Funny that the fins / airfoils / Wavegrinder etc. thread would pop back up today…
Maxmercy dropped by a couple days after Christmas with a little gift for me. We’d been talking back & forth a lot about my results with longboard quads, as well as about my experiences with the Spitfire (which is all great, except for noseriding). Cant as it relates to longboard-specific games like noseriding, turnability with rake vs. low drag with upright fins, etc. He’s got an engineering / aircraft background and knows how to play with foils and surface areas on the computer to find perfect Reynolds numbers - and all kinds of other stuff that goes over my head. All I have to do is ride the things and say what was good and what didn’t work.
John (MaxM) made a pair of fins with some CAD and some crazy technology ideas, not to mention a lot of good old-fashioned wet work in the lab. I’ll leave it to him to explain the process (and give the things a proper name).
So he came up with some calculators that I could play with to try & get fin sets with the same surface areas as my ‘normal’ big, pivot-type single fins. But maybe with the low drag of the Spitfire and some cant in the front pair for noseriding hold. Seemed like a good combination to me…
So I took these for a spin on the 30th. They were fast in a straight line (very little drag or toe-in) and great for noseriding, but hard to turn. Too much cant with the 8*, I think. And maybe too much overall surface area…
So I decided to try my hand at grinding one out. I didn’t have any fin blanks, and didn’t want to take the time to lay one up. I’m short on sunlight these days, and its too cold for epoxy. But I did have this old, giant, 10.5" True Ames Heritage fin that was all beat up from hitting rocks…
So I laid out the biggest airfoil-shape fin I could. Perfect trapezoids make the max. thickness line a lot easier to follow (30% on this one) and a tip chord in the 66% range of the base chord made for decent looking Reynolds numbers in John’s calculator, so I just went for it.
Drill some holes in unnecessary spaces…
And pop a carbide blade in the jigsaw to connect the dots
Here’s how I clamp the base tight to avoid splitting while drilling the screw hole in the front tab. This has come up a couple of times in recent other threads, so I figured it wouldn’t hurt to take a photo.
Now clamp down the belt sander and set the calipers to 3/8" and true up the base…
Install the hardware. Hey! Looks like a fin! Or an airplane wing or something…
Pop it in the board and things are starting to look very airfoilish. One thing’s for sure - I don’t want to fall on this cluster. Might have to blunt some of those corners just for piece of mind… after I surf it at least once. If its the best day ever, they may have to stay sharp.
Maybe I’ll glue up some balsa & redwood scraps. I know I’ve seen someone do this before. I don’t think it was RichardMc, but a woodworker of that quality (far above mine) who takes stock fins and replaces the guts with wood for beauty and weight loss. If anyone remembers who does that, please let me know as I’d like to properly credit him…
Re-foil the wood interior (the original 3/4" leading and trailing edges make nice foiling guides) and start to glassin’!
2 layers of 10 oz volan on each side should look about right. Glass one, hotcoat, trim the flash, fill the edge cavity with a bead of resin, surform it smooth, and glass & hotcoat the other side. Thanks to Troy & Austin for some tips on those processes
Hopefully, I’ll get to grind everything & sand & polish in the next few days.
Not to mention get to surf the jet airliner. Supposed to have the biggest storm of the last few years around here for the next 3-4 days, so please don’t hold your breath for ride reports, but if I get to the finishing on the Jenny Craig fin, I’ll post the results.
I’d love to hear how the airplane wing thing goes, and if you play with the side fins as well. Nice work saving an old fin in the process of making an new one. I need to make a thruster center fin for a FU box. This thread was at just the right time. Might have to make myself a twofer too.
Thanks, Mike - you’re one of the innovators who inspires garage guys like me. You doing any trapezoid fins you can share?
Johan, that’s it - John Cherry. Thanks! I’ll edit the post above as well.
Hey Pat, just go buy a big pivot fin & cut a small center out of it. If you follow the outline even more closely, you should be able to get a 6.5 or 7" (not including base) out of a 10-10.5" like that Heritage. And then fill the big guy back in, its super easy and I know you have extra balsa & glass sitting around…
Now clamp down the belt sander and set the calipers to 3/8" and true up the base…
Install the hardware. Hey! Looks like a fin! Or an airplane wing or something…
Pop it in the board and things are starting to look very airfoilish. One thing’s for sure - I don’t want to fall on this cluster. Might have to blunt some of those corners just for piece of mind… after I surf it at least once. If its the best day ever, they may have to stay sharp.
those airfoils are cool as all get out! Nice thread Ben.
Benny, Here’s a 15 1/2" Ninja fin I designed in 1988 for hi speed sailing. This fin was used up to speeds of 32 plus miles per hour in Hood-river Oregon. Keep up the great design work Benny. Mahalo,Larry
Larry, I remember all of those from a thread you did maybe a year ago showing all kinds of interesting stuff from over the years. The lateral ribs on the first one would seem to address the same downwash-redirection that Wavegrinder was talking about?
And yes, when I started playing with John’s calculator, high-speed windsurf fins suddenly made all kinds of sense to me. Hopefully, he can explain how he made it, I can’t even begin to understand the embedded formulae. But it has graphics which are pretty cool, so as you change numbers, the shape you’re seeing changes too. I like shapes.
I’m looking forward to the ride report too. Unfortunately, they said 75kt winds for tomorrow or Friday. Guess I’ll wait a bit, you know, only because my wife says I should.
