Gaussian Curvature Ranges / Bending Plywood

Discussion in 'Boatbuilding' started by Yakaking, Oct 30, 2010.

  1. Yakaking
    Joined: Oct 2010
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    Yakaking New Member

    Hello!

    I am designing a small boat in Rhino. I would like to build that boat of plywood. In the Internet I found some designs and the following design looks like my achieved design:

    http://svensons.com/boat/?p=RunaboutsInboard/riviera

    If you have a look at its frames

    http://svensons.com/boat/?f=RunaboutsInboard/riviera/riviera2_frames.jpg

    you can clearly see that the side planking of the boat is bended in two directions. I modelled exactly that hull in Rhino and of course the enroll command says that the surface is bended in two directions. The gaussian curvature analysis tool of Rhino says, that it ranges from 2.588e-07 to -2.588e-07 (auto range) with some red areas. My question is, do you have any experience in biulding boats of plywood bended in two directions and know what gaussian curvature ranges are still developable with plywood, so that I can enter those ranges in Rhino?

    I hope I could express my problem :)
     
  2. rasorinc
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    rasorinc Senior Member

    They are applying it in cut strips. (cold- molding)
     
  3. Yakaking
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    Yakaking New Member

    You mean they use several stripes of wood for planking?
     
  4. rasorinc
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    rasorinc Senior Member

    Yes. DOUBLE (MULTI-) DIAGONAL planking is more involved; used when compound shapes are incorporated into the hull (i.e.“bulbous forefoot”, “reverse curve”, “round bilge”). Uses strips of plywood or solid wood veneers laid over the hull in layers of opposite diagonals, glued together, most often with epoxy. Use epoxy only.
    COLD-MOLDED is a term that can be used interchangeably with above--These strips can be made for the total length of the boat and can be thin 3/16" or up to any width that will bend to get your desired thickness for the bottom and sides. The first layer is vertical about 25* off plumb and the second layer( if final layer) is horizontal. The pieces are around 4" wide X any length. If 3 layers needed the second is laid opposite the first and then the horizontal (third) layer. Much stronger than the original plywood. Most all of the classic mahogony speed boats were and are made this way with only the final layer being a horizontal layer of mahogony.
     
  5. Blue Salt
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    Blue Salt New Member

    don't know about the Gaussian curvature ranges, though "tortured ply" construction suggests very thin sheets of ply can be forced to bend very reluctantly in two dimensions. Would cold-molding you boat be worth considering?
     
  6. gonzo
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    gonzo Senior Member

    The best and probably only good way to learn about materials is to build something with them. Get some plywood and see what it does. Otherwise, you will only be playing in theoryland. You will find that different plywoods have completely different characteristics; some of them unexpected.
     
  7. Landlubber
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    Landlubber Senior Member

    ...basically a sheet of ply can only be bent in one direction...if you wish to do a second direction the ply will be "tortured", it does not like it, in thicker sheets you will NOT be able to physically do it anyhow.
    ...the above articles on cold moulding /thin diagonal moulding tell the story of what has to be done.
     
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    As Lubber has said, generally, plywood will not accept much compound curvature. The easiest way of handling compound shapes is to slit the plywood at along the axis of one of the curves. This lets the plywood "spread out" into the compound shape. Of course the triangular voids, caused by the slits will need to be filled, but they wouldn't be especially large on this particular design.

    Speaking of this particular design, it's not a very good hull shape unless you will be in calm water, with a light load all the time.
     
  9. apex1

    apex1 Guest

  10. Yakaking
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    Yakaking New Member

    Okay, so slitting the plywood is the technique. But what is wrong with the hull shape?
     

  11. PAR
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    PAR Yacht Designer/Builder

    There's nothing wrong with the hull shape. Most "sheet goods" will not conform to compound curves, so if you plan on using sheet goods to cover compound curves, this issue will crop up.

    To defeat this issue, a boat can be designed with conical or cylindrical shaped panels. This avoids the compound curve issues, but does limit the shapes available to the designer.

    Slitting the plywood is just one technique to work around compound curved surfaces, but not the only.
     
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