# Theories for estimating bulb drag

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

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### SailDesignOld 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

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### Dan SJunior 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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### Tim BSenior 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.

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### Raggi_ThorNav.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.

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### jehardimanSenior 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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