Theories for estimating bulb drag

Discussion in 'Sailboats' started by Dan S, Sep 29, 2006.

  1. SailDesign
    Joined: Jan 2003
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    SailDesign Old Phart! Stay upwind..

    If you want to do a simple estimate, which the subject line implies, then a prop of similar diameter to the bulb will do quite well.
    If you want to get fancier (which your text implies), then I would try all of the methods above - but that's not "estimating" in my book.
    Too high-falutin' fer me.
    Steve :)
     
  2. Dan S
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    Dan S Junior Member

    Thanks, for the help everyone, attached is the foil that I will be using. After I finish this project, I need to go back and get a more in-depth knowledge of fluid dynamics. I’m pretty sure I could write, a 3d solver (it’s what I went to school for “computational physics”), if I had a better understanding of the problem.
     

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  3. Tim B
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    Tim B Senior Member

    3D solvers are not easy to write, and considering the number available (do a search on the web) there isn't a huge amount of point. Find one that is already written and use that.

    If you want to use a CFD method to analyse the bulb, you will need to do the calculation with the fin attached. The flow pattern with/without the fin will be totally different.

    You can also achieve lower drag by making the bulb non-axis-symmetric. Have a look around a marina at the racing yachts to see what I mean. That will need 3D CFD to analyse.

    Good Luck,

    Tim B.
     
  4. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    Maybe another question could be, what profile do you recommend for a bulb with circular sections? We used a naca 0018. I see that many use a profile with maximum thickness further aft.
     

  5. jehardiman
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    jehardiman Senior Member

    I concur with Raggi's point; I think the bulb you showed needs to be fuller aft. It looks like you used an inviscid soloution.
     
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