Abstract: A device for increasing the motive force of a sail acting on a sail craft and at the same time decrease the risk of having the craft turned bottom up after a capsizing are obtained by fastening to the uppermost portion of the sail (3) a certain amount of flexible buoyancy foam (8) which is shaped such as to form together with the mast (1) and the sail (3) an efficient airfoil profile having a smooth contour. On account of its thickness, this airfoil profile will further counteract the short circuit flow of air from the pressure side to the suction side of the sail. The buoyancy foam (8) has at the same time a volume which is large enough to keep the sail (3) and the mast (1) floating at the water surface after a capsizing of the craft. In order to make the folding of the sail (3) easier prior to putting it into a sail bag, the buoyancy foam may be split up into a plurality of buoyancy zones separated by folding zones.