Ship Motions in Average Sea States (1)

Abstract

It has become a established theory that the ship motions in irregular waves are estimated from the transfer functions of ship motions and the Neumann spectra. But, according to this method we can only know the information of ship motions in irregular waves to be defined by spectra. Therefore it has a weak point, because we can not directly compare the actual ship performances in a certain wave height or a certain wind wave scale with the estimated qualities from it.This paper makes a proposal to exclude that fault of the conventional method. In the first place, the author investigated the relation between wind velocity and mean wave height in average sea states. Next, he tried on to substitute the assumed partially developed Neumann wave spectra for the average relations between wind velocity and mean wave heights. Hereupon, the significant wave heights of the assumed spectra are equal to the average wave height of actual average sea states. Moreover, he calculated the ship motions in that assumed spectra by the ordinary method. Then, it is considered that the calculated values by this method are equivalent the ship motions in average sea states.The obtained results show that the estimated values by the use of the fully developed Neumann spectra develop a tendency to over estimate, and that estimations give the inverse qualities at comparative calculations between various ship forms is possible.Then he predicts the critical ranges of the navigation in the averge sea state arising from severe ship motions or seasickness. Moreover, the cumulative occurance frequency of ship motions are predicted.The used ship forms for calculations are four kinds of destroyers. In this report, pitching and bow vertical accelerations in head seas are calculated.