It has been pointed out that a wave-induced steady heeling moment is one of the dominant causes for a large inclination of a lower hull type semi-submersible in addition to the wind-induced heeling moment. The occurrence of the wave-induced steady heeling moment is attributed to a fact that an upward steady force acting on a lower hull becomes larger as the lower hull is closer to the water surface. In this paper, it is reported that the reduction of the upward steady force and the steady heeling moment derived from this force in regular beam waves is achieved nearly satisfactorily by fitting simple attachments (fins) on the lower hulls. The experimental results of the vertical steady force acting on a submerged cylinder with and without the attachments and of the heeling moment acting on a scaled model of a two-lower hull eight-column semi-submersible with and without the attachments are shown.
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