When dealing with downwind sails, streamlines visualization can be extremely useful in order to detect whether (and where) the air stream smoothly follows the sail surfaces or detaches to generate huge vortices and turbulence. Even if wakes behind downwind sails can be usually accepted (and often cannot be avoided), one of the main goals in order to increase the driving force is to keep the flow stream attached to the sail surface, and this can be obtained by means of a careful sail design and a suitable trimming of the sail sheets as function of the wind conditions (speed, direction, …).

(Image courtesy of CMCS (Chair of Modeling and Scientific Computing-EPFL-Lausanne))