In this study a U-type tank, consisting of two lateral reservoirs connected by means of an horizontal channel, is considered as a passive anti-rolling device for ships. Forced oscillation tests have been conducted at CEHIPAR for the U-type tank on a moving platform for different frequencies and also for different amplitudes of the pure roll forced motions. The most important aspect was to obtain a large set of roll restoring moments, for this passive configuration, which gives a discrete distribution of the roll restoring moments along the entire range of wave frequencies of interest. Moreover, from free decay tests an estimate of the U-tank internal damping was obtained. A detailed account of model tests is presented and the method of analyzing the experimental results discussed. Herein the experimental data of the U-type tank is compared with results from a linear frequency domain model, where the level of nonlinear effects with the amplitude of the forced motions is thoroughly addressed.

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