4.1 The comparison in different L/U

　

 The model for simulation with center skeg and large strut in a pod propulsion system (Hereafter called a pod ship) has 8.4deg loop width in spiral characteristics in L/U=14.1 and it is almost same with that of ship A which has a conventional hull form and a propulsion system in L/U=31(Hereafter called a conventional ship).

　

 As mentioned at section 2.3, a conventional ship can comply with the resolution A.751(18) if her helm area is increased 20% from prototype and her loop width is modified to 6deg. On the other hand, a pod ship can almost comply with the resolution if strut area of pod system is increased to 2.5 times as large as the prototype. Then, the loop width is modified to about 6deg. In addition, second overshoot angle is the severe for both ships in criteria of course keeping and yaw-checking ability of the resolution A.751(18).

　

 It results from the facts that a pod ship has almost the same loop width as a conventional ship in order to comply with the resolution. Therefore it is concluded that the level of criteria of course keeping and yaw-checking ability in the resolution A.751(18) for a pod ship is the same as for a conventional one. However, the level of criteria in the resolution MSC137(76) for a pod ship is a little severer than for a conventional one.

　

4.2 The comparison in same L/U

　

 The criterion of 2nd overshoot angle in 10deg/10deg zigzag manoeuvre on course keeping and yaw-checking ability is different between A751(18) and MSC137(76). Accordingly, a pod ship and a conventional one should be compared for the same ratio of ship length and ship velocity (L/U). Then 10deg/10deg and 20deg/20deg zigzag manoeuvres were simulated by a conventional ship that is a series model for rudder area after the ship velocity changed.

　

 The relations between the overshoot angles and loop width are shown in Fig.18-20 with those in a pod ship of the series model for strut area. In addition, the criteria of course keeping and yaw-checking ability are shown in the figures according to the resolutions A751(18) and MSC137(76), but the level at L/U=14.1 is almost the same.

　

 It is found that the loop width of a conventional ship is narrower than of a pod ship in 1^st overshoot angle in 10deg/10deg and 20deg/20deg zigzag manoeuvre in order to comply with the resolutions, but it is the same in 2^nd overshoot angle in 10deg/10deg. It is therefore concluded that the criteria of 1^st overshoot angle in 10deg/10deg and 20deg/20deg zigzag manoeuvre are severer for a conventional ship than for a pod ship and that the criterion of 2^nd overshoot angle in 10deg/10deg is the same for both ships.

　

 On a conventional ship, the relation between spiral loop width and overshoot angle is the same for various ship hull form but overshoot angle is larger for the ship when her L/U is smaller [8]. Accordingly, the criteria of the resolution is severer for a conventional ship in L/U=14.I than in L/U=31 even though the level of criteria of resolution A.751(18) is almost same for a conventional ship in L/U=31 and a pod ship in L/U=14.1. The reason is why the angular velocity of helm angle is the same in the different L/U for the same ship.

　

 In this paper, the relations between the loop width in the spiral characteristics and overshoot angles in modified zigzag manoeuvre are shown by the simulation for both ships with a conventional propulsion system and with a pod one.

　

 It is concluded that the differences and values of overshoot angles for loop width show the most remarkable in 10deg/10deg and 20deg/20deg Z manoeuvre and that they are the better item to evaluate clearly the difference of course keeping and yaw checking ability of a ship.

　

　

　

　

 Moreover it is concluded that a pod ship in L/U=14.1 has almost same loop width with a conventional ship in L/U=31 to comply with the criteria of course keeping and yaw-checking ability in the resolution A.751(18). Therefore the level of the criteria for a pod ship is almost the same as for a conventional one.

　

 It is however concluded by the comparison in same L/U between a pod ship and a conventional one that the criteria of 1^st overshoot angle in 10deg/10deg and 20deg/20deg zigzag manoeuvre are severer for a conventional ship than for a pod ship and that the criterion of 2^nd overshoot angle in 10deg/10deg is the same for both ships.

　

 The reason is why overshoot angle is larger for a conventional ship when L/U is smaller because of the same angular velocity of the helm angle. Consequently, the criteria of the resolution is severer for a conventional ship in L/U=14.1 than in I/U=31 even though the level of criteria of resolution A.751(18) is almost the same for a conventional ship in L/U=31 and a pod one in L/U=14.1.

　

[1] Hans Otto Holmegaard Kristensen "The Manoeuvrability and Propulsion of Double-Ended Ferries-Design Considerations, Construction and Service Experience", 6'th SVA Forum (Shiffbau-Versuchsanstalt Potsdam), pp1-28, 1998

[2] S. Motora and M Fujino " II. Characteristics of an unstable ship", proc. of The 2^nd Symposium on Ship Manoeuvrability, The Society of Naval Architects of Japan, pp.41-60, 1970

[3] R. Kurimo "Sea Trial Experience of the First Passenger Cruiser with Podded Propulsors", proc. of Practical Design of Ships and Mobile Units, The Netherlands, pp.743-748, 1998

[4] T. Nobukawa, T. Kato, K. Motomura, and Y. Yoshimura " Studies on Manoeuvrability Standards from the Viewpoint of Marine Pilots", Proceedings of MARSIM & ICSM 90, pp.59-66, 1990.

[5] M Fujino "Keynote lecture: Prediction of ship manoeuvrability : State of the art", Proc. of MARSIM'96S, Denmark, pp. 371-398, 1996

[6] K Kijima etc. "ON A PREDICTION METHOD OF SHIP MANOEUVRING CHARACTERISTICS", Proc. of MARSIM'93, pp. 285-294, 1993

[7] S. Fujii and K Nomoto " I. Experimental Method on Ship Manoeuvrability", Proc. of The 2^nd Symposium on Ship Manoeuvrability, The Society of Naval Architects of Japan, pp.1-39, 1970

[8] Y. Yoshimura "INVESTIGATION INTO THE YAW-CHECKING ABILITY IN SHIP MANEUVRABILITY STANDAR", Proc. of Mini Symposium on Prediction of Ship Manoeuvring Performance, Japan Marine Dynamics Research Sub-Committee, pp.11-19, 2001

　

 The author graduated from Kyushu University in Japan and was employed in Ship Research Institute in 1974 that changed the name to National Maritime Research Institute in 2001. The author has engaged in the research of ship dynamics and especially in the investigation on the interim standards for ship manoeuvrability (Res. A.751(18)) from 1994. In addition, the author attended to the meeting on the revision of the resolution A.751(18) at IMO-DE subcommittee.