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 .