PURPOSE: To reduce the oscillation of a hull by forming an upper and a lower cushion chambers between both side walls in a boat through a movable partition plate airtightly and level-adjustably supported at its periphery so as to adjust the draft of a hull easily, thus preventing the cushion air leak from the lower ends of the side walls.

CONSTITUTION: A movable partition plate 1 airtightly and horizontally supported at its periphery is provided between the side walls of a hull, and the front and rear parts of the movable partition plate 1 are elevatingly suspended through winches 2 and lanyards 3 provided at an upper deck 01. Flexible bags 9, 10 are further mounted at the front and rear ends so as to form an upper stage cushion chamber 11 in a space between the upper deck 01 and the movable partition plate 1, as well as flexible seals 12, 13 are mounted at the front and rear end lower faces of the partition plate 1 so as to form a lower stage cushion chamber 14. In the on-cushion state, the level adjustment of the movable partition plate 1 is performed, as well as air for flotation is fed into the upper and lower cushion chambers through plural air holes 7 provided at the partition plate 1. This air spring function enables the movable partition plate 1 to follow the fluctuation of the sea level, thus preventing the transmission of oscillation to the hull.