Hello Guys, I am a fin nut and have been working for some years on the hydrofoil principle, and on bulb fins.
I have built boards with bulb fins and tunnels and was surprised and excited to see others working along the same lines. I put bulbs on fins to reduce tip turbulence. Bulbs are able to reduce tip turbulence when it is most likely to happen: namely when the tip is moving laterally and has more water pressure on one side than the other. The fatter foil proportion of the bulb is less likely to cavitate, or less likely to cavitate catastrophically.
About hydrofoils. Horizontal fin surfaces are very difficult to control if they are placed far below the surface of the board. We had interesting results a few years back with large horizontal wings built onto singlefins, but there were severe handling problems due to
1) The depth of the wing. When the wing starts to lift it will keep on doing so until it reaches the surface of the water (assuming it is directed that way) As it does this it jacks the tail out of the water and control is lost.
2) The straight wing. A straight horizontal wing will twist the board unexpectedly in the tube because water flowing from one side of the wing to the other will suddenly press on the top or bottom surface, twisting the board.
We have solved these problems completely using a half pipe tunnel which is attached to the bottom of the board, not the fin (therefore controlling the distance it can lift) and which can't twist the board due to the circular shape. A nice spinoff of this fin is that it has no tip and therefore no tip turbulence (the old 'annular' wing principle). It works like a charm.It is very fast, has tremendous drive, amazing holding power and is absolutely viceless. So far we have done several hybrid versions with a single in front of the foil, and a "pure' tunnel. The tunnel offers good lateral resistance once it is moving fast, but can be sideslipped at low speed (fun). The hybrid version doesn't sideslip at all. I have some pictures and videos of the boards moving fast. Some pictures are on my site at <a href="http://www.woodensurfboardbuilder.com/" class="bb-url">www.woodensurfboardbuilder.com</a> There is a lot more information I can offer on how to set these things up. Roy 'bowser' Stewart
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