PURPOSE: To enable a boat to slide stably with stability increased by forming a floating space provided with great buoyancy by means of the bulwark at the peripheral section of a hull composed of hull members and deck members, and thereby forcing a hull center line to be positioned above the draft surface when the boat rolls sideways.

CONSTITUTION: The bulwark 8 is formed in such a way that a floating space 80 composed of a hermetic space is enclosed inside by a rising section 31 and a turned-down section from the upper end section of the rising section. When a boat is subjected to great tilting moment, the bulwark 8 at each of both hull side sections is provided with the floating space 80 with great buoyancy, stability is thereby great so that excellent stability is exhibited for the various kinds of motion at the time of sliding. When the boat is tilted at 90° or at an angle less than 90°, ventilation pipes 4a and 4b are positioned above draft surfaces 91 and 92. When the boat is tilted at an angle equal to or more than 90°, the upper end sections 40 and 42 of the ventilation pipes 4a and 4b are positioned below draft surfaces 93 and 94 respectively, however, the lower end sections of the ventilation pipe are positioned above the draft surfaces 93 and 94, which situation thereby prevents water from coming into an engine room 12.