The results in Table 5 indicate that in deep water the MER tends to increase with decreasing ship speed, and/or reducing water depth, but other results, such as those of Gertler and Glover [1], do not indicate a consistent trend of MER with speed.

　

　

　

 Mariners have reported that in shallow water ships become more sluggish or difficult to steer as ship speed becomes small. This could be due to an increase of MER with decreasing speed, but also could be due to a less favorable shape of the r -δR curve at slow speeds as discussed by Asinovsky [38] (here r is the yaw rate and δR is rudder angle).

　

 Note that a small MER indicates, but does not guarantee, good controllability. Figure 5 presents r' - δR curves for three hypothetical ships having an MER of six degrees, here r' is the non-dimensionalized yaw rate r. Ships with minimal values of (dr'/ δR) for rudder angles greater than the MER will be largely uncontrollable, while ships with large values of(dr'/δR) for rudder angles greater than the MER can be controllable even with a relatively large MER.

　

　

6.3 Turn, Accelerating Turn, Coasting Turn

　

 Table 7 summarizes the three types of 35°starboard turns at slow speed. In order to make the comparison easier, all of the Turn and Coasting Turn tests started with the same approach speed of 6.5 knot, and the Accelerating Turn started DIW with a SAHD bell. Note that when the propeller is overloaded, the propeller wash is stronger. Thus the rudder is more effective and resulting in smaller turning Advance and Transfer, such as during an Accelerating Turn. On the contrary, when the ship is coasting, the rudder is less effective; thus the Advance and Transfer become larger during a Coasting Turn.

　

　

6.4 Overshoot, Accelerating Overshoot, Coasting Overshoot

　

 Table 8 summarizes the 20°/20° Overshoot and Accelerating Overshoot tests in both deep and shallow water. In order to make the comparison easier, all of the Overshoot tests started with the same approach speed of 6.5 knot, the Accelerating Overshoot started DIW with a SAHD bell. For the same reason discussed in the last section about rudder effectiveness, the yaw checking performance is much better during acceleration. Note that there is no data for some of the Coasting Overshoot tests at 6.5 knots, because the rudder could not recover the heading.

　

　

 A Suggestion of 5°/5°or 10°/5°Overshoot Tests for Piloting Information. During the planning process of Houston Ship Channel Study [39], a question was raised within SNAME H-l0 Panel about what slow speed tests are directly helpful or linked to a piloting task? Based on the two studies by pilots, Yamazaki [36] and Knierim [40], and also the fact that pilots have to handle a vessel within constricted channels, the following Overshoot tests using a small rudder angle were recommended:

　

　

 For directionally unstable ships, especially the ones with MER greater than 5 °, these small rudder command/small heading change overshoot tests can provide important information. These tests could indicate the level of stability of the vessel based on the overshoot response or lack of (see Eda and Landsburg [41]) while staying within the confines of the channel (more rudder would be necessary to recover if no response). Some pilots would normally prefer, however, to quickly gain a feel for the responsiveness of the helm through a 10° or 20° initial try of the rudder. The 5°/5°or 10°/5° tests may find their way into the basic test list through gained experience with their indicating value.

　

 We have focused our thinking and improved our understanding about the issues of slow speed maneuvering test and vessel performance. We need to move further ahead to pursue the objectives laid out in the beginning of this study. The following are several areas for follow-up work involving also some new challenges:

　

7.1 Further Analysis and Refinement of Suggested Slow Speed Maneuvering Tests Using Simulation

　

 The simulation analysis of these suggested maneuvers showed that there are still many issues that can be addressed through this approach. This is a more manageable effort, compared to the sea trials, and can be pursued at different scales with flexibility.

　

7.2 Assess the Suggested Tests in the Field

　

 The choices of basic and additional slow speed maneuvers are mainly based on a subjective view and experience working in a ship handling simulator environment. They have been tested on computer only. Virtual reality is not reality - it is always a concern that this study might become just an academic exercise and lose touch with the real world. In order to avoid that trap, it is necessary to assess the suggested slow speed maneuvering tests in the real world. Only through that process can we be assured that the test procedures are logical, the performance data are collectable, and the indices are measurable and useful. To do that, the difficulty is not so much in technical areas; rather, finding funding and achieving coordination of field tests are much more challenging.

