The sail (3) has two surfaces (12,13) and passes around the mast (2). A wishbone, passing around the mast and sail, is secured to the mast. Each surface of the sail is fitted with a number of parallel battens (14,15) in conventional pockets extending from fittings (18) on the mast to the leaches (10, 11) of the sails (12, 13). The battens are fixed to the mast in special handed hinged fittings (18). Spacers (19,20,21) separate the sail surfaces. The line (22) stretches between the mast and the pulley (25), through which runs a line (26) whose ends are connected to the leaches (10,11) of the sail surfaces. The profile of the aerofoil may be adjusted by rotating the fittings (18) by tension on a line stretching from the wishbone to the fittings, so bending the battens. Further adjustment is by tensioning the line (22) to pull on the leaches of the sail.