PURPOSE: To make it unnecessary to replace a stretcher support beam every time a fitting position is changed by fitting thin rails on both sides of a lengthwise direction of a boat inside a cockpit of a rowboat and providing an expansion stretcher support beam.

CONSTITUTION: Support rails 90 on both sides are fitted to both side walls 31 of a cockpit which is curved in a longitudinal direction, i.e., thin support rails 90 with openings are fixed, a sliding member 99 is fitted into the rail, and the spindle 92 penetrates a terminal section 91 of a stretcher support beam 9 and is screwed by a nut 98. Since the section 91 is thin, it is easy to fit it by bending along the curves. The length of the stretcher support beam 9 can be changed in response to the distance by conncting a plug-in pipe of a different diameter for expansion, therefore even when the fitting place is changed, it is unnecessary to replace the beam 9.