A flexible wing-sail (4) for wind-propelled vehicles includes a mast (3) to be rotatably mounted in a vertical position on the vehicle, a plurality of flexible sail panels (26, 27, 28) carried by the mast, and a spreader assembly (20) secured to battens (21) in the sail panels for securing them to the mast and for imparting to them an airfoil shape having a leading edge fore of the mast, and a trailing edge aft of the mast. The spreader assembly includes a fixed spreader unit secured to the sail panels at the bottom of the wing-sail and fixing it to the bottom of the mast, and a plurality of slidable spreader units secured to the sail panels at longitudinally spaced locations and slidable along the mast to permit hoisting and reefing the wing-sail.