PURPOSE: To improve the sea kindliness and navigating ability of a boat by bending a thin flat plate passing a keel line, a chine line and a stern line into a curved plane to form a ship's bottom, and dividing a section enclosed with the three lines through a straight line, and mutually bendably connecting dividingly formed flat plates.

CONSTITUTION: A ships bottom 14 is composed of bottom parts 22, 24, which are divided into a plurality of bottom elements 22A, 24A by a plurality of straight lines P. Each straight line has its one end on a keel line K and the other end at an intersecting point between a chine line C1 or C2 and a stern line S1. The bottom elements 22A, 24A are mutually bendably connected in the straight line P and on the keel line K. In the folded state of a boat, the bottom part 24 on one side overlaps with the bottom part 22 on the other side, and the ship's bottom 14 therefore becomes flat plane-shaped, but in its unfolded state, the bottom parts 22, 24 form a quadratic surface which bends gradually as it approaches a bow side, and are excellent in sea kindliness and navigating ability. A rubber boat body is detachably attached onto the ship's bottom 14.