PURPOSE: To improve the lift and efficiency by forming a circular cavity at each position corresponding to two vertexes of an isosceles triangle-shape cross section, and drawing a sail capable of being wound in each cavity through a slot formed in one side of each cavity.

CONSTITUTION: This mast Al is formed of a tubular main body having an isosceles triangle-shape cross section having a base part 4 curved outside and a round vertex 5 facing to the base part 4, and each circular cavity 6, 7 is formed corresponding to the other two vertexes of the triangle. Radius of each cavity 6, 7 is set so as to be smooth in relation to two sides 8, 9 of the triangle, and has a slot extended along the whole length of each mast. Inside of each cavity 6, 7 is provided with rotary support core 10, 11 for winding up sails 12, 13, and the sails 12, 13 drawn from the cores 10, 11 are lockable with two rails 16, 17 provided on a boom 15.