Currently ready to foil out a 5.5" rear fin for an 8’6 semi with a 2+1 fin set-up.

Any feedback on keeping the foil a bit thicker vs. the thinner foil found commonly on the Future or FCS fin sets…?

Currently ready to foil out a 5.5" rear fin for an 8’6 semi with a 2+1 fin set-up.

Any feedback on keeping the foil a bit thicker vs. the thinner foil found commonly on the Future or FCS fin sets…?

Bill Thrailkill has a few ideas about this.

Quote:

Bill Thrailkill has a few ideas about this.

Bill has mentioned a preference for full foiled fins.

Keep in mind that when he was riding big waves the rockers were nothing close to today’s rockers. They were much flatter. I’ve ridden reasonably large waves on boards with flatter rockers, and my hat is off to all those guys who went so large with so little rocker. It is hard enough with a banana rocker gun.

My preference is thin fins. Full depth, short in chord, and close to standard thickness and rake. And 3.5 inches of rocker on a 9"6" or longer gun. 3 or 4 fins, but gimme at least one on each rail, I’m gonna need it.

I’ve been wondering about that too.

I think that in the air, lift for a foil goes up at about the square of the speed, that is, double the speed and you get 4 times the lift, whereas in water, it goes up at about the cube of the speed: double the speed and get 8 times the lift (any tech guys know a better rule of thumb for an approximation?).

This means that for a big wave if the speed is greater, you can get away with a smaller fin and/or foil.

What is really disastrous is when laminar flow breaks down and suddenly the fin has virtually no lift at all (spinout). It seems to me that could happen more readily on a thicker foil than a thinner foil.

Quote:

I’ve been wondering about that too.I think that in the air, lift for a foil goes up at about the square of the speed, that is, double the speed and you get 4 times the lift, whereas in water, it goes up at about the cube of the speed: double the speed and get 8 times the lift (any tech guys know a better rule of thumb for an approximation?).

This means that for a big wave if the speed is greater, you can get away with a smaller fin and/or foil.

What is really disastrous is when laminar flow breaks down and suddenly the fin has virtually no lift at all (spinout). It seems to me that could happen more readily on a thicker foil than a thinner foil.

AFAIK, thicker foils provide laminar flow at higher angles of attack than thinner foils. The fluid has a better chance of bending around, without releasing, a more arcing curve than a less arcing curve.

Also, in bigger waves there is more water moving faster up the wave, and that changes the relative flow more perpendicular to the fins (higher angle of attack).

So, in theory (at least currently in my mind), thicker fins (to a point of course) would be more efficient in bigger waves.

P.S. I could be very wrong… hydrodynamics/aerodynamics/flow-theory are often very unintuitive.

how big of a difference are we talking about in thickness ? There is a reason almost all fins you see in the shop are with in a few mm’s of each other, with the notable exception of nose rider fins.

12% thickness wrt chord length

vs

5-6% thickness

ie: in a 7 inch fin, 12% is over 0.8 inches thick.

Compared to 0.35-0.4 inches thick in a standard

7 inch fin.