Surface Roughness of Foils (again.... sigh)

[Chris123]

So, Last year's Scientific American on whale fins seems to imply that, in the age old question of "how smooth is smooth enough?" foils are somewhat of a a special case...

Thinking it to be of no practical consequence to me, I was just about to start the sailing season by polishing my Laser rudder and daggerboard to within an inch of their lives, as recommended by a firend who's a sailor and a famous applied math guy who deals with turbulence a lot, when I came across this article:
Miklosovic, Schultz, Esquivel. Effects of Surface Finish on Areodynamic Performance of a Sailboat Centerboard

Well. actually, I read only the abstract and first page because I'm not a paid subscriber, but it certainly implies that I ought to think again before getting out the 1500 grit sandpaper.....

Thoughts?

[Skippy]

Two points: First, I'm not sure the features on the whale fins are really roughness. They may be smooth, moderate-size waves that induce somewhat more orderly behavior. I have no idea for sure, that's just a thought.
Second, I was shocked a while back to discover that my Laser foils seem to have turbulators, bands of vertical ridges down the max thicknesses. I figured the Reynolds number is about right, so I would keep them.

Skippy, how do they work against ,say, smooth foils in racing?

[Skippy]

I have no idea, I'm just guessing based on the numbers. Re = 2 m/s * .3 m / 1E-6 m^2/s = 600,000. Supposedly that's not enough to induce transition of the boundary layer to turbulence, so turbulators seem reasonable in that case. Or did you mean the whale fins?

Thats what I meant..I'm interested in the whale fin story-there's another thread here about that I think.

[Skippy]

http://www.boatdesign.net/forums/sailboats/designing-rudder-6888.html#post44559