　

7.3 Trade-off between Ideal and Reality

　

 One interesting realization during the study is the difficulty of balancing the ideal of comprehensiveness with economic and operational reality. Considering the 11 basic tests, if we do conduct the tests in both deep water and in shallow water, and for both loaded and ballasted draft, the total number of runs will be 44, not mentioning the tests that can start out from either starboard or port side. That number seems to be beyond reach when considering how many IMO suggested definitive maneuvers are actually tested during a sea trial. The issues need to be addressed include:

　

　

7.4 Promote the Usage of Slow Speed Maneuvers and Performance Indices

　

 Today's simulator training and research environment is very competitive. Compared to a decade ago, simulator users are much more knowledgeable, sophisticated, and demanding. The simulator users always apply pressure for better and cheaper simulation modeling with a short notice. To answer that demand of improving quality, cost, and response time, one important link is the ready access to credible maneuvering performance information of the targeted vessel during model development and validation process. It cannot be over emphasized that maneuvering data of a "loosely controlled test" could be misleading and counter-productive. In that regard, we need to promote the testing of standardized slow speed maneuvers and reporting the performance via a commonly recognized indices protocol.

　

7.5 Establish Slow Speed Maneuvering Performance Databases

　

 One step to establishing a knowledge base on slow speed maneuvering performance that is useful for research, development, regulation, etc., is to build a database similar to the USCG database of standard definitive maneuvers [6]. The database must, how-ever, include much more information about the key ship parameters that are essential for comprehensively analyzing ship maneuvering performance. Requiring slow speed maneuvering tests to be included as part of a vessel's Delivery Test Program may never become practical, but the technology of dGPS, voyage recorder, and onboard information systems are very close to making it practical to extract information from "tests of opportunity." The challenge will be to establish a depot to store the maneuvering performance information and manage the usage of it.

　

7.6 Delivery of Customized Slow Speed Maneuvering Information to Different Types of End Users

　

 In order to have the information useful to the end user it should be presented in a comprehensible format in the user's working context, be it on paper, through computer display, or as useful rules of thumb. The performance information summarized in Appendix B and C of [33], with samples included in the Appendix of this paper, was mainly described from an engineering perspective. Further R&D will discover better ways to present the information in a vessel's operational setting.

　

 It can not be over emphasized that the information provided to the pilots has to be simple and easy to read, understand, and grasp its critical implications. Pilots have very little time to become familiar with a ship before having to begin critical maneuvering. The thorough review of detailed documents such as maneuvering booklets or wheelhouse posters is usually not feasible. The best way of helping pilots to make an immediate assessment of ship capabilities would be to provide a limited set of quantitative measures which characterize maneuverability and the broader issues of controllability. The IMO maneuvering standards could provide a means for addressing this difficult task. The trials performance of a ship (e.g., Advance and Transfer in a Turn, Overshoot Angles in a Zig-zag, etc.) as a percent of the allowable value defined by the IMO standards could be provided to and used by pilots to make a first assessment of a previously unknown ship.

　

 In conclusion, there is a growing recognition of the needs for characterizing and communicating the slow speed maneuvering performance. It needs to be determined if the current IMO maneuvering performance standards can be used to identify ships with poor slow speed maneuvering performance. Through the pilot study, we have tried to appreciate the complex issues involved. Though the challenge to reaching the objectives is significant, we shall pursue the mission steadfast. We appeal to the international community to help coordinate the effort to accomplish the mission in a systematic way.

　

[1］ Gertler, M. and Gover, S., "Handling Criteria for Surface Ships", David Taylor Model Basin Report No. 1514, Washington, D. C., 1961.

[2] Panel H-10, "Proposed Procedures for Determining Ship Controllability Requirements and Capabilities", SNAME Proceedings First Ship Technology and Research Symposium (STAR), pp. 4/1 - 4/34, August 26-29, 1975.

