Need a Formula to figure out the "Blow" on Aluminum Hull Plates and Side Plates

Discussion in 'Boat Design' started by jdnelson97, May 24, 2011.

  1. jdnelson97
    Joined: Mar 2011
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    Location: Ferndale,Washington

    jdnelson97 Junior Member

    Hello,

    I am looking for an accurate CAD process or mathmatical way to figure out the "blow" when creating an Aluminum Hull and Sides (or any devolpable surfaces) . I am currently using Rhino. I have created a few new hull shapes and have flatened them out and had them CNC cut. But they tend to be off when the boat goes from straight to curved. The software does not compensate on how much the Aluminum bows out for example: From the shear line to the chine edge. The software projects a straight line between the 2 planes. This works on straight sections of the boat, but when it starts to curve, I need to figure out an accurate way to come up with what builders call " The Blow" in the sides and bottom plate. Any help would be appreciated.

    Regards
     
  2. philSweet
    Joined: May 2008
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    philSweet Senior Member

    See attached
     

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  3. micspoko
    Joined: Jul 2010
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    micspoko Senior Member

    in rhino use a plugin squish for developable surface of course you need properly divide plates to do this
     
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  4. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    See my responses in your other thread about this: http://www.boatdesign.net/forums/boat-design/need-help-aluminum-lofting-38220.html

    If you create a developable surface in Rhino using DevSrf it will properly model the curvature of the surface. Loft can also create proper surfaces but it can also cause problems as I discussed in the other thread.

    It is very easy to create a surface between two curves which looks okay but is not actually developable. Check the Gaussian Curvature with CurvatureAnalysis. It should be a very small number everywhere on the surface. If it isn't modify the surface so it is.

    Using Squish to get around problems when trying to flatten surfaces which are not actually developable is likely to cause problems later. Use UnRollSrf instead.
     
  5. jdnelson97
    Joined: Mar 2011
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    jdnelson97 Junior Member

    Thanks Phil. That answered my question.
     
  6. Kiwifinn
    Joined: Oct 2008
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    Location: Auckland NZ

    Kiwifinn Junior Member

    DCockey is right on, Loft can be a bit of a handfull and often takes a few goes to get good robust results. Interstingly I've found that sometimes the resulting surface differs when clicking the stern end of the chine and keel lines compared with the bow end. Go figure.

    A robust developable surface has a Gaussian curvature of Zero.

    Squish only works on small animals as the tool button clearly points out =)
     
  7. micspoko
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    micspoko Senior Member

    I have been involved in to conversion where curved plate were developed usuing commend squish in rhino and all the plates have been well developed
     
  8. Kiwifinn
    Joined: Oct 2008
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    Location: Auckland NZ

    Kiwifinn Junior Member

    Interesting, I will have to try out squish on plates I know unroll well and compare results.
     

  9. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    If the surface is developable, then Squish should give the same results as UnRollSrf.

    The problem with using Squish instead of UnRollSrf is Squish will flatten almost any surface, but if the surface isn't developable or sufficiently close to be developable the result probably won't be close enough.
     
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