A composite iso-stress sail structure comprises a sail body having an expected iso-stress line, when in a chosen sail shape and under a loading within a chosen range of loadings. An iso-stress element is laminated to the sail body material to create an iso-stress portion at a corner of the sail body. An edge of iso-stress portion is shaped to be at least generally parallel to the iso-stress line. The iso-stress portion extends from the corner along at least one of the sides of the sail distances greater than 20% of the lengths of the sides, respectively.