In the previous papers, the authors presented a method for calculating vertical motions and vertical wave loads for a high speed craft of medium size by treating it as rigid body. While the size of high speed craft has increased significantly, the influence of fluid-elastic interaction has to be investigated cautiously. In this paper, rigid and elastic representation calculations have been performed on an existing large size high speed craft, respectively. A series of tests has been carried out to measure vertical motions of two wooden models of different scale ratio. Another series of tests has also been carried out to measure verticalbending moment distribution along ship's length of an elastic back-bone model. The theoretically predicted results were compared with the experimental results and the experimental conclusions of others. The experimental inference shows that these two methods of theoretical calculation are accurate enough for practical application. Even for a high speed craft of large size, the correlation between magnitudes of vertical acceleration at bow and the maximun bending moment is confirmed by experiments and theoretical calculations. Hence the existing formula for ship length below 24 meters may be extended to large size high speed craft through some modification. A proposal is presented here in order to provide a tool for structural design.