Wolfgang Lessacher Fin Design Technology

Fins always travel with the leeward side slanted forward. There's pressure on the leeward side and low pressure on the windward side behind the fin. Occasionally the low pressure increases and moves enough to cause turbulence and the fin spins out.

A concave surface on the leeward side keeps the flow relatively straight and keeps the high pressure from moving. This provides the best possible circumstances for one direction. Concaves on directly opposite sides of a fin do not work, but when transferred diagonally from side to side, everything is fine. The concave reduces the low pressure and as a result the fin causes less drag.

To compensate for a difference in how the foil works close to the surface vs. deeper, the concave surfaces are of slightly different size. This makes the fin work equally on both tacks. The concave near the surface is about 10% larger than the concave near the tip of the fin.

When you land small jumps in chop or waves, air is drawn under the board and onto the fin. The cut out at the base of the fin prevents the air pocket from attaching to the fin and allows it to be ejected behind the board.