Abstract: A pod propulsion system is noble and popularizing because it is very useful in manoeuvring a ship in port. On the other hand, it was reported at the meeting of 44th Design and Equipment Subcommittee of International Maritime Organization (IMO-DE44) that a model ship with pod propulsion systems was neither able to carry out 10deg/10deg zigzag manoeuvre nor 20deg/20deg zigzag manoeuvre as required according to IMO resolution [1] and that a kind of the modified zigzag manoeuvre was alternatively proposed in order to evaluate the course keeping ability of ships with pod propulsion systems. In this paper, the relations between spiral loop width and overshoot angles in modified zigzag manoeuvre are shown by the simulation for both ships having a pod propulsion system and a conventional one. Then they are compared with those in 10deg/10deg and 20deg/20deg zigzag manoeuvre and it is concluded that the overshoot angles in 10deg/10deg and 20deg/20deg zigzag manoeuvre is better item than those of modified ones. In addition, the level of the criteria on course keeping and yaw-checking ability is described for both ships with a pod propulsion system and with a conventional one.

　

 A pod propulsion system was developed for an icebreaker in 1993 and widely equipped to many ships including cruse vessels. The ship equipped with the novel propulsion system is characterized in good controllability and has often the buttock flow shape at stern in order to improve the propeller efficiency. The stern profile is not good for course keeping ability [2] but most of ships are reported to have the good course keeping ability as well as the turning ability because they have often two propulsion systems that give the large lateral area [3].

　

 On the other hand, there is an example that a ship with pod propulsion systems but with low L/B ratio and high B/d ratio did not comply with IMO Resolution A.751(18). Accordingly a kind of the modified zigzag manoeuvre (25deg/1deg Z) was alternatively proposed at IMO-DE44 in order to evaluate the course keeping and yaw-checking ability of ships with pod propulsion systems.

　

 The course keeping ability is closely related to safe operation of ships and evaluated by the loop width of spiral characteristics that is affected by hydrodynamic forces acted on the hull. The hull form is therefore very important for attaining the good course keeping ability.

　

 In this paper, the course keeping and yaw-checking ability of a ship equipped with a pod propulsion system is described, compared with a conventional ship. The relation between overshoot angle and loop width had already been shown [4] and the overshoot angles in 10deg/10deg and 20deg/20deg Z manoeuvre are adopted in the resolution A.751(18) and MSC137(76) as the items on the course keeping and yaw-checking ability.

　

 Accordingly, the overshoot angles in various zigzag manoeuvres are simulated for both ships with a conventional propulsion system and with a pod one. Then the relation between the overshoot angles and loop width are described and it is shown that overshoot angles in 10deg/10deg and 20deg/20deg Z manoeuvre are better than those in modified zigzag manoeuvre and that both ships can almost comply with the resolution A.751(18) for same loop width. It is conclude from the fact that the criteria of the course keeping and yaw-checking ability in the resolution is same level for the both ships that have different ratio of ship length and ship speed(L/U). In addition, the level is discussed when the ships have the same L/U.

　

2.1 The models for simulation

　

 The hull form, especially stern profile, is important in a conventional ship equipped with a single propeller and rudder in order to comply with the resolution. The models for simulation are built up based on the experimental results [5]. They have same principal dimensions but different stern profiles as shown in Table 1 and Fig.1.

　

　

　

　

　

　

　

　

 Ship A has a V-shaped stern profile and ship B has a U-shaped one. Ship C has the medium stern profile between ship A and ship B. The effects of different stern profiles to the manoeuvrability are shown by means of simulations using the models. The ships are imaginary tankers and their velocity (U) is 11.3 knot in the simulation. The coordinate system is shown in Fig.2 and the motion equations are described by the Kijima's expression [6].

　

 The spiral characteristics are shown in Fig.3. The horizontal and vertical axes respectively mean helm angle (δ) and nondimensional yaw rate (r'=rL/U), where r is yaw rate in turning. The positive value expresses turning to starboard.

　

 It is clear that these ships have almost same turning ability and different loop widths. In the three ships, ship A has 8.9deg in the loop width and the widest. Ship B has almost 0deg and the narrowest. Then ship C has 2.9deg and the medium between ship A and ship B. The loop widths result from the different stern profiles and it is clear that the stern profile is important for the course keeping ability.

　

 The heading angles (ψ) of a ship in 10deg/10deg zigzag manoeuvre are shown in Fig.4. The horizontal and vertical axes respectively express time and heading angle. It shows that ship A has the largest overshoot angle and that ship B has the smallest one in three ships. Then Ship C has the medium one. The relation is the same as that of the loop width and the time that the heading angle reaches the peak. Accordingly, the overshoot angle expresses the course keeping and yaw-checking ability. It had therefore been adopted as the item of the ability in the resolution A.751(18) and MSC137(76).