. I find it helpful to cold mould the forward third of the bottom panel of a planing chine hull, two layers over stringers which gives me much more freedom as a designer to choose what I feel is the appropriate shape in that area for the boat.

    John Welsford
    An expert is but a beginner with experience.

  9. #9
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    Default Re: Plywood query

    What John said: "A conical projection around that bow section generally leads to a somewhat bulbous entry" The "cupping" shown in frame profiles is a natural result of conic projection connecting a curved chine and/or keel shape. If that is not desired, John has also suggested corrections, either in the design stage or through abandoning the use of sheet goods in sheathing that part of the hull.

  10. #10
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    Rockland Maine USA and Woodbridge, Suffolk, England
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    Default Re: Plywood query

    It is true as John says that a conical projection tends to produce convex sections, especially as you approach the bow – and one can use this in "reverse" by drawing somewhat convex sections which generally helps lead to a developable surface.

    But both conic and cylindrical projections are developable and a combination of the two produces poly-conical developable surfaces and also super-poly-conical developable surfaces. So a surface does not have to be totally conical to be developable.

    And in practice, plywood is indeed somewhat plastic and will conform to non-developable surfaces to some extent. I noticed this recently helping a friend restore a Buzzards Bay 14: we laid ¼" ply over the foredeck, epoxy bonded to the existing ⅝" mahogany planking. The surface was clearly not truly developable but the ply pulled down quite nicely with a combination of good wetting out before bonding, tie-down straps and screw fastenings. A lot depends on the size of the panel and the thickness of the ply.

    Cheers -- George
    To be truly free to live, one must be free to think and speak.

    A C Grayling

  11. #11
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    Default Re: Plywood query

    Quote Originally Posted by upchurchmr View Post
    The other classic on wood construction (with epoxy) is Gougeon Brothers on Woodenboat Building .
    Available as a free download.
    http://www.westsystem.com/wp-content...k-061205-1.pdf
    Thanks for that. I thought I had it saved somewhere but apparently not. I'll have to upgrade my internet connection to grab that (8+meg), long overdue anyway.

    Not sure we can get that stuff here (NZ) and I had read somewhere that it's really expensive. There must be some useful pointers in the document anyway whatever I choose.

    Thanks again

    Cheers

    Norm

  12. #12
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    Default Re: Plywood query

    Quote Originally Posted by normil View Post
    Not sure we can get that stuff here (NZ) and I had read somewhere that it's really expensive.
    We can get WEST in NZ - even Burnsco has it, but it's cheaper from one of the online suppliers (fibreglassshop.co.nz or others)

    There are cheaper alternative brands but all epoxy is relatively expensive.

  13. #13
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    Default Re: Plywood query

    Quote Originally Posted by OliverBendix View Post
    We can get WEST in NZ - even Burnsco has it, but it's cheaper from one of the online suppliers (fibreglassshop.co.nz or others)

    There are cheaper alternative brands but all epoxy is relatively expensive.
    Thanks for that. Just googled "Burnsco" seems we have one fairly local, though that "fibreglassshop" seems to be a logical destination.

    Thanks
    Norm

  14. #14
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    Default Re: Plywood query

    Quote Originally Posted by john welsford View Post
    A conical projection around that bow section generally leads to a somewhat bulbous entry, it can be modified by shaping the stem to avoid that but that has to happen in the design stage. I find it helpful to cold mould the forward third of the bottom panel of a planing chine hull, two layers over stringers which gives me much more freedom as a designer to choose what I feel is the appropriate shape in that area for the boat.

    John Welsford
    John san, Howdy neighbour!
    How's things on the mountain? Where's your nearest body of water for your sea-trials?

    If that sounds presumptuous I apologise, it's that I've read so many of your posts and been in and out of your website so often it feels like I've known you forever.

    I nicked a copy of your rowing/seating/rowlock position sketch a while back to help with my intended boat layout. It's still here somewhere?

    Speaking of rowboats (at the risk of going off topic) why can't a rowlock-hole in the gunwales(?) be big enough to serve as a rod holder? I'm all for multi-function even though such things seldom do all functions well. Such is life.

    Thanks for your pointers

    Cheers

    Norm

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