Tunnel fin/annular wing talk

Hey Roy, It's Rich.

I'm a fan of fin lift too. I always semifoil my twinners and like a lot of angle, with the idea to keep the tail snug into the wave at speed. I get the rail-to-rail advantage, so now I'm wondering about that bottom section of a tunnel fin. If that surface is foiled to produce lift, does that not tend to lift the tail out of the water? And what of the cost of that lift in terms of drag?

Like I said, I am just curious...

Hi Rich,

Thanks for the questions

Regarding the middle section of the tunnel, it will produce upwards lift no matter how it's foiled, We've had good results with double foiled sections and also with single foiled ( nearly flat on the inside or 'up' side) as well. Because the tunnel is an enclosed system, pressure is more evenly distributed inside the tunnel than it would be with a planar non enclosed wing. For that reason it's easiest ( in my opinion ) to understand and visualise the lift via the Newtonian method, i.e. redirection of water flow and mass, rather than Bernoulli's system of pressure differences, although both systems ultimately give the same answers.

Whether or not it lifts the tail out of the water depends upon the angle the fin is set up at.

For example If the tunnel fin is set up parallel to the bottom, then it's running at the same angle of attack as the bottom in the fin area. So, one might expect the fin not to lift the bottom upwards. In fact it does lift the bottom though, because of the difference in the lift characteristics of the planing surface of the bottom and the tunnel. The tunnel produces more lift at lower angles of attack than the planing surface in the tail and thus lifts the board in the tail area even when running parallel to the bottom.

Adding a positive angle of lift between the tunnel and the surfboard planing surface in the fin area increases the force lifting the board.

One might expect that the tunnel, acting as an underwater lifting foil, will just keep on lifting the tail upwards if it generates enough lift to do so, in other words jacking the tail out of the water. What happens in reality is that as the tail lifts slightly, the fore and aft pitch angle of the surfboard changes. . . the surfboard pitches forward. Because the tunnel is fixed to the bottom of the board this automatically lowers the angle of attack of the tunnel. Thus a state of equilibrium is reached where the tail is riding slightly higher than it would be without the tunnel, and the tunnel is taking some of the load. The system ( if adjusted correctly ) becomes self tending.

This self tending aspect doesn't completely determine the angle of attack of the surfboard because rider mass can be redistributed to alter the trim of the board. Thus the rider can trim the board nose up and the tunnel and planing surfaces working together will lift the board out of the water, in much the same way as a surfboard usually behaves when trimmed nose up, with some added lift from the tunnel. When trimmed nose up the efffect is not to lift the tail up more than at low angles of trim, what happens is that the whole board is projected forwards and upwards in the direction in which it is pointing. . .. this is because the tunnel and the surfboard tail have a fixed relationship to each other.

The self tending aspect is greater on longer and/or heavier boards, but is still there on lighter boards.

As far as lift vs drag goes, the tunnel has a better lift/drag ratio than the surfboard planing surface because it is effectively a wing of infinite span. Tunnels also have low induced drag. So, whenever the tunnel does part of the job of producing the necessary lift which is usually done by the planing surface of the board, there is a drag advantage to be had.

According to some tunnel analyses tunnels are better at low speeds because they have high surface area, ( skin friction drag due to surafce area predominates at high speed and induced drag at low speed) Such analyses make a couple of misleading assumptions, one is mileading because it doesn't apply to the surfboard application of tunnels and is based on the areonautical assumption that the wing must provide all the necessary lift ( not the case with surfboard fins ), the other is an anomaly in the way annular wing area is measured when comparing lift/drag characteristics with planar wings..

Testing identical boards with and without tunnels it's immediately apparent that the tunnel equipped boards are faster as well as haveing more drive and hold . I'm not claiming that tunnel finned boards will always be the fastest in all situations, as there are other ways of achieving low drag, but they certainly work and are a force to be reckoned with.