Originally Posted by Jean-Marc
Your last result is not surprising to me at all. I bet you tried a modern Select 32 cm fin and this explains why you have no more spinout issues with a modern fin on a modern board. So, I would dump the 20 years old Concrete Wave 28 cm fin and get a new 28 cm modern design ASAP. Your antique fin is simply no match anymore with your modern board + sail combo.
I think so too, as with the old one there was absolutely no way to fix spinout. The only solution was either to bare off basically onto a run (difficult to do overpowered) or to sheet out, lose speed and then accelerate again.
I'm not sure how old the fin really is as I can't find any information about it online. I guess that in itself is quite telling of it's ancientness...