Twisted Developable Surfaces

Discussion in 'Boat Design' started by Tedd McHenry, Nov 8, 2021.

  1. Tedd McHenry
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    Tedd McHenry Junior Member

    I've been looking at lines drawings for stitch-and-glue kayaks. Some of the lines drawings (if accurate) depict surfaces that have twist, as opposed to purely conical or cylindrical surfaces. It seems reasonable to me that the thin plywood used in kayaks could sustain a small amount of twist, and I'm wondering if there are some guidelines as to how much? (I.e., allowable twist angle per unit length.) I've been working on kayak hull shapes that are purely conical or cylindrical and it can be done, but many designs seem to use twisted surfaces, as well.
     
  2. duluthboats
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    duluthboats Senior Dreamer

    In my experience developed surfaces can twist, but not warp.
     
  3. Tedd McHenry
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    Tedd McHenry Junior Member

    By warp do you mean twist and curve simultaneously?
     
  4. DCockey
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    DCockey Senior Member

    All cylindrical and conical surfaces are developable, but not all developable surfaces are cylindrical or conical.

    Developable surfaces can have stations which are not parallel. If so the stations will be curved, not straight for a true developable surface. (And stations of cylindrical surfaces will also be curved if the axis of the cylinder is tilted.) However if the curvature is not too big it can be ignored. How big is too big depends on the material and it's thickness as well as the geometry.

    If you are drawing your surfaces using pencil and paper then you are pretty much limited to designing with simple conical and cylindrical shapes. If you are using software such as Rhino then there may be methods/commands available which will find the developable surface which fits between two edge curves. Using such software you can create a pair of edge curves, then find the corresponding surface. If that surface is not satisfactory the edge curves can be modified and the surface updated. Of course it can become more complicated depending on the shapes involved.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Twist and warp are different behaviours of the same thing.

    Twist - is rotation about an axis where all the sections remain in-plane.
    Warp - as above, but the sections are no longer in-plane.

    A tad more about it HERE.
     
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  6. BlueBell
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    BlueBell "Whatever..."

    Build it Tedd!
     
  7. gonzo
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    gonzo Senior Member

    There is also tortured plywood where the material is stretched and/or compressed. That allows to create surfaces that are not considered developable with a flat panel.
     
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  8. jakeeeef
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    jakeeeef Senior Member

    Did anyone else read the thread title and hope he was going to make a shape shifting hull by twisting panels on the fly?

    Displacement shape at displacement speed, planing shape at planing speed.
     
  9. gonzo
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    gonzo Senior Member

    That is already in the market.
     
  10. Tedd McHenry
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    Tedd McHenry Junior Member

    From the above comments it appears that nobody here is aware of any quantification of the amount of twist that a developable surface can sustain, but that it's more a matter of experience and trial-and-error learning. @DCockey , I've been experimenting with the method you described in "Developable surface - exact ruling lines from edge curves." It occurred to me that it might be possible to gain some understanding of how much twist will work by trying to find ruling lines between twisting edges, using that method. Do you think that would lead me to some insights?
     
  11. BlueBell
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    BlueBell "Whatever..."

    Maybe.
    But you'll have a much better understanding Tedd, after you build it.
     
  12. DCockey
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    DCockey Senior Member

    The amount of deviation from developable which is acceptable depends on:
    • Overall shape of the surface
    • Material properties
    • Material thickness
    • Smoothness and fairness of surface required
    • Locations, stiffness and strength of support structure
    • Deviation from designed shape which is acceptable
    • Fabrication technique and limitations including amount of force which can be applied.
    • Other factors
    There is no simple, universal answer.
     
  13. DCockey
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    DCockey Senior Member

    That method should find ruling lines with zero twist between pairs of curves.
     
    Last edited: Nov 15, 2021
  14. DCockey
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    DCockey Senior Member

    Three surfaces.jpg

    Three surfaces, only one is developable.
     

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  15. Tedd McHenry
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    Tedd McHenry Junior Member

    No doubt. But I'll waste a lot less time with trial and error if I make some attempt to understand what I'm doing before I start building.
     
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