AN EXPLORATORY STUDY TO CHARACTERIZE SHIP MANEUVERING PERFORMANCE AT SLOW SPEED
Wei-Yuan Hwang (U. S. Merchant Marine Academy, USA)
Bent K. Jakobsen (Computer Science Corporation, USA)
Roderick A. Barr (Hydronautics Research, USA)
Vladimir K. Ankudinov (Designer & Planners, USA)
Nathan R. Fuller (USCG Retired, USA)
Louis C. Vest (Houston Pilots Association, USA)
Michael A. Morris (Houston Pilots Association, USA)
Andrew W. McGovern (Sandy Hook Pilots Association, USA)
Alexander C. Landsburg (Maritime Administration, USA)
Abstract: A common language, standard procedures, design tools, criteria, and standards have been established for describing, measuring, predicting, and assuring a vessel's maneuvering performance. The emphasis, however, has been on maneuvering at cruising speed and in deep water. The call to expand performance assessment to slow speed operations in shallow waters has evolved from simulation and real experiences. Slow speed maneuvers are not the central concern for design of most ships but they are crucial to safe operations. The Technical & Research Program of the Society of Naval Architects and Marine Engineers (SNAME) through Panel H-10 (Ship Control-lability) performed a study of the issues of characterizing slow speed ship maneuvering performance. The objective was to identify and develop a set of slow speed maneuvers based on relatively simple and yet effective test procedures. The effort focused on the inherent slow-speed maneuvering performance; it did not cover factors such as human control, environment, proximity to moving or fixed objects (except the bottom), and external assistance. The effort was intended to be a first step of an iterative process leading to refined solutions to the needs for slow speed performance evaluation. This paper summarizes the study, the findings based on computer simulations of maneuvers for open deep and shallow water, and the practical issues for pilots in judging a vessel's slow speed maneuvering performance, especially in the restricted environment, such as a narrow channel.
Since its inception in 1 956 discussions among the members of Society of Naval Architects and Marine Engineers (SNAME) Panel H-10 (Ship Controllability) have focused mostly on developing the criteria and design tools needed to assure'adequate ship con-
trollability. At the 1960 Symposium on Ship Maneuverability, Gertler and Gover  suggested criteria which reflected the understanding of that time as derived from analysis of ship trials and model test data. In the 1970's a more integrated focus on the whole ship system, including human operator and the ship operating environment (waterway properties, weather, aids to navigation, electronic charts, propulsion characteristics, etc.) occurred. This holistic view, and particularly the focus on the man-machine aspects of the system, has lead to significant improvements in ship design and operating safety, although the development of widely accepted numerical performance criteria has been difficult to achieve because of practical considerations.
Panel H-10 has made many contributions to improving ship operational safety and to developing maneuvering performance standards, for example:
・Proposals of maneuvering criteria, adequate design processes, and tools, e.g., Panel H-l0 , Landsburg, et al , Cojeen, et al 
・Advocating better approaches to provide and communicate the vessel maneuvering information to bridge personnel and pilots, e.g., Landsburg, et al 
・Providing information for developing international maneuverability regulations and establishing guidelines, e.g., Barr, et al 
・Supporting the U.S. Coast Guard through various activities to help in establishing practical national and international design criteria and guidelines
Through the deliberations of Panel H-10 and activities such as the SNAME cosponsored May 2000 International Workshop on Channel Design and Vessel Maneuverability (Gray, et al ) it has become clear that an increased focus on slow speed operations in shallow water is essential.
The performance criteria recently established by the IMO are a significant step forward in setting numerical criteria for assessing ship maneuvering performance, but the standards address only operations in deep water at relatively high speeds. Today's challenge where a real difference can be made is to address common interests and needs of all operational facets of the maritime industry. Pilots need to know before the vessel arrives how well it will maneuver in their port. Owners need to know what to specify to insure that a vessel will work well in the ports and waterways it will visit. This paper describes an exploratory effort that begins to address the issues of slow speed maneuvering performance from technical and practical viewpoints. The goal is to come up with performance tests, criteria, indices, and tools for designers, builders, regulators, and mariners to assure safe operations in increasingly congested waterways.
