PURPOSE: To prevent a flow of wind by a mast from separating at a sail surface, by setting a mast section down to a swelling triangle consisting of a large curvature small arced part at a front end part and a small curvature arced part connecting a symmetrical pair of large curvature small arced part to each small arced part.

CONSTITUTION: A profile form of the section of a mast 10 for a sail board or a yacht is set down to a swelling triangle consisting of a large curvature small arced part 11 at a front end part and small curvature arced parts 13 and 14 connecting a symmetrical pair of large curvature small arced parts 12 at the rear to each small arced part. A fore-and-aft sail body 20 is attached to the mast 10, while both front and rear panels 21 and 22 and a sleeve 23 are tightly stretched whereby the fore-and-aft sail is set up. A fore-and-aft sail front edge 20a comes to a pointed form, and a first half part 13d and a latter half part 23e also come to being formed along a streamline, whereby any separation due to generation of a swirl in a flow of wind at the mast is effectively reduced. And, resistance by the wind is made smaller, thus propulsion efficiency is improvable.