PURPOSE: To protect the hull and maintain the hardness of the edge of the hull by a method wherein the cross-section of a top rail is formed to be the shape of U opened downward, both corner parts are formed unitedly so as to be swollen outside and to have a large thickness, and the opening thereof is made in the upper end of the bulwark.

CONSTITUTION: A top rail 5 formed to be a member having a cross section in the shape of U opened downward through the extrusion forming of high-density foamed polyethylene and the like is fitted in contact with the vertical part 31 in the upper end of the bulwark 3 and the horizontal part 32 thereof, by means of bolts 6, etc. The corner part of the top rail 5 and the lower part on both sides thereof are swollen outward to form swelling parts 51 and 52, thereby the hardness of the top rail 5 being increased. The other end of the top rail supported by bulwark stay 4 provided at fixed intervals. By this, the shape of the cross section of the top rail is not changed even when a large bending stress acts on the top rail and the high hardness is maintained, whereby the hull can be protected from any abstructions.