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SUP fins help steer and provide stability to your stand up paddle board when moving at speed. They help somewhat with tracking in a straight direction while flat-water standup paddling, but the real advantage is they allow you to stand up paddle surf without spinning out when you turn. While a lot of the pros and cons of SUP fin configuration is up for debate, it's pretty well agreed to that single long stand up paddle racing fins (8-10 inches) are best for flat water cruising, and a 2 + 1 or three fin set-up (which is a 6-8 inch center fin flanked by two smaller side fins) is typically best for most stand up paddle surfing. The best stand up paddle boards have a removable fin configuration that allows the flexibility to install paddle board fins specific to your use, which can change from day to day.