You make a jump or you surf over choppy water. If you come in the water ,two things. Leewards pressure,
On the other side from the coming in the water more vavuum. That takes a mixture of air and water from
the boardbottum to the end of the fin, and goes down to the tip,make bubbles and sometimes spin out.
Have you a cut out at the base of the fin,there goes the water and air mixture in an angle of 30-40° behind the fin from leeward,the pressureside, to the other side aned take all it away from the fin, so it cannot
come something to the tip.If you saw what in Netherland happend ,sorry I stopped in the half, so I cannot say what it was, but it is to easy to say it was the Windsurfer. I hope that some of you try to make such a cut out at the base,take a old fin and try it.It is not so that I want to work for you here fins, I want only to show some People what we can do against spin out. Please look here:
There can you see the cut out. And how the diagonal concaves are. Wolfgang