PURPOSE: To improve the draining efficiency by producing a negative pressure range behind a cornered part formed by bending the floor upward at the rear end of the ship's bottom during cruising and by forming a drain port connecting the storing chamber and the outside of the rear of the ship above the cornered part.

CONSTITUTION: A storing chamber 10 independently partitioned is formed in the stern section facing the transom 1a. An opening 11 is formed on the bottom floor of the storing chamber for accommodating a propeller 9. An opening 14 for taking in and out the rudder 13 also is formed on the bottom floor of the storing chamber. The propeller 9 is raised and lowered by the operation of lifting shaft 7. The rudder 13 can be raised or lowered by a steering handle 13a. A drain port 15 is formed on the rear end wall of the storing chamber 10. A cornered part 30 is formed by bending floor upward at the rear end of the bottom floor of the storing chamber 10. This cornered part 30 functions to produce a nagetive pressure region P during cruising around the drain port 15 so that draining efficiency is improved.