Windsurfing Fins, check out our huge selection of Black Project, MFC, RRD, True Ames, Select Fins, in Slot Box options in Thruster, Tri Fin and Quad Fin sets with US and power box base options. In-line and closeouts, save big! Windsurfing fins. Windsurfing fins Matching fin design and size to your board, sail size and sailing conditions will optimize windsurfing performance. A fin generates lift as it travels through the water, while your sail does the same when harnessing the wind. Balancing these lifting forces affect control and performance of your board. Too large of a fin can cause excessive lift and drag. Too small of a fin contributes to poor planing and cavitation (spin-out). Cavitation feels like your fin is no longer attached as your board slides sideways at high speed. This page features Fins with Slot Box type bases.