5. LOW SPEED MANOEUVRABILITY CRITERIA
It is one thing to make a plea for low speed manoeuvrability criteria; it is quite another thing to define them.
At the outset it has to be admitted that the chances of running standard, or any other, manoeuvres with every newbuild in shallow water is virtually zero; the problems associated with such tests are well demonstrated in Reference 6. It is very difficult to find a sea area with a sufficiently constant restricted depth in which to carry out turning circle trials and even more difficult to find a similar area big enough to contain Z-manoeuvres. Add to this the need to explore the effect of another variable - water depth - and it becomes clear that formal trials are very unlikely to be a viable proposition. So, manoeuvring criteria based on such trials are unlikely to be of any value.
However, are formal trials really necessary? Perhaps a number of indices, based on past performance and model test data, focussed on low speed manoeuvres of the type listed in Section 5, would suffice. Furthermore, if there were sufficient confidence in the predictive powers of simulation models, the low speed confined water handling qualities of a new design could be determined by computer.
Therefore it is suggested that a combination of a simulation model and suitable indices, derived from past best practice, be used as a way of assessing the low speed manoeuvrability of vessels. This is analogous to the present criteria, which themselves are based on best past practice, but relate to standard manoeuvres carried out as part of a new ship's trials programme.
What form should the indices take? Perhaps they could be broadly classed as follows:
5.1 Indices Derived from the Geometry of the Ship
These could be determined at the preliminary design stage, and in may cases are (or can be) used in preliminary design. Examples are:
・Rudder area ratio, aγ/LT
・Lateral area ratio, Ay/LT
・Lateral and longitudinal above-water aspect ratios, 2Ay/Lpp2, 2Ax/Lpp2
Where aγ is rudder area
L is a suitable length
T is draught
Ax is end-on windage area above
Ay is lateral windage area above
Lpp is length between perpendiculars
5.2 Operational Indices
These would relate to the operations the vessel may have to undertake at low speeds and could include:
・Limiting Froude Depth number, v
gh. This would indicate whether transit speeds expected of the vessel were possible, bearing in mind increases in resistance associated with sub-critical speeds in shallow water
・Lateral thruster power per square metre. Figure 3 shows a plot of lateral thrust unit power per square metre of lateral underwater area. It may be noted that those ships of high windage with self-berthing abilities have values greater than 1.0 kW/m2. Other vessels of similar windage (such as laden container vessels), which are not expected to self-berth, have values less than 1.0 kW/m2
・Depth/draught ratio. This will limit operations due to squat and motions. Limiting values are often set by ports for safe operations; if this compromises cargo carried, it can become an important low speed index
Figure 3 Lateral Thrust Unit Power per unit of Lateral Underwater Area
Lateral Thruster Power
Although data on stopping ability in shallow water can be collected on the ship when in service, the ability to stop under control within a specified distance in shallow water should be subject to scrutiny at the design and development stage. Criteria for head reach and lateral deviation should be set, both being of importance in confined waters. Stopping should be from harbour speed (say 10 or 15 knots) and, the track reach criterion should be less than that in the present IMO Criteria, say 7 to 10 ship lengths. Lateral deviation should be small, ideally less than a beam and certainly less than one ship length. This would allow controlled stops to be undertaken in approach channels.
Clearly the ability to measure such performance in the real world is, as already mentioned, unlikely to occur on a routine basis. It would seem reasonable to assume, however, that a combination of physical and computer modelling could be used. It is well-known that scale effects and the proper modelling of power train characteristics give stopping tests using physical models a limited validity. However, perhaps this could be overcome with computer control of the model offsetting these problems, or the physical model could provide suitable information for a simulation which is then used to predict stopping performance. Handling techniques would be appropriate here, with the use of rudder angles in excess of 35°being explored. Such techniques have been tested with simulation in the past, and have resulted in satisfactory full scale stopping behaviour.
Breasting must be carried out at very low forward speed and the criterion should be related to any opposition provided by the wind, or possibly, current. The criterion could easily be couched in terms of a desired beam wind (and/or current) speed, wind speeds of 15 to 25 knots seeming to be appropriate.
