Favorite Foil Section

If you build 'boards and rudders, what's your favorite foil section? NACA 00XX? Something more modern like the 6 series? How about the International 470 class legal barn door?

I'm hoping to give a local Shopbot some practice (it seems lonely), and am wondering where to begin the process of messing with designs...

Will you make a plug and a female mold or carve a core and laminate over it?
I use NACA 00XX for boards. For rudders there is something called DAG section. This was used on A Class and all of Marstrom's boats including Tornados. It was explained by Martin Fischer a few years ago on his catamaran site but it seems to be gone now. I do not have a mathmetical description and always used my Tornado rudders to copy the section and scale it up or down as needed. The section has a smaller tip radius than NACA, max. thickness is 45% aft, and there is a slight hollow in the after half of the chord. The theory is that this shape delays ventilation and the onset of stalling at higher angles of incidence. I have an aquaintence that works at one of the big design firms at Annopolis and he told me they sware everbody to secrecy when it come to their library of foil shapes. This might explain why details are so hard to come by from open sources.

Kevin is right about people holding foil sections "close". The DS12 uses NACA sections because they are easy for people to generate and because I wasn't going to give more developed foils away for free.

If you really want to learn about foils contact RG, he's the man on this stuff.

Thanks for the replies guys; sorry I'm late getting back but after waiting a few days, I had given up on the thread.

KC, I was thinking of making a core with a local shopbot and laminating over it -- probably a carbon uni "spar cap" and a couple of layers of glass over the whole thing. I noticed that softwood (pine, cedar, etc.) adds about a pound over foam for my design and does a lot more to prevent failures due to local buckling, not to mention dings. This old style seems to have advantages, especially in a shallow water venue with lots of sandbars, weeds, pipes sticking randomly out of the water, etc.

That "DAG" foil design you describe sounds interesting: things which I had thought would worsen high-angle performance (sharper LE, larger adverse gradients in the back) improve it? Then again, there are so many things going on, one wouldn't expect undergrad level fluids to be much of a useful guide...

For a recreational business, there sure are a lot of closely guarded trade secrets. But, one has to respect that people who have innovated need to, well, eat. Holding onto design details seems a bit more polite than starting a one-builder class. <img src="http://www.catsailor.com/forums/images/graemlins/wink.gif" alt="" />

I have carved them out of both cedar and balsawood. Used a regular old plunge router. I would mark the depth contours onto the blank based on NACA foil generator program results. I would divide it up into six to ten steps and the result would look like a topographic map. After routing as close as possible to the lines, I sprayed flat black paint on the blade. Then, used a rotary sander with 40 grit discs to rough shape the foil - using the black paint as a guide. Finally, did the finish shaping with 80 gtit flat sanding block. You can actually get pretty acurate results this way. Does the shopbot carve the finished contour or do you still have to finish the shaping by hand?

And also, recommend whatever you laminate to the outsid of it, include a layer of 5 oz. Kevlar underneath it. This adds a tremendous amount of damage resistance.

Kevin

Hmm, interesting. Balsa should make a superb core, shouldn't it...the attraction of the shopbot NC router is that it gives you the right shape almost directly from CAD drawings. A bunch of the guys here use it to make female molds from high density foam; they cut the molds with the 'bot, then spray on a coating ("duratec" or something), finish sand, add mold release, and get out the vacuum pump. Pretty soon they'll be making blades for small wind turbines that way.

I've heard about tricks with saws and routers and such; yours seems better refined than what I'd heard before and I'll plan to give it a try sometime. Hopefully in a few weeks some "hey look guys" pictures...though maybe not showing the old sweatshirts as bleeder ply. Thanks again for the advice.