Refers to the shape and geometry of the inside and outside faces of the fin. Foils directly affect the flow of water over the surface of the fin. Different foils create variations in water flow and have a direct link to the overall performance of the fin and the board. (See diagrams below)
FLAT: A flat inside face combined with a convex outside face. The traditional flat sided foil offers an even combination of drive, pivot and hold and provides a very consistent, reliable feel over a wide variety of conditions.
INSIDE: A sophisticated hydrodynamic foil consisting of a convex outside face, a rounded leading edge and a concaved inside face. Inside foil increases the efficiency of water flow over the surface of the fin adding lift and reducing drag. The result is a fin with more options through increased hold and speed.
50/50: A symmetrical foil used on all centre fins where both sides are convex. Even water flow on both sides creates stability and control.