Memory effects: In this part of the parametrical study, the container ship is numerically tested for different heading angles, hence different encounter frequencies. Here, the motions of ship were compared with and without the radiation terms, described in Equation 16.

　

 Fig. 12 shows the purse seiner trawler vessel for the condition and parameters given in Table 2 with the memory effects included from Equation 16 for Froude number is 0.3 and heading angles are 0,30, 45, 60 degrees, respectively. The initial yaw angle was set to be the autopilot course from the wave direction. The ship was tested for same conditions and without the memory effects (Fig.13).

　

 Here, the difference between the numerical model with and without the memory effects is not significant. However, for controlled ship motions, the yaw and rudder angles are sensitive to the wave frequency effects as was mentioned by Ankudinov [13] and there is slight deviation for yaw motions without the memory effects (Figure 13).

　

 Comparisons between the 6 DOF and 4 DOF numerical models considered in this paper displayed that the difference between them is not very significant. However, the difference increases with increasing wave steepness. Also, 6 DOF model displays slightly stiffer control characteristics. Additional parametric studies with more environmental and control parameters are required before reaching definitive conclusions. These are currently underway.

　

 The numerical investigations focusing on identifying the effect of frequency, showed no significant difference either although small differences exist for yaw and rudder, and, control characteristics are sensitive to the effect of frequency for the controlled ship motions in astern seas. Furthermore, the full incorporation of hydrodynamic memory effects using 3-D numerical model thus including the aft effect can improve the numerical accuracy.

　

 Investigations to assess the motion of the vessel in long and short crested irregular waves are being carried out with the effect of frequency and its contribution to the accuracy of numerical modelling in random waves. The effect of control parameters on the vessel motions is also important. Use of more sophisticated autopilot models, therefore, is being attempted.