Shaper's Notes:

Flex is so crucial. Not just the amount of flex, but the quality of the flex. Where is the fin bending? Base-to-tip flex? Torsional flex? How quick does it rebound?

Small mushy day: Try fins with softer flex. Try front fins with some base and extra area. A more pivoty, triangular outline, a heavily cambered foil, and a back fin with a little less area and depth, and move it up if your system allows.

The surf is cranking: Run smaller fins up front, preferably with a little more rake, and stiffer flex. It would be good if they had a slight radius to the leading edge. Place a larger, more dominant fin farther back on the tail for control and holding power.

Lighter surfers in smaller weaker surf will usually benefit from a fin with softer flex. Softer fins will help the board feel looser. It will help to initiate turns but isn't necessarily a dynamic solution. Soft, as in "bendy" plastic, helps start a turn but there isn't a lot of return. You may find yourself double-pumping to try and keep your rail set or struggling to project where you want to go.

Another symptom of a fin that is too soft is that it will wash out on turns, especially on a cutback or coming off the top. Coming off the top you should be able to accelerate. A soft fin will slip a little and throw your timing off. A quality glass or composite fin may have a firm base but a "softer tip" and what you should be looking for is a crisp rebound on that flex. A fin that is soft, generally speaking, spills a lot of the power you put into a turn and slows reaction time out of the turn.

Conversely, larger surfers in more powerful surf will be happier with a more rigid fin. Too rigid is no good. A turn is a complex thing that takes everything working in harmony to maximize the return on effort. A good hull will have certain flex attributes and it should be married to a fin with complementary qualities. As you load the board in a turn, so too should the fins be storing energy. As you follow through on your arc and start to unweight, the power of the wave and your energy being returned should flow together. You should feel a launch out of the board and the fins should contribute to that launch. Read more about fins from Rusty.