A steerable wing-type sail system for a wind powered craft. The system includes first and second secondary airfoils and that are spaced outwardly to the sides of the plane of the main wing, and that are positioned rearwardly of the trailing edge of the wing. The secondary airfoils are selectively pivotable so as to steer the main wing in one direction or the other. The main wing is also provided with a pivoting flap at its trailing edge, which pivots simultaneously with and in the same direction as the secondary airfoils. The secondary airfoils are carried on elongate horizontal booms mounted near the mid-span height of the main wing. The secondary airfoils pivot about vertical axes at the distal ends of the booms, and are operated by control cables that are retracted and paid out by linear actuators or similar mechanisms. The craft may be multi-hull vessel, such as a catamaran. The control mechanism for the secondary airfoils may operate the airfoils in response to signals received from one or more onboard sensors. The system is suitable for use on an autonomous unmanned surface vessel (AUSV).