Plywood hull shape definition

Discussion in 'Boat Design' started by Alexanov, Jan 21, 2017.

  1. Alexanov
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    Alexanov Senior Member

    Hi, does enybody can recommended some books of guidance for hull surface definition for boats with plywood hulls? Idea is to make computer model as close as it possible to real plywood material shape and minimize any adjustments. Any opinions will be appreciated.
     
  2. TANSL
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    TANSL Senior Member

    If you get all surfaces to be developable, the development of the parts can be totally accurate.
    Probably what I say is too obvious.
     
  3. kerosene
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    kerosene Senior Member

    how to guarantee what you model on computer is truly developable is the question.
     
  4. TANSL
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    TANSL Senior Member

    In most CAD programs there is a specific command to generate developable surfaces. Normally, it is also necessary that the user has knowledge about the types of surfaces and how to generate them in order to use the adecuate command.
     
  5. philSweet
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    philSweet Senior Member

    There are several existing freeware that can be gotten to work if the user has the necessary background. What none of these does is provide force calculations or solve the energy balance to calculate the distortions induced in the panel. The ability to model a panel shape that is actually being wrapped over a framework or joined at a floating chine would be very nice to have. You would need a set of non-isotropic panel models for each different plywood construction and species - so that's a good fifty material descriptions just for starters (but paramertizing them isn't that complicated). Basically, you need to start with a decent verified FEA, and add a front end and a decent non-isotropic material model for the plating.
     
  6. Alexanov
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    Alexanov Senior Member

    Try to make a simple test. Take paper, fix opposition edges to be straight and twist them. Then you will see result. But gausiuan curvature of such twisted surface is zero and surface should be developable. So it is not so simple question. But a lot of boats built with such shape. I am sure should be some information how to make it right way...
     
  7. TANSL
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    TANSL Senior Member

    I do not know where the shots are going to, but maybe the attached file can be of help; it deals with how to make the hull with conical surfaces.
     

    Attached Files:

  8. philSweet
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    philSweet Senior Member

    The problem with working with developable surfaces directly is that they tend to be mathematically under restricted. For instance, if you have two panels and join them along an edge, there are normally more than one shape that can be had. You end up either needing to do an energy balance and find the shape with the smallest distortion energy, or you need to create a fully enclosed manifold, or you need to add sufficient additional constraints. This is not an easy problem to attack.
     
  9. TANSL
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    TANSL Senior Member

    Nobody says that it is easy but, on the other hand, there are techniques to develop plates that are not developable. They have been used for a long time on steel ships, especially in welded construction, and are approximate enough.
     
  10. Alexanov
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    Alexanov Senior Member

    Thank you. It can be usefull. I am just afraid it is not possible to keep c2 continuity between patches with this method.
     
  11. TANSL
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    TANSL Senior Member

    Yes, there is precisely the secret to getting a really smooth hull. Many people do not know but c2 continuity is fundamental. That is why I believe that, in addition, of methods and formulas, it is essential the experience and knowledge of the user. In some cases, adding a new knob, even if it is imperceptible, can be a solution.
     
  12. Wayne Grabow
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    Wayne Grabow Senior Member

    I do not know how to make a computer model of plywood hulls, but I do know how to design accurate plywood-sheathed hulls (with shape restrictions). It involves simple algebraic projections connected by common ruling lines. The projections are either of constant slope, x:y or x:y:z, (in a x,y,z coordinate system) or conic projection from a chosen focal point. The key ingredient for me is to have a predefined chine from which to base most of these projections.
     
  13. Wayne Grabow
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    Wayne Grabow Senior Member

  14. Wayne Grabow
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    Wayne Grabow Senior Member


  15. rwatson
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    rwatson Senior Member

    The shape restrictions make for unfair curves in that canoe by the look of it. Tehre are several 'kinks' in that hull.
     
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