In the maritime community, common languages and standard procedures have been established to describe and measure a vessel's maneuvering performance. The performance criteria are also being slowly established in the international community, through the International Maritime Organization (IMO) , ,  For a review of IMO's work on ship maneuverability, refer to Daidola, et al .
However, these common languages, performance evaluation procedures, and standards focus on cruising speed maneuvering behaviors. They provide rather limited information about a vessel's or a model's performance at slow speed. Thus a common ground is desirable for evaluating or comparing ship maneuvering performance in slow speed conditions. These conditions are usually not the central design concerns of most ships - many ships spend very small part of their expected life navigating in the harbor environment. The constrained waterway conditions, however, are important for safe operations and it is critical that the vessel can be handled properly in confined waters with other traffic to avoid potential disaster.
In recent years, the need for expanding the performance indices from cruising speed range to slow speed range has gradually evolved from the needs of operations and simulator facilities:
・Shortcoming of most standard definitive maneuvers and performance indices that do not provide mariners adequate information about a vessel's slow speed maneuvering performance
・Lack of standardized maneuvers and common performance indices for objective math model validation in the slow speed range of ship operations that most simulator training scenarios or research issues are concerned about
・Validation of ship model slow speed performance relies heavily on expert subjective evaluation using the simulator instead of fast-time simulation. Consequently, it is difficult to conduct thorough and effective ship model acceptance tests. There is a hidden cost to simulator operations when slow speed performance is not validated comprehensively
・Lack of common information exchange protocols and criteria among different fields of the maritime industry such as ship design, shipping, piloting, traffic control, port operations, survey, regulation, for objective decision-making about tug deployment, operation hours permitted, berth access, etc.
The objective of this project was to identify and develop a set of slow speed maneuvers, their test procedures, and performance indices. IMO  provided a philosophy for its proposed deep water maneuvering standards; they should be simple, relevant, comprehensive, measurable, and practicable. Following a similar guideline, the maneuvers, procedures, and indices for slow speed should try to satisfy the following requirements :
・When maneuvers are tested systematically, the information should allow characterizing a vessel's slow speed performance relevant to operation
・Test procedures should not be too complex
・Performance indices should be easy to derive, intuitive, quantifiable, and of practical use to both operational people and technical people.
A comprehensive set of slow speed maneuvers from the math modeling point of view, will be too time consuming and costly to develop and even involve uncomfortable risk. Therefore, when developing a comprehensive set of slow speed maneuvers and performance indices for simulation, we should prioritize these maneuvers so that when only few tests are affordable during the field test, the effort can focus on the most revealing maneuvers.
1.3 Scope of Effort
This project is a pilot study intended to be the start of an iterative process that will eventually lead to a refined answer to the needs of slow speed performance indices. This project does not explore what level of performance is acceptable nor does it classify the performance; standards are issues beyond the scope.
The study has focused on the intrinsic slow speed maneuvering performance in open deep and shallow water, it does not include the factors such as human control, environment, proximity to moving or fixed objects, external assistance, physical contact, etc. However, it is recognized that
・Slow speed operations are vulnerable to current, wind, and wave conditions
・Proximity to ship traffic, banks, bottoms, obstacles, etc. and the ways of approaching to or departing from them can affect the hydrodynamic forces and the effectiveness of control devices.
・The strategies applied to deal with a situation vary widely among the mariners with human control playing a crucial role in slow speed operations.
・Control through physical contact or connection, such as tug assistance and dredging anchors are important vessel control strategies.
2. UNIQUE ASPECTS OF SLOW-SPEED SHIP MANEUVERING
Several aspects of slow speed ship maneuvering are described in the following sections. They are not inclusive, but are provided as the background information for exploring the candidate slow speed maneuvering tests and indices.
