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.
