I’ve decided to attempt to make some fins from some shop scraps for a couple of boards in the works and am now questioning the thickness of what I have right now and am looking for advise from the experts here. I’ve searched through the archives and read many posts on fins and foiling and see that the advise can be all over the board. I am making a retro fish and a MiniSimmons-ish board. One set of fins is 7-1/4" x 5-1/4" (185mm x 133mm) and the other is 9-1/4" x 5-1/8" (235mm x 130mm). The main issue I have is the thickness and I’m really questioning it right now. Both sets are 7/16" (11mm) thick. After glassing them on they’ll be 1/2" thick. I don’t have a lot of experience with Keel fins but have researched the Naca foils and what others have done but these just feel too thick to me. The numbers tell me I’ll end up with about a 7% foil but is that too thick for a single sided fin or is that more for a double foil? Most of my longboard fins are about the same thickness but they are of course doublefoils. At this point if I need to thin them down it wouldn’t be a big deal.
Also, I’ve put the widest point of the cord length at about the 30% mark. I still have some shaping to do on the foiling but does it look like I’m on the right track as far as shaping goes?
My best performance results, (speed, stall resistance, turning) have been achieved with single foiled fins in the 6% to 7% range, AND double foiled fins, in the 9% to12% range. For best performance, the leading edge MUST have a blunt rounded shape. The foregoing is based on experience, not theory. My fastest big wave gun ever, had a classic keel fin 6.25 inches deep, with a 10.25 inch long base, and a base thickness of one inch. Highest camber at 30%, to 40% max.
Thanks McDing! These are straight off the sander and I have a bit of handwork to make them right and get rid of a small flat spot but think they are close. Was looking for a little guidance to see if I was on the right track before I went too far. I did a set of two for each style but only took pictures of one of each
It’s interesting that most production fins are sharp on the leading edge and all my fins are thinner. I’m sure they may go faster when in a straight line but knowing what little I do about airplane wings and stall angles… particularly aerobatic planes with extreme AoA… it makes perfect sense to round off that leading edge. I’ll round them off like you and Bill say and can’t wait to try them out!
I’m no expert on the aoa, percent of foil etc…I usually just foil to what feels right. I also generally have a nice production fin of a similar shape to reference off of. So that is of no use to you. Those fins look right to me. I’m sure they would work. I think the cant, toe in and positioning on the board will all have equal effect on their effectiveness.
On a side note. Those look like zebra wood and maple? Did you lam veneer together? Also what are you glassing them in? Epoxy or poly? I have done tons of experiments with some of the more exotic hardwoods and gotta day they can be risky. The tannins in the woods can really do a number on the cure of the resin. Epoxy is generally better but even some woods will inhibit that.
Thanks Parsurf for the comments. Yes, I lam veneered a blank together to a thickness of 7/16" and then cut out the shape of the fins. I didn’t have a production fin to go off of but I did a bunch of searches for similar fins and took particular note of how they were foiled as well as do the numbers on NACA foils and did my best to make a true shape with no flat spots. Still a little hand tuning to do but they’re close I think.
They are Wenge and Maple. I agree that some of the exotics can play havoc with resins. I’ve experienced plenty of that in my years as a boatbuilder and that thought was in my head when I made these. Both Wenge and Maple are pretty benign as far as that goes without a lot of oils in either one of them and thought that the contrasting colors might look good. Oh, and I’ll be using epoxy on these.
