I'm with Roger on this one. I weigh 80 Kg and would be on a much smaller fin then 36 cm in 20-25 knts. For that matter, I would also be on a smaller board and sail. Depending on water state 4.5-5.0 with 85 liter wave board in rough water or 5.0-5.5 with 93 liter Futura in flat water. In any case the biggest fin I would be using is 30 cm.