Runabout developable surface

Discussion in 'Boat Design' started by Bento, Jul 28, 2014.

  1. Bento
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    Bento New Member

    Dear all, I'm new to the forums and currently in the beginning stages of designing a runabout that (one day) should be relatively simple to build.

    My main question at the moment is how I can be sure that a surface is 'developable'. The first layer over the frames would be marine plywood with a thin mahogany planking finish layer on top.

    If one would look for example at the added picture, how can you be sure beforehand that the shape of that bow would be possible to form out of one piece of plywood.

    [​IMG]

    What would be the preferred CAD/CAM software that I could use to make templates for these plates.

    Thanks in advance!
     

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  2. Rurudyne
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    Rurudyne Senior Member

  3. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    If you can make a paper pattern you stand a good chance. If you can lay a piece of craft paper over the ribs without it tearing, chances are you should be able to develop it. Weather you can develop it in the material you want is a different story.

    Curves in two different planes will make creating a pattern in paper impossible.

    I do a lot of 3D drawing in Rhino. In Rhino when you issue the command to develop a surface it will refuse to do it if it can't be done.
     
  4. Bento
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    Bento New Member

    Thanks for the fast answers!

    When designing in Rhino is it possible to let the program 'help' you to make a surface that is developable? Otherwise, if you can't allow for a small degree of non-developability it might be impossible to create a surface that IS developable.

    Thanks again for the input!
     
  5. Russ Kaiser
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    Russ Kaiser Exuberant Amateur

    To my knowledge, Rhino doesn't have a tool that assists in making developable surfaces. The version I use I got when I took a drafting class. It's an academic version that I can't afford to rev. Even the old version is a fantastic program.

    Looking at your bow I think you're going to have trouble with that last section. Your best bet is to work in smaller pieces.

    Good luck, it looks like very nice work.
     
  6. TANSL
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    TANSL Senior Member

    A developable surface can only have a curvature. For example, if we materialize an X axis and a Y axis in the hull area under consideration, there can beonly bend in direction "X" or direction "Y". But there are approximate methods for developing an "non developable" surface with sufficient precisión.
     
  7. Skyak
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    Skyak Senior Member

    The primary consideration is the surface can be described by two curves and at every point on the surface a line can be drawn on the surface through the point from one curve to the other. The surface is conic.
     
  8. philSweet
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    philSweet Senior Member

    For a single chine craft, it is best to pick your chine line first. The keel line and bow profile is then whatever happens when a developable surface is constructed through the chine which gives you the volume properties you want. You can not pick a chine line and keel line and expect to find a developable surface to fit between them. Similarly, the topsides are developed up from the chine, and you cut a sheer as best you can.


    http://www.boatdesign.net/forums/attachments/boat-design/83555d1376573679-4-8m-skiff-rabl.pdf
     
  9. DCockey
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    DCockey Senior Member

    Rhino has two tools for creating developable surfaces from a pair of lines.

    Loft is a command which creates a developable surface if Style is set as Developable in the command's menu screen.

    DevSrf is a free plug-in for creating developable surfaces which appears to have a somewhat more sophisticated algorithm. http://wiki.mcneel.com/labs/devsrf

    UnrollSrf and UnrollSrfUV are Rhino commands which unroll developable surfaces to flat surfaces. The target surface does not have to be "exactly" developable. The commands report whether the area of the unrolled surface matches the area of the developable surface within a tolerance, and if it does not the difference in area is reported.

    There are no differences between Rhino purchased for the regular price and Rhino purchased for the educational price other than license restrictions.
     
  10. DCockey
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    DCockey Senior Member

    All conic surfaces are developable, but not all developable surfaces are conic.

    A ruled surface is a surface which can be drawn as a set of straight lines. All developable surfaces are ruled, but not all ruled surfaces are developable. A helical surface is an example of a ruled surface which is not developable.
     
  11. peterjoki
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    peterjoki Junior Member

    I have recently learned; through my own project with a naval architect, that a single sheet of plywood can be bent and twisted only into "ruled" surfaces. I'm sure that some cad software will give you the option to constrain your shapes within these rules.

    If you want to create more complex shapes you can opt for narrow strips of plywood instead of large sheets. This requires more fitting, but it isn't so hard with some practice.

    Check "ruled surfaces" on google and wikipedia. I learned something new.
     
  12. DCockey
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    DCockey Senior Member

    "You can not pick a chine line and keel line and expect to find a developable surface to fit between them."

    You can not pick a chine line and a keel line and be guaranteed to find a developable surface to fit between them. Using tools such as Rhino's Loft and DevSrf you may be able to find a developable surface.
     
  13. DCockey
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    DCockey Senior Member

    There are a wide variety of ways to describe a developable surface.

    A concise mathematical definition is a surface with zero Guassian curvature everywhere. Not very useful for designing boats though.

    Another definition of a developable surface is a ruled surface without any twist. A ruled surface is comprised of a set of straight lines which do not intersect (unless the surface is flat or at the edge of the surface). At any location on a developable surface there is a straight line which goes through that location and lies on the surface between the edges of the surface. There is no twist along the line which means the direction perpendicular to the surface is constant along the straight line on the surface. A physical test for a developable surface would be to take a straight and flat narrow batten and see if can be made to lie flat against the surface while passing through any selected location.
     
  14. DCockey
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    DCockey Senior Member

    Any surface which has both concave and convex curvature is not a developable surface. An example of such a surface is the side of the hull near the bow in the first post's photo. The waterlines are convex which the stations are concave.
     

  15. Rurudyne
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    Rurudyne Senior Member

    Yep.

    What I've done with DelftShip was to use relatively finely broken up intersect planes at the offset for my rabbet down the length of the boat. I then extended either a flat plane with my desired dead rise aft and one or more conical sections fore through that plane, each edge coincident with its neighbor, with a common reference edge being the chine. This gave me control points representing my hoped for developed surfaces.

    With this limitation ... one thing I don't like about NURBS is that while it draws pretty pictures it doesn't actually draw surfaces through the points as if they were offset coordinates ... but that's what a program like HullForm is good for, which isn't NURBS and can't intersect planes like that but which does draw lines through offset points you enter directly, and which has a good set of hydrostatic reporting features.

    It does seem cumbersome to me too.
     
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