Made some using epoxy and some fibre glass with plasticine as the mould. They came out ok, bit patchy where the plasticine got mixed in with the resin though. (kind of matches the dodgy finish on the plywood board I made!)

Wondering if anyone has better idea for the mould?

Cheers, Warren

plaster of paris?

Hi Salty,
I've made quite a few using modelling clay which is a bit harder than plasticine. I push the original fin into the clay to make the mould with a covering film of vaseline making it easier to get the fin out. Then reapply a film of vaseline again to the fin to line the mould more (use sparigly to aviod spiking). Once set, extract and use a belt sander to level the base. Hardest part is drilling the screw threads stright. Very fast to make but not as strong.

made a few with a silicone mould, there are profesional 2 component silicone mixes available, at a cost though. So I used silasic, works ok but you got to apply thin layers to the original, which I coated with a thin film of wax (rubbed a candle on) put some fiberglas into the mould and filled with epoxy resin. push a stainless screw into the soft mix, this will come out ok when the resin sets, wrap a wetted out glas fibre around the screw for extra strength if you want, grind off the top, one home made fin work well,

Ray

The silastic mould that RayQ describes works ok. I tried using "all clear" hoping that it would be easy to see any air bubbles etc when putting the glass & resin in. The "all clear" took ages to set properly. I ended up with a much better mould using the cheapest silastic that Bunnings had.
Waxing the fin to be copied is easy with a mixture of beeswax and eucalyptus oil (used as a timber finish).
Although the moulded fins come out ok, I found the best way to make fins is to cut them from a sheet of polycarbonate (as suggested by Tobes).
Polycarbonate is sometimes sold under the brand name Lexan; just cut to shape with a jigsaw, taper with a belt sander, finish with some hand sanding, drill & tap the bolt holes and you have one bulletproof (literally) fin.