PURPOSE: To facilitate the rising of a tilted sail by forming a boom into an arch while stopping rotatably to a mast and forming a slit for passing the boom in the sail.

CONSTITUTION: An arched boom 3 is projected to any one direction while the sails 4, 5 are inflated in the reverse direction to perform the sailing, while for the wind of the reverse direction the boom 3 is rotated and passed through a slit between the sails 4, 5 and projected reversely. Here a projection 71 of a bush 71 will contact against a stopper 8 to hold the boom horizontally. When tilting the sail the water is removed from the slit between both sails 4, 5, thereby it can be rised easily by means of a rope 29. Since the boom 3 is locked to the opening of a tubular section of a stopper 6 fixed to a mast while utilizing the force for stretching the sail, the structure of the stopper is simplified.