No dragging leashes, smaller board with flatter rocker, a smooth bottom surface and a single fin with a full fillet around the base?
"Interference drag
When the air flows around two objects connected together, such as the fuselage and the wing, there is always more drag than would occur around the two objects separately. This is because the boundary layers from each object interfere, causing more drag than would otherwise happen.
This situation is particularly bad when the objects meet at an angle of less than 90 degrees.
The ideal solution is to reduce the number of junctions, and keep those, which are required, at angles greater than 90 degrees.
If that is not possible a fillet between the two objects… will help reduce the interference drag."
The guys who ride glass-ons for the performance they feel are right, due to exactly what you said, Bruce.
If anyone wants to temporarily try it yourself on inserted fins, demould wax around your fin and do a temporary microballoon/cabosil fillet/radius using a half inch dowel or similar. No need to glass, just make it neat and crack it off when you want.
Another big no-no on this subject is allowing any sort of flow between the surfaces, ie. where wings attach to fuselage, where fins attach to the board, etc. The performance lost in this area is in double figure percentages, so in a surfboards case it would be noticable to most surfers.
I think I have suggested before, that the fin system companies should start working on a radius insert that would solve this. It’s actually fairly simple and could be moulded out of silicone and fitted to all current removable fins.
There are still a lot of steps forward that have not been taken yet.
i think this is very true and an awesome topic. Having just switched from removable fin systems in my old board to glass ons in my new board has made a world of difference. Both boards are similarly shaped and are within 1 inch of each other. (5’10, 5’9). The 5’9 is the one with the glass ons. This board has alot more glide, thrust, and builds much more natural speed. After riding this board and one other with glass ons, I have realized that the many removable fins are definitely not the answer when it comes to performance.
