The stability performance of small ships is an important factor for safety. Current stability regulation for the small ships of which capacity is 12 people or less has two kinds of criteria. Since there are both simplified and original criteria based on the ship length, the problem of reversal phenomenon concerning the capacity of ships occurs. The simplified criterion applied to the ship length of less than 3.3m allows more capacity than the original criterion applied to the ship length of 3.3m or more.
In this paper, the appropriate requirement for the margin freeboard that can be evaluated by one criterion is discussed to solve this problem. The margin freeboard in longitudinal waves is one of the requirements of stability criteria and is derived from wave steepness. Winds and waves in the Japan coastal areas where small ships navigate have been analyzed, and the appropriate required value for the margin freeboard are proposed.
