Recently, the maximum speed of small passenger craft is increasing. In case of the craft, the encounter wave period becomes shorter with increase of forward speed, and very large upward vertical acceleration is caused when its bow goes into the water surface. It is known that it causes not only bad ride comfort but also failure of hull or injury of passengers in some cases. For development hull form, it is necessary to estimate the response of vertical acceleration for its forward speed and sea

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... condition. For safety operation, it is important to estimate statistical short-term or long-term prediction of occurrence of un-desired large vertical acceleration. The purpose of this study is to investigate the above-mentioned characteristics of vertical acceleration of small passenger craft and to propose an estimation method based on measured data. First, in order to measure accurate vertical acceleration by partly captive model test, data sampling and data analysis methods are discussed. Next, the vertical acceleration on a hull is measured in irregular waves, and the characteristics of the vertical acceleration for wave height, wave period and forward speed are investigated, and its probability distribution for it amplitude is also investigated for the results in irregular waves. Moreover, the same measurements for two additional different hulls are carried out, and the effects of hull form is investigated. Finally, an estimation method based on measured data of the passenger craft whose some particulars are informed is discussed.