We have covered some of the basics in our first fin article, so now let's continue and talk about fin flex. Fin flex is the amount of movement of a fin from side so side. The stiffer the fin is, the less movement from side to side it will have, and thus the less Flex.
Stiffer / Lesser Flex
Fin foil is the curve of the fin from front to back and from tip to base. The foil and fin thickness determines how much and where on the fin that flex takes place when turning. Different foils give the fin its different performance characteristics.
The fin on a single-fin surfboard and the center fin on a tri-fin will have a foil on both sides. This ensures the fin reacts evenly when turning in both directions.
The outside fins on a tri-fin surfboard are a little different. They have a foil on the outside and will be flat on the inside. This flatness on the inside aids in a quick release when transferring from rail to rail.