A tunnel fin question: Is the tunnel a venturi ?

 " In looking at your fin configuration I would assume that a primary function of the "tunnel" is to create speed by funneling the water, compressing and then releasing it. The right angled portion of the fin would appear to be an answer to the tunnel's tendency to force the board to track while it generated speed - so the "wag" of the right angled portion of the fin would be to create a snap out of the turn. On it's own this would create drive coming out of the turn but, with the presence of the speed-and-track-inducing tunnel it helps overcome the tracking tendency and returns the board to an easier to turn and smoother turning board than if the right angled fin existed alone. "

Thank you

Hello J,

Actually the tunnel is designed to work without compression of the water or to keep compression to a minimum.   If you are thinking of the 'venturi' effect it's of no use in surfboard and fin design as compressing the water merely creates drag with no benefits. Constricting water flow  doesn't speed up the  flow rearwards creating thrust as venturi  theorists suppose, instead water  is dragged along with the surfboard. In other words with a constricted flow the water accelerates in the opposite direction to that which is supposed.

Essentially the tunnel fin is a low drag fin which provides lift in the vertical plane as normal fins do as well as in the horizontal plane as winged fins and other 'hydrofoil' fins ( including canted side fins ) do.

I'll be happy to explain how they work in detail.

Regarding the 'tracking' idea tunnels are the 'loosest' fins with the least tendency to track which I've ever ridden. The boomerang fin is not there to prevent tracking, it's there to provide more lateral fin area. Tunnels work well in larger sizes but at the size shown above  ( 8 inch diameter) they need some added fin area in order to get sufficient lateral resistance ( unless one wants to do a controlled 'lala' sideslip ).  At ten inches in diameter I find that a tunnel is fine on its own.