Abstract: A sail envelope has an inner open area between two spaced sail surfaces with spaced battens supported in line from the head of the sail envelope to the foot, between the mast in the open area at the front of the envelope and through sealed openings at the leech of the sail envelope. A batten support strip supports, spaces, and guides the center portion of the battens and a batten socket support strip supports and spaces the batten sockets along the mast. Each batten socket permits pivoting of its cooperating batten. Constraining devices place a compression force on each batten externally of the leech to place a curvature in each batten which is transmitted to one side of the sail envelope with the other side being drawn straighter. The boom attachment opening in the sail envelope is sealed and a drain is located in the foot of the sail envelope. Friction reducing material is used between the mast, sail envelope, and batten sockets where desired.