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

[Triman]

Hy to all,
I found this interesting thread, but unfortunately most of the links provided by Tom are no more available. Sure you can give me some tips about keel profile: we are building a 7m (23') Sportboat (similar kind of Melges 24) and are looking for the best keel profile (this is a classic vertical blade with a lead bomb, 2m long, 350mm chord). We are considering a NACA 63009, but like suggested by Tom, this can give stall problems; the Eppler could be an alternative. Any suggestion?
Thanks and good wind

[daiquiri]

This site works ok: http://www.ae.illinois.edu/m-selig/a..._database.html
It contains .dat files and pictures of nearly all of the most commonly used airfoils.

In addition to Tom Speers' always inspiring words, I'd just like to add that airfoils with a cusped trailing edge will at some point have to be cut, round or beveled along the trailing edge for manufacturing reasons. That operation will increase the drag of the foil.
Airfoils with thin trailing edges will also be less structurally resistant, so take account of that fact too, particularily if you need to carry a lead bulb. I'm referring to FX series here.

The increase in base drag due to cutting, beveling or rounding the trailing edge can be approximately evaluated with this formula taken from Hoerner's "Fluid Dynamic Drag:



As for the drag bucket, it will exist and will give you benefits in terms of drag reduction as long as you make sure that the foil surface is clean and smooth. In that way, there will be a laminar flow zone over the forward 30-50% of the airfoil surface, which is the reason for the drag bucket.
If the surface is not smooth or if it becomes contaminated by a marine growth, the laminar flow area (and the relative drag bucket) will disappear and will leave you with a conventional foil, with an ordinary (smooth and higher in value) drag curve.

In practice, it means - if the boat will stay in water for long periods, there will probably be no advantages of using airfoils with a drag bucket. Then you could as well opt for a simpler foil with a thicker trailing edge, like the said NACA 0012, NACA 0015 or similar.

If you intend to haul the boat out of water and clean the hull after every (or so) use, then you will see an advantage of laminar foils' reduced drag.

[tspeer]

Originally Posted by Triman

Hy to all,
I found this interesting thread, but unfortunately most of the links provided by Tom are no more available. Sure you can give me some tips about keel profile: we are building a 7m (23') Sportboat (similar kind of Melges 24) and are looking for the best keel profile (this is a classic vertical blade with a lead bomb, 2m long, 350mm chord). We are considering a NACA 63009, but like suggested by Tom, this can give stall problems; the Eppler could be an alternative. Any suggestion?
Thanks and good wind

Eppler E836


E837


E838


Comparisons between a NACA 63a010, E836 and NACA 0012 are attached. If the laminar drag bucket can be achieved (in water, with bottom paint), the E836 has the same minimum drag as the thinner NACA 63a010. The NACA 0012 has drag above a lift coefficient of 0.5, and a higher maximum lift at low speeds.

So it comes down to which is more important to you - laminar flow for less profile drag when sailing downwind, or a keel that is less likely to stall out in a downspeed tack?