Abstract: The front body (2) has holes (3, 4) for receiving the ends of the wishbone boom, and an arched surface (7) intended to be applied against the mast. An arched clamping piece (9) is provided at each end with a metal rod (10, 11), each of which is intended to pass through a hole in the front body (2). The free end of these rods is threaded (13, 16) and projects beyond the body (2). The rod (10) is straight and the rod (11) is bent. A nut (17) is intended to cooperate with the thread (16) on the bent rod so as to ensure that the mast is held, with play, between the two arched parts (7, 9), while another nut (14), cooperating with the thread (13) on the rectilinear rod, enables the windsurfer to lock the device on the mast after having slid it to the height which suits him. This device can be fitted to the mast, with play, before the sailboard is put in the water and enables the height on the mast to be adjusted while sailing.