Questions about Tunnel fin annular wing surfboard fin

. .. . a conversation with 'SB'on the Tree to Sea surfboard design forum.Roy, Can you explain a bit about how a tunnel fin performs and why? I'm not sure what to expect from it or what one would hope to achieve by using it. Also what is gained by combining the tunnel fin with the cut away Spitfire fin?It's an interesting subject, and there's quite a lot to it. The tunnel plus one setup can best be understood by trying a tunnel plus one equipped board without the tunnel and then with the tunnel. The tunnel plus one has more drive, hold, lift and speed than the single.So, I suggest that you install your tunnel after riding the new board with the singlefin alone.As far as the theory and practice of tunnels go, I'll get into that below :)If the idea is to provide lift in the tail then I would suppose the idea would be to foil the inside (being the top) and leave the outside (being the bottom) flat. The tunnel fins do provide lift, but the notion that this ideally requires a foil curve on the 'top' of the wing but not underneath is not correct. It's best to treat the lift generating properties of the wing using the Newtonian lift concept, which is that the wing generates lift by redirecting flow ( and thus water mass ). This is conceptually more useful than the Bernoulli principle ( although ultimately both theories produce the same answers )Now, because wing sections produce lift with 'inverted' foil sections and symmetrical sections ( and even more so with enclosed systems i.e tunnels ), it isn't the case that the foil needs to be flat on the inside and foiled on the outside in order to produce lift. In fact a tunnel which is flat on the outside is not ideal as it stalls too easily under negative angles of attack ( This occurs as the tunnel receives flow from one side of the board or the other: one side is always at a negative angle of attack )So the short answer is that both kinds of tunnels ( those foiled on both sides, and those foiled on the outside only ) produce lift.. . and either will work well. In practice it is almost impossible to foil a tunnel so that it has no foil on the inside, as this would entail a sharp 90 degree edge at the leading edge. Any rounding off of this edge introduces a foil to the inside of the tunnel by moving the apex of the leading edge towards the outside of the tunnel.The foiling isn't critical at all, as long as it's fair. A double sided foil works very well, and produces a bit more vertical lift than a single sided foil .That would indicate that either a) the intent is not to create lift after all or b) lift is created by some force other than foiling.As mentioned above, the best ( in my opinion ) way to visualise the tunnel lift is via the Newtonian treatment of lift, via flow redirection and displacement of mass . If you want to see it in terms of Bernoulli's theory that works too, but one has to realise that the pressure differentials which produce lift exist even with flat non foiled wings and inverted wings due to the angle of attack of the wing. . . . thus the classic example of the flow taking a longer path over the wing due to the curve on top of the wing is misleading as once the flat topped wing has a positive angle of attack the flow on top of the wing is forced into a longer path and lower pressure situation anyway.By the way it's good to keep in mind that the tunnel will respond (by producing lift) to angles of attack from the side as well as from below. . . in other words it produces lift horizontally and vertically and every angle in between. It doesn't just make lift upwards like a 'hydrofoil', even though that is one of its main functions.Just looking at the vertical lift component, the tunnel produces lift in the same way as the botto of the surfboard. It does so more efficiently than the surfboard bottom though because it is effectively a wing of infinite span, unlike the surfboard bottom.I've been studying the photo of the squidfish above as well as photos of the Makaha on your website and I do seem to detect a foil pattern on those tunnels. It seems to be a standard foil pattern (thicker along the leading edge and tapering down to a thinner trailing edge) and appears to be equal inside and out. Yes that's right.