Before several of the Alter Cups I worked with, I made a point to get the acceptable recommended wind range to safely sail the particular boat from the manufacturer. These guidelines were then given to the PRO. Concrete data makes decisions easy.

I've always heard that the A-cat class limit is 22 but I don't know that I've seen that documented anywhere. I do know that I would agree with that being the upper limit on an a-cat. It just gets silly after that.

Regarding the F18 and Nacra 20, though, I think 24 or 25 is still "raceable"....but sea state will play into how "raceable" the course is at those levels. My rule of thumb is that if you can reasonably control the risk of capsize and it's still a matter of your skill level and choosing how hard to push for speed, it's raceable. For instance, at 30, you can no longer control the risk of capsize on a 20 and it's only a matter of "when"...at 24-25, you can still usually avoid capsize unless you cross that line and push a little too hard.