PURPOSE: To enhance the strength of broadside of a hull by a method wherein the top rail is bent inward to provide a U-shaped cross section and is overlaid on and connected to the upper edge of a side plating.

CONSTITUTION: The top rail 3 being U-shaped in cross section and provided with a lower horizontal part 6, inside and outside vertical parts 7, 9 and an upper horizontal part 8 is so positioned that the lower horizontal part 6 is overlaid on a horizontal part 5 at the upper edge of the side plating 1, and the rail 3 is fixed by integrally fastening with a bolt and nut 4. In this case, the top rail 3 is fitted while maintaining the U-shaped opened inwards, so that the section of the top rail 3 cannot be seen from the outside of the hull.