PURPOSE: To increase the rigidity of the keel part by swelling-out the hull to the outer part of a ship, forming the keep part integrally, and laminating fiber-reinforced plastic materials in the vicinity of the keel part and on the surface of the core material arranged along the inner surface of the keel part, forming the reinforced part.

CONSTITUTION: On the ship bottom part 2 of a hull 1 made of fiber-reinforced plastics, a part of the ship bottom part 2 is swelled-out to the outer part of the ship in projected stripe form, forming the keel part 3, and a core material 4 is arranged over nearly the whole length of the keel part 3 along the inside of the keel part 3. Over the upper surface and both sides of the core material 4, and over the bottom part and the inner side part of the ship in the vicinity of the both sides of the keel part 3, the reinforcing parts 5 are laminated, and sealing is performed between the hull and the reinforcing parts. The reinforcing part is formed in cylindrical form as follows: The core material 4 is arranged in the keel part 3, and over the core material 4, the glass fiber fabric having unsaturated polyester resin in nonhardened state impregnated is laminated. Rubbed elements 6 are fitted along the outside of the keel part 3 by means of tapping screws 7.