Best Foil Shape for Rn 250,000 to 750,000

Anybody have any recommended shapes for foils in this renoylds number range.

[tspeer]

For what pupose?

There is no single "best" shape. The modern approach is to use a program like XFOIL (http://raphael.mit.edu/xfoil/) to design a custom section for each application. One often uses an existing shape as a starting point.

Its for a 12ft Skiff centerboard, 300mm and speeds from 5-7 knots

[tspeer]

In the NACA sections, the venerable 0012 is actually a pretty good choice. I wouldn't use one of the NACA 6-series sections without modifying it with XFOIL - too subject to leading edge stall at low speeds.

Other more modern choices would include the Wortmann FX L V-152 K25 (nice wide drag bucket - out to Cl = 0.5, min drag Cd = 0.009 @Re=1M) and FX 71-L-150/20 (narrower drag bucket to Cl=0.4, but lower min drag Cd = 0.007 @Re=1M). These were actually designed for the vertical fins on sailplanes. You can use XFOIL to examine them over your Reynolds number range.

The sections, along with wind tunnel data, are documented in Wortmann, F. X, "Stuttgarter Profilkatalog I", Friedr. Vieweb & Sohn, 1981.
Coordinates can be found at

FX L V-152 K25 http://www.aae.uiuc.edu/m-selig/ads/afplots/fxlv152.gif
http://www.aae.uiuc.edu/m-selig/ads/coord/fxlv152.dat

FX 71 L-150/20 http://www.aae.uiuc.edu/m-selig/ads/...s/fx711520.gif
http://www.aae.uiuc.edu/m-selig/ads/coord/fx711520.dat

There are also some Eppler sections that might be appropriate. Check out his book, "Airfoil Design and Data," Springer Verlag, 1990.

Paul Bogataj published some modern symmetrical sections designed for keels and boards in a Sailing World article in recent years, but I don't have the precise reference.

You can get coordinates for just about every airfoil known to man at http://www.aae.uiuc.edu/m-selig/ads/...database.html. Another good site is http://www.nasg.com/afdb/index-e.phtml - it has section data as well as shapes.

I have been recommended the Eppler 836 and have been analysing using the Design foil software. XFOIL has been consistantly recommended but it seems to have a very step learning curve.

[tspeer]

The E836 isn't a bad choice either - Eppler designed it for use as a keel section. He puts a lot more emphasis on cavitation that I would, given the speeds these boats go. Although a skiff probably needs to consider it more than most monohulls. The drag bucket on the E836 goes out to a Cl of 0.5, so that's comparable to the Wortmann sections, but the Eppler code shows bubble warnings on the suction surface outside the bucket.

Other Eppler sections to consider are the E520 (15% thick) and E521 (13.8%). These are more vertical tail sections. But they have a more gradual movement of the transition point and don't show bubble warnings until much higher lift coefficients. You could proably scale them to the thickness you want.

Yes, there is something of a learning curve to XFOIL, but it's nothing like learning to use the Eppler code! It's really not that bad. You really need to use a code that can handle laminar separation bubbles and has been validated against test data. XFOIL is the only download-able code that meets these requirements.

[nico]

I am interested in the article from Paul Bogataj, published in Sailing World, does anybody has an idea about precise reference?

Thank u
Nico