Michlet/Godzilla Hull Design - Rocker

Discussion in 'Software' started by coreym, Feb 18, 2012.

  1. coreym
    Joined: Aug 2007
    Posts: 29
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    Location: Texas

    coreym Junior Member

    I'm helping some friends design a canoe by using Godzilla to optimize the hull before bringing it into FreeShip to extrude it, add hard chines, and perfect the overall shape to make it developable. Obviously, I would like to make as few changes as possible in FreeShip as that risks straying from the optimal shape. It doesn't help that it's been few years since I've done this and I'm a bit rusty.

    The problem I'm having is that Godzilla is creating a hull design in which the keel line drops at the bow and the stern. I want a couple of inches of rocker at each end of the canoe and I want a nice consistent curvature of the keel line. I thought I constrained the keel line to convex curvature only via the f2 and f4 shape factors. See my shape factors below:

    Code:
    # f0 - Bow Waterplane Shape: 
      0.50, 1.00, 0.50
    
    # f1 - Bow Cross-Section Shape: 
      0.10, 0.20, 0.15
    
    # f2 - Bow Sideview Shape: 
      0.50, 0.90, 0.70
    
    # f3 - Stern Waterplane Shape: 
      0.45, 1.00, 0.45
    
    # f4 - Stern Sideview Shape: 
      0.50, 0.90, 0.70
    
    # f5 - Bow Relative Length: 
      0.50, 0.50, 0.50
    
    # f6 - Stern Relative Length: 
      0.50, 0.50, 0.50
    Godzilla (or perhaps my inability to properly set it up) has a tendency of creating rectangular bow and stern shapes (sideview). In the image below, the yellow line is what I'm trying to get. If I were to show you the full profile view, you would see the keel line is somewhere between parabolic/elliptical (as constrained by the shape factors) but then deviates below this line at station 0' and 20'.

    [​IMG]
     
  2. Leo Lazauskas
    Joined: Jan 2002
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    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Hi, Corey.
    I'd have to see the input file to see if you have set it up correctly.
    Also, what hull series are you using? It seems like you are using series 7,
    so I'm puzzled why you are getting a drop at the stern and bow.
    It might be that Freeship is not reading the offsets correctly.

    Leo.
     
  3. coreym
    Joined: Aug 2007
    Posts: 29
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    Location: Texas

    coreym Junior Member

    Leo,
    Thanks for the response. I'm using Series 7. I've been importing the file into FreeShip by going to file -> import -> surface. The in.mlt file is attached.

    I'm ultimately going for something along these lines:

    [​IMG]
     

    Attached Files:

    • in.mlt
      in.mlt
      File size:
      6.3 KB
      Views:
      253
  4. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 149, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    I had a quick look at you input file.
    It's generally Ok, except
    1. You are only using 11 stations and 11 waterlines. You need to use many more
    2. Ntheta = 128 might be a bit low. I'd use at least 256.
    3. Don't put blank lines in the file, and try to start each line in the 1st column.
    Yours worked, but I advise against it.
     
  5. coreym
    Joined: Aug 2007
    Posts: 29
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    Location: Texas

    coreym Junior Member

    Leo,

    Thanks for catching the # of stations and the value for Ntheta. The processor is strong with this one, so running 512 works fine! Would you recommend higher?

    I still get the odd shaped keel line at the first and last stations. If I run the maximum of 81 stations it's negligible and can be fixed without altering the performance. 81 stations definitely affects run time, but I can just let it go overnight! Of course, it's only necessary once I've gone through many iterations of perfecting the input file.

    Thanks again. I really appreciate your development and support of this awesome program!
     

  6. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 149, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    My pleasure.

    The problem at the first and last station is really just a consequence
    of using equally-spaced stations and waterlines. It makes no difference
    in the resistance estimates. It is, however, a bit messy when you try to
    use the table as input to other programs, like Freeship etc.

    For your work 41x41 is Ok, and Ntheta = 256 is fine.

    More important than trying to get deadly accuracy for minimum resistance,
    is including reasonable, useful constraints on the hulls.

    Once you have completed your first round of runs, look at quantities like the
    GMT of the hull you have found. This tends to control the transverse initial stability.

    You could now change the limits on GMT and try to find a hull with
    the best resistance for a better GMT.

    If you repeat the exercise using different limits on GMT, you could plot a
    curve of, say total resistance for your design speed on the x-axis, and the
    GMT for the same hull on the y-axis. This will allow you to see the trade-off
    between transverse stability and drag.

    Which hulls are a resonable compromise of stability and resistance?
    Sorry, I can't help with that - it's more of an NA thing :)

    There, are of course, many other factors to consider, depending on your
    particular design problem. Some, like GMT, will be important, others (such as GML) less so.

    Leo.
     
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