Abstract: A boom for a sailboat is controllably flexible to assume a predetermined specific airfoil-derived contour and to impose an aerodynamically efficient shape on the large lower portion of the sail. The boom is formed of five segments connected by four flex joints. The lengths of the segments and the hinge angles assumed by the flex joints are determined by piecewise approximation of a shape producing a pressure distribution corresponding to the pressure distribution of specific proven airfoil contours, for instance the Joukovsky airfoil profile. Flexure of the boom is achieved by tensioning cables extending along the sides of the airfoil boom. Controlled boom flexure can also be achieved by continuously varying the cross-sectional moment of inertia of the boom to match the radius of curvature of the boom with a specific airfoil profile or by inserting a controllably bendable flexible plate in a pocket along the sail foot.