Mark has made a couple of interesting comments about tip stall in connection with forward sweep. I'm sure that windsurfers have experienced the problem he described and I have been trying to understand what the cause might be.

I've been surfing with vertical fins on my surfboards since 1969 (only exception is if I surf in kelp). By vertical I mean the leading edge is 90deg. to the bottom. Usually the trailing edge angles or arcs forward making the tip narrower than the base. I have tried fins with the leading and trailing edge parallel (rectangular plan shape). I have even tried fins with forward sweep (yes, the tip leads the base!) I got on to that idea by reading about a US fighter jet that was built with forward sweep to the wings. It was done for faster response in dogfights. The article also stated that forward sweep required very stiff wings or they would twist adversely.

My experience with forward sweeping fins is that they are unstable, same as conventionally swept fins and for the same reason I describe in my book on surfboard design. This is a different problem than tip stall. My guess is that if the windsurfer has a problem with tip stall, caused by the tip leading, it's actually a problem of the tip flexing and possibly twisting, thereby increasing the angle of attack at the tip. The tip might be stalling or cavitating. A stiffer fin should eliminate the problem.

With the tip trailing the base (rear sweep) fin flex will decrease the angle of attack at the tip. This will also decrease the lift a bit but it will be gradual.

Bob