PURPOSE: To prevent a combined motion of bouncing and pitching by making an opened section over upper and lower water lines on the rear hull of a strut of a half-merged catamaran and positioning the center of floatation of the strut just above the center of buoyancy of a lower hull and the strut.

CONSTITUTION: A half-submerged catamaran is composed of symmetrical left and right submerged lower hulls 1, struts 2 which are raised on them, and a cross deck 3 which connect the upper ends of each strut 2 over a water level 6. Opened sections 7 are made in the rear hull of the struts 2 over the upper and lower water lines 6 so as to position the center of floatation F of the struts 2 nearly above the center of buoyancy B of the lower hull 1 and the strut 2. The positioning of the center of floatation F above the buoyancy B prevents the combination of bouncing and pitching, thereby improving the moving performance of the hull.