Hey guys digging this thread up cause I wanted to thank all for your advice and
share the knowledge I learned during the process. As well as show off the fact that
I ACTUALLY F'N DID IT!
Here's how I did it.
Measured up a few windsurf fins, decided most were pretty close to a NACA 0010. Also the Naca Equations are easily available online, it's symmetric (ie works both ways) and is reasonably thin. Model airplane guys seem to like this one, and these guys run similar reynolds numbers...besides, it was my first try, this is about whether or not I can, Not trying ot build a supercavitating foil here.
Making a plug:
Got to know my router....how many turns of the adjustment screw = how many millimeters change in depth of cut.
Translated the equation of a Naca foil into incremental steps on the router and # of turns so I could accurately reproduce the foil. This is why my fins don't taper, cuz I built my plug on a router table..I'm undecided if there are significant advantages to a tapered fin. typically it's a structural thing on airplanes. I don't think the loading on a kiteboard of a solid glass fin is really a concern......If it is, I can make it out of carbon......So screw it, I made it straight because straight was easy to make in my garage.....in hindsight, using a hotwire cutter to a fixed point you can make a tapered foil very easily..... will try this later....I've got other things to build like a race board. http://www.youtube.com/watch?v=jZkTCeoN79MI
I cut the basic outline out of 2 pieces of scrap 1/4" divinycell Then ran if over on a router table to get the foil . Piece (a) goes through left to right, piece b goes through right to left. Adjust router position, adjust cutting depth and repeat....when done I had 2 squared off mirrored shapes each resembling a fat stepped foil. gently sanded off the steps, hand foiled the tip and planed off any extra thickness....giving me two mirrored ½ foils....put them together and I have my desired fin shape.
I layed these up separately with a few layers of fibreglass to give them some structure so I could use them for plugs.. I used 2 layers of 6oz top and bottom.... layed them up on a flat mirror....
Found THE BEST mold release Loctite frekote FMS sealer and frekote release. . Not terribly expensive, and lasts a long time. (not the spray can) WAY faster than waxing, 1 wipe, and your cured epoxy will fall off under gravity and surface finish is PERFECT.. ...found PVA gives a pretty crappy surface finish While wax is WAY too labour intensive as the release if often times difficult.
Once glass was cured I shined em up
Here’s my plugs
A coat of mold release, and a few mold indexing bumps then covered 1/2 a fin with an epoxy tooling surface coat. (ie molding surface) and reinforced with 10 layers of scrap 6 oz glass I had from building boards. After that’s cured. I stuck the plug halves together and built the second half of the mold ontop of the first. Here’s a pic of the finished mold
Here’s a link to the basic Idea on the how to but on something more complicated. http://www.youtube.com/watch?v=UgKvDw1E60E
Once I had a mold, I made a few templates of decreasing size and cut out lots of glass (olfa rotary cutter is your friend) Wet layed enough glass in each mold half to fill it (ish) (like 30 layers of 6 oz). Keeping this much wet lay-up organized is nearly impossible so once cured, I pulled the part out and anded the flat side until it was flat with the mold surface using a belt sander. Did the same with the other side. Then put the parts back into the mold. And squeegee’d on a thin mixture of milled glass (for strength), resin(for bonding)and cabosil(to prevent sagging) So that the mold was perfectly filled. Put the two halves of the mold together to bond the pieces in the perfect shape and alignment and vac bagged the closed mold to squeeze out any extra resin.
Now I had a 50cm foil.......that's stiff and strong as all hell.
Needed a fin head, this was done according to these instructions.http://www.kiteracing.com/Pot_a_Fin.html
For mine I used an epoxy and milled glass mixture. Tried resin and carbon hair but it wouldn’t pour and trapped a lot of air leaving a large void in my casting which had to be repaired. (the one fin with the black fin head
Used 4 degrees rake,... cuz 4 degrees produces the optimal lift drag ratio on a flat planning vessel. (Savitsky) So figured this would be a good angle to have my fins going straight to get the best Ligth wind performance.
Used 4 degrees of cant.....cuz I heard that’s what the fast guys run
Used zero toe, cuz toe would counterproductive. on a raceboard.
Went with 42cm side and 40cm rear fin to start I highly suspect my untapered fins will make the board extremely “overfinned” but for 25 bucks worth of materials I can make more and cut em down smaller
It’s a small part, so not a-lot of actual work but a hell of time spent waiting for epoxy to cure
2 layups for the plugs
2 layups for the mold
3 lay ups per fin.
3 epoxy pours for the fin head
So 16 days spent just waiting for stuff to cure.
I’ll be experimenting with how to do the fin mold it in one shot. Bladder molding and resin infusion both look promising to save a few steps.
Obviously if I had a CNC machine I woulda just cut em out of G10, but this method has a lot of technical advantages
1) I can play with hollow cores or foam cores to save weight.
2) I can play with ridiculous high aspect ratios and use carbon uni for strength I don’t believe they make a carbon equivalent of G10.
3) If there is some desired stiffness, I can use lay up regimen and materials rather than screw with geometry and compromise the foil .
4) This is cheaper, I use about 1 yard of glass per fin (8 bucks) and not a whole lot of epoxy
Certainly not the last set I’ll build.
Know there’s a bunch of board builders out there, thinking about a raceboard but balking at the fin cost, I’d say it’s TOTALLY doable just takes time. (an hour a night for 16 nights)