As mentioned above, the stern profile is very important for course keeping ability. A stern profile of buttock flow is the configuration cut up the aft body like a ship hull without a center skeg as shown in Fig. 11. This profile looses the hydrodynamic side force at the stern and it is predicted that such a ship is unstable in the course keeping ability.

　

　

 Accordingly, the relation between loop width and overshoot angle is discussed on the ship with a pod propulsion system by the same method as on a ship with a conventional one.

　

3.1 The models for simulation on ships with a pod propulsion system

　

 Before the models for simulation are built up, it is necessary to investigate the manoeuvrability of ships having a buttock flow stern profile and equipped with a pod propulsion system. Therefore, free running model test was carried out using the model ship without center skeg at an outdoors basin of National Maritime Research Institute. In this case, the turning rate of the pod angle in simulation and experiment is 3 times as fast as that of a conventional ship, which is required by SOLAS Convention.

　

 The principal dimensions of the model ship that is an imaginary tanker are shown in Table2. The spiral characteristics are shown in Fig. 12. The horizontal axis expresses the pod angle and the vertical axis expresses nondimensional yaw rate. The marks in it express the experimental results. It is clear that turning ability is extremely good and that loop width is very wide, that is, the ship has bad course keeping ability.