[3] Landsburg, A.C., Card, J.C., Crane, Jr., C.L., Alman, P.R., Bertsche, W.R., Boyleston. J:W., Eda, H., Keith, V.F., McCallum, I.R., Miller, Jr., E.R., and Taplin, A., "Design and Verification for Adequate Ship Maneuverability", SNAME Transactions, Vol. 91,pp. 351-401, 1983.

[4] Cojeen, H.P., Landsburg, A.C,, and MacFarlane, A.A., "One Approach to the Development and Achievement of Maneuvering Standards," Proceedings, Int. Conference on Ship Maneuverability Prediction and Achievement, RINA London, 1987.

[5] Landsburg, A.C., Card, J.C., Eda, H., von Breitenfeld, H.C., and Kneirim, T., "Proposed Shipboard Maneuvering Data", Proceedings of Fifth STAR Symposium, SNAME, pp.241-262, 1980.

[6] Barr, R.A., Miller, E.R., Ankudinov, V., and Lee, F.C., "Technical Basis for Maneuvering Performance Standards", USCG Report CG-M-8-81, 1981.

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[9] IMO, Res. A.601(15), "Provision and Display of Manoeuvering Information Onboard Ships",1987.

[10] IMO, MSC.137 (76), "Standards for Ship Manoeuvrability", December 2002.

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[12] IMO DE 34/4 (with annex 1- 3), 5 June 1990

[13] Chislett, M.S., "A Generalized Math Model for Manoeuvring", Proceedings of MARSIM '96, Copenhagen, Demark,1996.

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[18] International Towing Tank Conference, "Report of the Manoeuvrability Committee," Proceedings of the 22'nd ITTC, 1999.

[19] Miller, E.R., Jr. and Ankudinov, V., "Model Test and Simulation Correlation Study Based on Full Scale 'ESSO OSAKA' Maneuvering Data," MARAD, MA-79-SAC-00097, Oct. 1980.

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[26] Norrbin, N.H., "On the SNAME Slow Speed Maneuvering Performance Indices Study-Comments on the 1998 October Focus Group Summary", Summary paper, Sweden, 1999.

[27] Groh, H., "Ship Maneuverability and Controllability at Slow Speeds", draft paper to be published, August, 2002.

[28] Ankudinov, V., Jakobsen, B.K. and Mazurkiewicz, J., "Hydrodynamic Aspects of Slow Speed Maneuvering in Deep and Shallow Waters", SNAME H-l0 Slow Speed Maneuverability Workshop, Kings Point, New York, 1998.

[29] Jakobsen, B.K., Mazurkiewicz, J. and Ankudinov, V., "Improved Ship Maneuvering Assessment Based on Integration of Advanced Modeling Technique", Proc.of MARSIM 2000, Orlando, FL, 2000.

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[33] Hwang, W-Y, "Slow Ship Speed Maneuvering Performance Study", Final Report to SNAME H-10 Ship Controllability Panel, 2003.

[34] Jakobsen, B.K., Technical Notes, 2003.

[35] Fuller, N., "Write-Up on the Minimum Effective Rudder (MER) Concept", Technical Notes, 2000.

[36] Yamazaki, Y., "Studies on Manoeuvrability Standards from the Viewpoint of Marine Pilots", Japanese Pilots Association, 1990.

[37] Crane, L., "Maneuvering Trials of the 278,000 DWT Esso Osaka in Shallow and Deep Waters", Exxon Report No. EII.4TM.79, New Jersey, 1979.

[38] Asinovsky, V., "Review and Analysis of Ship Manoeuvrability Criteria", Naval Engineers Journal, May 1989.

[39] Daggett, L.L. and Hewlett, J.C., "Ship Performance Measurements, Houston Ship Channel, Galveston Bay, Texas", Waterway Simulation Technology, Report for U.S. Army ERDC Contract No. DACW39-97-D-0001, D012, via Designers & Planners, 2001 .

[40] Knierim, T.G., "Maneuvering Information for the Pilot/Navigator: Its Source Value and Limitations", Marine Technology, Vol. 31, No. 2, pp. 123-144, l994.

[41] Eda, H. and Landsburg, A.C., "Maneuvering Performance Analysis During Preliminary Ship Design, Second International Symposium on Practical Design in Shipbuilding, Tokyo, Japan & Seoul, Korea, 1983.