I managed to hit a rock with my fin while windsurfing at the Tsawassen Ferry Terminal a couple of years ago. I opted to repair the fin with some carbon fiber and epoxy.
As you can see from the picture below, I broke off the tip to the fin. Since the fin is tapered, I first lay down a layer of play-doh to support the underside of my repair:
I put several layers of carbon fiber on the tip, overlapping the existing material which I had faired down a bit.
Next up was to cut a rough shape for the tip and then do a bit of sanding:
I added a couple of layers of regular fiberglass over top of the CF because I figured it would be easier to work with for finishing.
I used the fin sleeve as guide on what the shape the fin tip should be:
Next up was the final shaping of the outline and profile, along with some more sanding. I think I got the shape right and the finish is pretty good. I’ve used the fin a lot since doing this repair and I don’t have any problems with spin-out. Does it restrict my top speed? I don’t think so. I’d be more concerned about that if this was for a slalom board that depends more heavily on fin design.