PURPOSE: To expand and contract a boom according to the dimension of a sail when the sail is replaced by using each pipe member of a boom for two inside and outside telescopic pipes.

CONSTITUTION: A boom 5 is made by connecting both ends of two curved pipe members 8 with boom ends 8A. Each pipe member consists of telescopic pipes 9 and 10. In order to expand and contract the pipe members 8, for example, a board 16 or the like is inserted in the space of the booms 5 and the entire boom is twisted toward an arrow head. Besides, when an inside pipe 10 is simultaneously twisted toward the arrow head with both hands, the boom is deformed and the pin 14 of each outside pipe 9 is removed through the notch 13 of each inside pipe 10. As a result, thepipe member 8 can be expanded or contracted by relatively moving the inside pipe 10 against the outside pipe 9 in the longitudinal direction.