Twin singles or something

[Surfdad]

This is a longboard with 3 fin boxes installed, the normal one in the middle and then two on the outside. They've installed the two single fins in the outside boxes and the folks involved claim significant speed increases.

Could that be? Anyone want to have a stab at why that would be? I just keep seeing twice as much drag.

[NICKYPOO]

Something to do the increased pressure between the two fins? Since water cannot be compressed, the increased pressure (energy) needs to go somewhere. That somewhere is straight out the back of this "tunnel", resulting in "thrust".

...or, the increased pressure directly pushes against the inside of the fin, creating "drive".

I'm just throwing these out there. Maybe its neither, maybe its one, the other or both.

[Surfdad]

Yeah I don't know either, I wonder if it would work on rail fins? Those singles are double foiled, so in between them would look like an hourglass. That would lend credence to your theory. Water going between the fins would sort of get "squished".

I wonder if that would work on the rail fins? Those aren't typically double foiled. Huh, it sure begs to be tested!

[NICKYPOO]

If only there were someone around who could build a board for such a test.

[Surfdad]

LOL yes, if only! I am sure I have a test bed that I can stick some extra fins in. I wish I understood what was happening more. Well, worth a go! HEY! It would be a quad! I wonder what those folks that call James board a skimboard will say if it works, too many fins?

Just seems to me like it will create twice as much drag.

[Ewok]

Something to do the increased pressure between the two fins? Since water cannot be compressed, the increased pressure (energy) needs to go somewhere. That somewhere is straight out the back of this "tunnel", resulting in "thrust".

...or, the increased pressure directly pushes against the inside of the fin, creating "drive".

I'm just throwing these out there. Maybe its neither, maybe its one, the other or both.

I think your pretty close on your first guess, I would guess too that the increase in pressure increases the velocity of the water coming out of the fins and, as a minimum, reduces the drag of the fins to something less than a single which feels faster than a single fin.

[jbort]

Do the fins resemble airfoil wing shape thus generating "lift"? Just a whacky theory.

[Surfdad]

@ Ewok - that makes sense, I wonder if rail fins might do the same thing. It wold probably take two different foiled fins to achieve that though, as most of the high end rail fins are vectors and have a cupped inside foil.

@ jbort. Sort of. On the fins in that picture, they are double foiled, meaning either side is like the TOP of an airplane wing. So in between the fins, there are two convex surfaces, and like ewok and nickypoo point out that restriction, creates an increase in pressure and the speed of the water through the fins increases. At least that's the theory we are thinking