PURPOSE: To enhance the longitudinal stability of a hull, by making the cross- sectional area of the center section below the water-line of a hull integrally fabricated with a foamed plastic material member, etc., smaller than the cross- sectional area below the water line of the bow and stern of the hull.

CONSTITUTION: The cross-sectional area of the center section below the water line of an integrally fabiricated hull which is formed by covering the surface of styrofoam with foamed urethane is made smaller than the cross-sectional areas below the water-line of the bow and stern sections of the hull. That is, in the hull shape which is defined by a chine 91 and a keel line 94, the hull bottom section between the hull longitudinal center section and the hull side is made in a shape given by lines 92, 93. Since the displacement of water in the bow or stern section of the hull is larger than that of the center section of the hull, the bouyancy of the bow or stern of the hull may be made greater than that of the center hull. Accordingly, the longitudinal stability of the hull is satisfied even in a small size ship.