Non-Linearity in Sagging Moment and Shear Force of Fine Ships

Rational estimation of ship's longitudinal strength should be based on accurate evalution of bending moment and shear force. The non-linear strip theory and model experiments reveal that the external force by waves in rough seas differ from those obtained by the linear theory. Rules of classification societies, based on the long term prediction of structural responses, assume linearity in response and use the linear superposition methods. According to the recent studies, the effect of non-linearity needs to be incorporated in the rules of classification societies. In the present paper, calculations are performed for a fine container ship using the nonlinear strip theory. Numerical results are obtained for regular waves of various wave heights, wave lengths and wave encountering angles and also for irregular waves of ISSC spectra by the non-linear strip theory. These results are compared with the result of O.S.M. (linear theory). Results obtained are as follows: 1) Sagging moment increases with the increase of wave height due to non-linear response, and the moment is about 1.7 times than that of O.S.M. at the wave height 8m in regular head sea for the container ship. 2) On the contrary, the effect of non-linearity in hogging moment is not so significant as that in sagging moment. Therefore, long term prediction of hogging moment is obtained approximately by linear theory. 3) Approximate prediction of long term response of bending moment is performed taking into account the effect of the non-linearity and compared with the present classification rules. 4) Positive shear force distribution in the for'd part shows non-linearity in response.