Ship motions are influenced by the sea state. Conventionally the responses are calculated in the frequency domain. This method, however, is valid only for narrow band spectra. As the seaway becomes more nonlinear, the ship motions cannot be readily predicted using the spectral method. Experiments conducted by Dalzell, have shown that the Response Amplitude Operator (RAO) decreased with increasing sea state or non linearity. Conventionally in the shipbuilding industry, the ship motions are studied by the linear RAOs and the energy density spectrum of the seaway. This method does not take into consideration any non linearities in the system. These are ignored and the ship seaway system is modeled linearly. The following work analyzes ship motions in the conventional linear approach and compares it to time domain simulations using the technique outlined in the work, viz. UNIOM (Universal Nonlinear Input Output Method). Time domain simulation of the SL-7 container ship hull is carried out. A comparison of the most probable peak value of the different modes of motion indicates that the linear theory tends to overpredict.

This research was done to study the effect of sea severity on the vertical ship responses like heave and pitch. Model testing of a 175m moored container ship with zero heading speed was done for different sea states varying from very rough to very high seas. Transfer functions were extracted using Volterra model which constitutes both linear and quadratic part. The experimental linear transfer functions were calculated using Volterra linear model and were compared with linear transfer function from the hydrodynamic theory. Experimental second order transfer functions were also extracted using Volterra quadratic model and their behavior was studied for different sea states. After the extraction of linear and second order transfer functions total responses were reconstructed and compared with the measured responses. This also helped to investigate the contribution of second order part to the total vertical ship responses. In the last stage of the research a new semi- empirical method was developed called as ‘UNIOM’ for the prediction of the responses. Laboratory input waves and theoretical LTFs were used for the simulation of ship response and these were compared with measured responses.