PURPOSE: To reduce the occurrence of uncomfortability, such as overflow or leakage, and an adverse influence on restoration efficiency by communicating a draft body through fluid with a dent and the external part of a hull and taking water in a hollow vent body or a storage tank even during hard inclination or turn-over in running.

CONSTITUTION: A draft body 15 is provided to comprise a hull bottom 32; a transom 38; a dent 14 adjoining the transom 38 and extending at least a part of the lateral size or the width of the hull bottom 32; a horizontal step part forming the horizontal boundary of a dent 14; and at least a pair of access ports 40 and 41 the internal parts of which form the hollow draft body 15 or a storage tank and which have a further limited cross section size compared with the enlarged sectional area of the draft body 15. The draft body 15 is communicated through fluid with the dent 14 and the external part of the hull. Thereby, even during hard inclination or turn-over of a boat hull during running, ventilation of a low pressure section adjoining the dent 14 without water entrance access to the internal part of the hull is practicable, water is taken in the hollow draft body 15 or the storage tank.