The running track is of no relevance in windsurfing. But for understanding the fin function on downwind, across the wind and upwind courses it is helpful to understand the flow details on this course first.

Furthermore we have not mentioned yet that the fin is of finite length and that there is additional flow around the fin tip. For the moment we pay no attention to this fact and imagine that the fin is of infinite length. The flow around such a theoretical body is called two-dimensional as the water particles do not leave their layer of movement.

Further down in the chapter on induced resistance, we will take into consideration the flow around the fin tip. We will then find out that the real flow is a three-dimensional one.