Fig. 6 shows time histories of the interaction forces acting on 2 ships in deep and shallow water. The interaction forces in h/d = 1.2 are much larger than those in deep water. Around x/L = -0.5 in shallow water large bow-in moment acts on Ship 1. Due to the bow-in moment, Ship 1 approaches to the stern of Ship 2 and ship collision occurs. Hydrodynamic characteristic of the yaw moment in overtaking is important when discussing the ship collision.

　

  Fig.7 shows the ship trajectories in the series when changing the ship size of overtaken ship (Ship 2) with keeping the size of overtaking ship (Ship 1). L2/L1 = 0.8, 1.0, 1.25 were employed as ratio of 2 ships. Table 1 shows the calculation conditions for different ship size. The actual initial lateral distance between ships (Sp)) is the same for different ship size, although the non-dimensional values are different. No ship collision occurs in L2/L1 = 0.8, however collisions are simulated in L2/L1 = 1.0 and 1.25. Particularly in case of L2/L1 = 1.25, Ship 1 collides with rear side part of Ship 2 at the beginning in overtaking. From this result, we see that ship collision becomes more remarkable when the overtaking ship is larger than the overtaken ship. The behavior simulated in right figure of Fig.7 resembles to the model test result for ship collision conducted by Dand[16], although calculated condition are different from Dand's one.