APPLICATION OF SHIP-HANDLING / MANOEUVRABILITY CRITERIA TO STERN CONFIGURATION AND RUDDER AREA DESIGN
Kozaburo Yamada (Universal Shipbuilding Corporation, Japan)
Abstract: The maneuvrability criteria for large full ships are discussed with investigations of ship-handling performance in operations including restricted waters. Practical application procedures based on these criteria are proposed for the stern configuration and the rudder area design. The main subject is the comprehensive approach in the manoeuvrability design how to select and decide the parameters of the ship's hull form and the rudder which represent the manoeuvrability with allowable reversed spiral loop widths which are given by the ship-handling and the manoeuvrability criteria. The special attention has been payed during the study to the stern bulb configurations which have been the trends of modern hullforms.
The manoeuvrability design of ships are generally done by naval architects in shipbuilders or design offices, and ship operators handle the newly built ships by their skills, capabilities and experiences with reference to the reports of sea trial results.
The author intends to recognize and evaluate not only the manoeuvrability but also the ship-handling performance -course change control and position control performed by operators - and is to apply these criteria obtained by this study to the manoeuvrability design, mainly for large full ships like VLCC which face to severe operational circumstances such as heavy traffics in restricted waters. The author is to pay attention to the dimensional figures of turning angular velocity in view of human feeling and ship's path in manoeuvring. Most of the figures and tables are based on the author's former study results .
The author understands that the reversed spiral loop width of r'〜δ(non-dimensional angular velocity〜rudder angle) characteristics (hereinafter loop width) mostly represents the indices for manoeuvrability and ship-handling performance, because the course stability and the change-of-course ability with response to steering are the most important in case of large full ships. Then, the design procedure in the initial design stage of ships are proposed how to decide the stern profile and the rudder area ratio in connection with the ship's form factor K and aft fullness γA when a certain allowable figure of loop width is given. The main subject at first is to newly define and utilize the projected skeg area ratio together with rudder area ratio, especially putting an eye on stern bulb configurations (hereinafter SB) with less skeg area which might spoil the course stability although they are the main stream in recent years. Then the comprehensive study was carried out to organize these parameters which represent the manoeuvrability. Practical design charts are proposed for the manoeuvrability in the initial design stage.
2. PERFORMANCE IN COURSE CHANGE AND AVOIDANCE IN VIEW OF OCCUPIED SURFACE BY SHIP'S PATH IN MANOEUVRING
2.1 Comparison of Sea Surface between Ship's Path in Manoeuvrability Sea Trials and Restricted Waters in Navigation.
Ship-handlings are composed of the course keeping and the course change/avoidance, one of which examples of course recorder sheet is shown in Fig. 1-1. When we define the ship- handling performance in the expression related to the manoeuvrability, the course keeping will be the course stability and the course change/ avoidance will correspond to the turning performance and the response to steering. Sea trials by shipyards are carried out in deep, open seas, but the ship-handling of large ships like VLCC is severely restricted in the complicated circum-stances such as narrow and curved seaways, shallow waters, heavy sea traffics and their mixed situations with each other.
A typical example in Malacca/Singapore Strait is shown in Fig.1. Most of the seaways are separated, east and west bound, by TSS (Traffic Separation Scheme) by IMO. The narrowest width of east bound seaway for Deep Draft (≧15m) Vessel and VLCC (≧DW150,000T) is only about 700m. No separations are applied in the Precautionary Area close to ports, where danger increases due to mixed traffics by various kinds and different sized vessels.
Results of turning test and 10°Z test of a VLCC are shown in Fig. 1-2 and 1-3 together with its simulated paths for different rudder area ratios of 70〜120% of the original. This shows that the transfer due to overshoot is about 700m and reaches up to 1,400m in case of 80% of the original rudder area, even with the small rudder angle of 10 degrees.
This simulated paths will mean, in other words, that the practical rudder angles for initial course change and check helms for course keeping adopted on passing fairway are not like those of 10°Z steering tests. Nishimura and Kobayashi's study will explain these circumstances  . According to their study, the total rudder angle for course change becomes smaller and the total rudder angle for checking turn becomes larger in accordance with the increase of the loop width of ships with course instability. They also point out that in order to limit the lateral deviation, the maximum manoeuvrability performance should be utilized in ship-handling, which means that such ships have little allowance. It is observed that the longer the ship's length, the more the rudder angles to limit the lateral deviation.
Fig. 1 Comparison of dimensional restrictions in Malacca Strait with maneuvring motions of VLCC.
2.2 Relationships between Overshoot Angle of 10°Z Test by IMO MSC.137(76) and Ship's Paths
Looking at the collected data of 10°Z 2nd overshoot angles in Fig. 1-4, some ships show the overshoot close to 40°in L/V≧30 range which is usual in VLCC and the others are over the figure and almost 50°. In addition to 2nd overshoot angles themselves, it should be noticed that such ships might have insufficient manoeuvrability in view of position control due to the excessive overshoot paths when they are compared with the restricted seaways. In Fig. 2, the comparison of the 10°Z overshoot angles of VLCC and ULCC with different sterns are indicated. This shows that the 2nd overshoot of the VLCC Ship V with SB is close to that of ULCC Ship E4 with Mariner stern, and that of ULCC Ship E5 with Inv. -G is much smaller than Ship V. The 2nd overshoot of 40°by IMO should be referred to the overshoot angles of the ships in Fig. 2, together with the operator's comments mentioned in Table 3 in 5.2. Even if the 2nd overshoot is marginally within the IMO standard, the ship does not always seem to have sufficient allowance in ship-handling point of view.
2.3 Navigation in Reduced Speed and Shallow Water Effect
Rate of navigating speed in the restricted water in Malacca Strait is regarded as about 20% after repeated voyages as shown in Fig. 1-5, and if the sea speed in open, deep sea is assumed to be 15kt, the reduced speed by 20% equals to 12kt which is the restriction by TSS. On the assumption of the design draft 20.5m, applied to Malaccamax class VLCC of about L=320m recently build, the requirement for UKC (Under Keel Clearance) of 3.5m is estimated to be kept on the basis of the water depth of 23m to which 2m tide added and 1m squat in Fig. 1-6  subtracted from, which result in H/d (water depth/draft) of 1.16. Fig. 1-7 shows examples of passages of a VLCC with UKC smaller than 3.5m, by which the rate of reduction in speed is noticed to increase according to the decrease of H/d, especially less than 1.2. It is well known that turning angular velocity in shallow water becomes about 50〜60% of that in deep sea as shown in Fig. 1-8 . Fig. 1-9 shows the increase in turning circle of "ESSO OSAKA" in shallow water .