Move the fin forward for smaller waves or if you want the board to make tighter turns and feel looser. The downside is less hold and more chance for spinouts.
Move the fin back for bigger waves, more hold, better nose-riding, and wider, drawn out turns.
To find the best position, start with the fin somewhere near the middle of the fin box.
If it feels too stiff and hard to turn, move it forward by 1cm and try again. Repeat until you find the optimum feel.
If the board feels too loose or spins out too easy, move it towards the tail using the same method as above until you find the best position.
A single fin setup is better for noseriding and classic longboard surfing. Adding side fins encourages the board to stay on edge for frequent turns and more performance type longboarding.
See our selection of longboard fins here: LONGBOARD FINS