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 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.%耐航性能向上を目指した船型開発および船体設計にとって，設計段階で波浪中航走時の上下加速度の大きさを知ることは重要である.また，運航管理において，統計的手法などによって大きな上下加速度の発生を予測することが事故を防ぐためには必要である.実際に，波浪中を高速航走する小型高速旅客船に激しい縦運動が生じ，乗客が負傷する事故が発生した.日本小型船舶機構は，この事故の原因が乗客の着座位置での非常に大きな上向き加速度もしくは乗客が座面から浮き上がる1.0[G]を超える下向き加速度が発生したためであることを示した.また，事故回避のためには，両現象のうち発生確率の高い後者に注目し，乗客の着座位置で下向き加速度が1.0[G]を超えない操船が望ましいと結論づけた.
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