Such a criterion would be largely of value for self-berthing ships and not only would they have to withstand the limiting wind, they would have to move against it in a defined water depth. This criterion could also be checked by means of model tests or simulation during the design stage. The model tests would, of course, need to be carried out in the wind tunnel as well as the test tank.
5.5 Kick Ahead
It is possible that a "kick ahead" criterion could be checked in shallow port approaches during builder's trials. Perhaps a specified shaft rpm should be applied for a specified time and the heading change over this time period noted. Physical model tests may not be helpful here due to scale and other effects, but, again, a combination of physical and computer modelling may provide the answer.
7 CONCLUDING REMARKS
The purpose of this paper has been to provoke discussion. It has not provided many answers; indeed, it may not have asked the right questions. But, in this age of standards, regulations and criteria, it seems extraordinary that the low speed manoeuvring of a ship should be defined almost by default, rather than design.
Many accidents to ships occur when they are operating at speeds lower than those for which they were designed and it therefore seems reasonable to plea for greater attention to operation in such conditions.
It is of interest to note that, at present, some criteria for low speed operation are not set by ship designers, and operators, but by those that have to deal with vessels in confined waters, namely the port and harbour authorities. Much thought and design goes into approach channels, swinging basins and multiple ship operation so that ships may operate in safety at low speeds in a range of environmental conditions. An important feature of these deliberations is the part played by low-speed manoeuvring and the allowances to be made. Often such allowances must be made with the most sketchy data and the benefits of suitable criteria in this context should not be underestimated. Safety, as well as efficiency, of operation is at stake and few artefacts are more hazardous that large ships difficult to control at low speeds in busy waters.
Lest it be argued that all low speed manoeuvring problems can be solved by the use of tugs, it should be mentioned that most tugs, world-wide, can only assist effectively at ship speeds less than about 6 knots. In most cases they work in harmony with the ship and their job is made more difficult if they have to make up for deficiencies in ship behaviour. Furthermore, entrance to some ports of the world demand complex manoeuvres of ships at low speeds in restricted waters, long before tugs are made fast. In some cases, it is true, escort tugs are available, but they are for use in extremis and not to help the ship to manoeuvre under normal circumstances.
It is therefore suggested that criteria for low speed manoeuvring be given serious consideration for the future. Some naval architects have accepted this need and designed their ships accordingly. The vast majority, however, leave low speed manoeuvrability in the hands of others who, by their shiphandling skills, maintain safe operations at low speed in shallow water.
International Maritime Organisation (IMO), "Manoeuvrability of Ships and Manoeuvring Standards" Report of the Working Group, Doc DE36/WP.3
2 Coates G. A. and Guicharrouse, M.:
"A Shiphandlers View" International Conference on Ship Manoeuvrability. Prediction and Achievement, RINA, London 1987.
3. Gertler, M. and Gover, S.C.:
"Handling Quality Criteria for Surface Ships" First Symposium on Ship Manoeuvrability, DTMB report 1461, October 1960.
4. Cojeen, H. P., Landsburg, A. C. and Macfarlane, A. A.:
"One Approach to the Development and Achievement of Manoeuvring Standards" Internation Conference on Ship Manoeuvrability, Prediction and Achievement, RlNA, London, 1987.
5 Barr, R. A.:
"Estimation of Ship Manoeuvrability using Ship Trials Data Bases" International Conference on Ship Manoeuvrability, Prediction and Achievement, RINA, London 1987.
"Manoeuvring Trials of the 278,000 dwt Esso Osaka in Shallow and Deep Waters" Exxon International Company report E11.4TM.79, January 1979.
Dr Ian Dand is Director of Hydrodynamics for BMT SeaTech Limited in the UK. A naval architect by profession, he has been involved with ship model testing, manoeuvring and simulation for over 30 years. He has been Secretary and Chairman of ITTC Manoeuvring Committees and Committees dealing with the Safety of High Speed Craft. He is a Fellow of the RINA and of the Royal Academy of Engineering.