PURPOSE: To improve torsional rigidity of the hull of a small boat made of fiber reinforcing plastic by building up external side boards from a chine thereof to sides of a deck with a lamination of fiber reinforcing plastic layers on both faces of a hard vinyl chloride bridging foaming material.

CONSTITUTION: A hull is principally made of fiber reinforcing plastic and has a V-shaped stem and a flat bilge at the stern. An external side board 30 of the hull extends upward from a chine 15 up to hull side of the deck 16. The external side board 30 is built in a so-called sandwich structure, that is a lamination of non-directional glass fiber layers 13 on both sides of a core 11 made of hard vinyl chloride bridging foaming material and fiber reinforcing plastic layers 14 piled thereon. The deck 16 also adopts the structure of sandwich lamination. Hence torsional rigidity of the hull may be greatly improved.