Multi hull vessels have emerged as popular alternatives to conventional mono hull ships for high speed crafts. However, the bridging structures connecting the hulls are vulnerable to various wave actions and the wave impact to the bottom of them is the most serious problems associated to the multi hulled vessels. In this study, prediction of relative wave elevations under the bridging structures is investigated for multi hull ships travelling with forward speed in waves. A computer code YNU-SEA of 3-D Green function method with forward speed has been developed and used to analyze the hydrodynamic radiation and diffraction forces and motion responses for high speed catamarans in waves. The results of present calculations are compared to those of the previous calculations as well as with the experimental ones. Numerical results reveal that the present computer code has the practicality to be a powerful tool for the accurate numerical computations of seakeeping problems for multi hull ships advancing in waves. Numerical calculations of wave pattern are also carried out including wave interactions between the hulls to analyze the effects of hull form on the free surface flow around catamarans advancing in waves. The analysis of the wave pattern allows the determination of relative wave height including radiation and diffraction waves. Finally, some discussions are made based on these numerical results which may be helpful for the accurate prediction of relative wave height and wave breaking load on the deck associated with multi hull ships.
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