・During berthing, departing, turning basin operation, and transiting at very slow speed, ship hydrodynamics usually involves a large drift angle β and yaw rate angle γ (Chislett ) as well as large ahead and astern propeller loadings, which can be handily characterized by the propulsion ratio η. Maneuvering forces, particularly rudder forces, are strongly dependent on η. Here
u: longitudinal ship speed relative to water
v: lateral ship speed relative to water
U: (u2+v2)1/2 in ship-fixed coordinate system
Jd: Propeller design advance ratio
J: operating advance ratio u/(nD), where n is propeller rpm, D is propeller diameter
・The fluid flow is quite complicated in the region where propellers, rudders and the hull interact with each other, especially when the steering devices and engine react to frequent piloting commands during slow speed operations where a vessel could have either headway or sternway while the propeller is thrusting ahead or astern. Illustrated in Figure 1 are the four quadrants representing the 4 combinations of speed directions with thrust directions, adopted from Harvald .
・According to Bishop, et al , the stability criteria that are relevant for a steady reference motion do not necessarily apply if the reference motion is not constant. Their analysis poses interesting questions to course-keeping capabilities at slow speed where acceleration/deceleration is frequent.
2.2 Onboard Vessel Maneuvering Control Devices
A vessel is usually equipped with one or more of the following maneuvering devices:
・Rudder - alone or combined with engine-kick
・Propeller-induced lateral forces
・Rotating propulsor, e.g., Azipod and Z-Drive
・Tunnel and other types of thruster
Their application at any time is contingent upon many factors: integrity of 'quipment, effectiveness, passenger comfort, safety, and style. For example,
・Due to the risk of breaking the chain and losing the anchor, anchors are used at very slow speed
・Thrusters start to lose their effectiveness significantly as ship speed goes beyond 〜3 knots.
・A vessel equipped with a flap rudder may use the flap only to maintain the course at cruising speed and take the benefit of the high lift rudder at very slow speed
2.3 Nature of Slow Speed Maneuvering Operations
In contrast to the relative stable ship speed and engine setting at cruising condition, slow speed operations involve various combinations of
- Vessel moving direction (headway or sternway, crabbing and/or rotating to starboard or port)
- Thrusting direction (ahead or astern)
・Regular command and execution of rudder, engine and thruster; vessel reactions are primarily during transient phases and seldom reach steady state
・The vessel is usually in proximity to traffic, bottom, and other restrictive physical environment
・The rudder/propeller configurations (e.g., fixed vs. controllable pitch; propeller(s) rotating direction), the dynamic response and operational constraints of steering gear, engine, and thruster systems all play an important role in deciding a vessel's maneuvering capability at slow speed and thus the strategy for handling the vessel
・For a given power plant and propeller, a change in throttle/rpm/pitch setting can have a significant impact on a vessel's slow speed performance
・Operations are vulnerable to environmental factors such as wind.
Fig.1 Illustration of Quadrants
2.4 Modeling Issues of Slow Speed Maneuvering
・Some math model platforms use 2 separate models and/or data sets to represent cruising speed and low speed maneuvering. If enough care is not exercised, the transition from cruising to slow speed may not be smooth. Sometimes blips can be detected on the digital displays of a simulator.
・Availability of comprehensive field data of slow speed maneuvers for validating the math model is rare
・Simulation results of slow speed maneuvers are not as widely published as those at cruising speed
・Universal definitive maneuvers and performance indices at slow speed have not been established
・Validation of slow speed maneuvering performance relies heavily on expert mariner's judgment and subjective assessment works for qualitative validation, but not for the quantitative validation
・Subjective model validation is usually done on a simulator as it is costly, time consuming, and difficult to achieve a thorough checkout.
・The modeling of engine response and the definition of throttle/rpm/pitch in combinator table have considerable effects on slow speed performance
・A realistic simulation of slow speed maneuvering requires the proper modeling of propeller and rudder forces not only when they operate within each quadrant of Figure 1 , but also during transitions
・Typical maneuvers during harbor approach, channel transiting, turning basin operations, and berthing, can be grouped into several patterns of operation within or between quadrants:
□ Within Quad. 1; e.g.,
- Accelerating ahead from stop or headway
- Cruising ahead
- Coasting turn
- Deceleration by reducing thrust
□ Within Quad. 3; e.g.,
- Backing from stop
- Kick turn astern
□ Quad. 1 → Quad. 4; e.g.,
- Crash stop from headway
□ Quad. 3 → Quad. 2;e.g.,
- Crash stop from sternway
□ Quad. 1 → Quad. 4 → Quad. 3; e.g.,
- Acceleration/Deceleration Combination
- Back & Fill with Fill First
□ Quad. 3 → Quad. 2 →Quad. 1 ; e,g.,
- Backing & Stopping
- Back & Fill with Back First
Table 1 is a summary of the quadrant distribution of IMO recommended maneuvers for the Maneuvering Booklet  (maneuvers in wind are not included). Note that these maneuvers do not cover the full range of operation in terms of the 4 quadrants illustrated in Figure 1. Additional tests are required to allow for more complete assessment of a vessel's performance over the full operational range.
