Experimental prototype Here is the motor boat used for the tests with leeboards added. The rig is composed of three pieces : - A kite–surf sail. The maximum size of 22 m2 is to be used for small boats. This one is able to launch and drive a second sail of large area. We have tested more than ten different sails and selected the best. - A sail-board mast. The lower part of the mast is fixed on the roof. The upper part is used only during the launching. - The reels on the same axis are mounted on the lower part of the mast to store the lines between the sails and the boat with a handle to control the launch. We have built and tested a dozen of different reels. An industrial prototype of the selected solution after tests is shown on the figure. The launching Here above is the 22 sq.meters kite-surf sail after inflation. With a wind speed of 10 knots inflation takes 2 minutes. After inflation, without any intervention and whatever is the wind the sail is stable. In these conditions it is possible to run at low speed. Then the reels control the raising of the sail. without any trouble. During the launch the brake lines of the sail are operational. Sail drive The sails are driven like classical kite-surf sails with the brake lines. Photos during the tests organized for Thomas Coville. - launching. - in operation. - come back of the sail. Overturning torque With a kite sail the overturning torque is 2 or 3 times lower than the traditional sails. It is possible to suppress the keeling as shown on the figure. But this disposition is reserved only in case of research of the maximum speed.