Flow field around the sail coloured with relative pressure. Inset in the lower left corner is about 1 m above the deck: Note how strong the suction on the leeward side of the mast is (the purple area). This gives the mast plenty of positive drive in the lower part. Close to the top of the sail (inset upper right), the low pressure area is shifted on the backside of the mast, creating drag insteadof drive. - Finn sail development cycle