・For certain propulsion-steering configurations, such as Azipod or twin screw, the modeling of steering forces as well as the interactions between propellers/rudder/hull for slow speed maneuvers is complex. As an example, the Aegis cruiser can gain sternway during a Twist maneuver .
Table 1 Quadrant Distribution of IMO Recommended Maneuvers in Calm Environment
|Maneuvering in Deep Water
||Max Rudder Turning Circle from FULL SEA
||Max Rudder Accelerating Turn from rest
||Man-Overboard & Parallel Course
||Turning at 0 Speed
||Turning with Forward Speeds
|Stopping & Speed Control in Deep water
|FULL AST from FULL SEA AHD
FULL AST from FULL AHD
FULL AST from HALF AHD
FULL AST from SLOW AHD
STOP ENG from FULL SEA AHD
STOP ENG from FULL AHD
STOP ENG from HALF AHD
STOP ENG from SLOW AHD
||FULL SEA to STAND BY ENG
FULL AHD to HALF AHD
HALF AHD to SLOW AHD
SLOW AHD to DEAD SLOW AHD
||From rest to FULL SEA
|Maneuvering in Shallow Water
||Max Rudder Turning Circle from HALF
||Desired information specified, but no specific maneuver described
|Maneuvering at Slow Speed
||Desired information specified, but no specific maneuver described
・Validation of simulation models for all shallow water operations is made particularly difficult by the lack of reliable shallow water ship trials data. The few existing shallow water trials data are for minimum water depth-draft ratios of about 1.2. The extensive and carefully conducted trials of the Esso Osaka did include a number of slow speed maneuvers such as coasting turns and zig-zags in deep and shallow water, Crane . Although many laboratories conducted model tests and simulations for the Osaka , only one is known to have tried (with relatively good success) to reproduce the slow speed maneuvers in both deep and shallow water, Miller and Ankudinov . Comparisons of simulated maneuvers (and even more important, the comparisons of forces measured in captive model tests and used to generate inputs to simulation models) raise questions about the adequacy of the methods, even for deep water and higher speed operation, Barr .
・When a model is validated for turning and zigzag maneuvers in both deep and shallow water, e.g,, l .2 depth/draft ratio, at cruising speed, sometimes the model at slow speed in water much shallower than 1.2 ratio, e.g., 1 .1, is deemed too sluggish by pilots. This modeling difficulty can be attributed to complex interactions between the ship and the bottom when underkeel clearance is very small, especially with the soft mud bottom; it is not only difficult to define where the bottom is, but a ship can also generate internal waves, PIANC .
・Stern walk/paddling effect is essential for slow speed operations; some mariners commented that the shallower the water, the more prominent the effect. Compared to the modeling of hull, propeller, and rudder, this interaction phenomenon needs more research for better math modeling
・Memory effect is the hydrodynamic effect on the local flow at some part of the ship due to the earlier flow at a more upstream part of the ship, e,g., the effect of vortices shed from the bilge keel onto the propeller and rudder. Fujino  and Scragg  explained that maneuverability hydrodynamic memory effects could be safely ignored for most deepwater maneuvers. But during slow speed maneuvers, the change of rudder angle, propeller rpm and direction can be quick. Besides, they usually happen in relatively shallow or restricted water. It is yet to be determined whether omitting the memory effects could have a meaningful impact on slow speed simulation.
・The limitations of current hydrodynamic math models mainly come from insufficient knowledge of complex flow phenomena. The modeling of maneuvering at low speed is one of the tasks affected by this deficiency. Progress in basic research in this area is essential to improve the understanding of physics and the simulation model. Innovative use of Computational Fluid Dynamics (CFD) may provide the promising tools .