8. LOW SPEED MANOEUVRING
Ships have to call in port regularly. A minimum sideways performance has to be guaranteed by the vessels. The low speed manoeuvring performance is, however, not a priori a safety criterion, but a mission criterion. Some vessels do not need low speed manoeuvring capability, as they are guided by tugs during the complete port entry process. For vessels like this (large tankers and large bulk carriers), escort tugs are required and are becoming more and more standard (see ).
For a larger group of vessels a minimum level of lateral control at zero speed is required. Owner requirements tend to pay more attention to this aspect. It was summarised by Quadvlieg and Toxopeus () that there was a minimum wind speed of 20 knots wind defined for ships leaving the quay. For ferries and cruise liners, 30 knots is used often, and even sometimes 40 knots. Another criterion that is seen more and more is the ability to turn on the spot within a square area of 2 ship lengths within a certain time frame. The way to verify this requirement is to perform model tests. Especially close to the quay, so many interactions are taking place that no satisfactory easy calculation methods are present. In , it was demonstrated that error margins of 100% can be found in this way. The best way to verify this is to perform static tests, eventually combined with bridge simulations.
9. STEERING IN WIND
When a ship slows down in wind, its heading will come up into, or pay off from the wind. This will depend on the design of the ship and the heading with respect to the wind. Roughly above a critical ratio of wind speed to ship speed, the ship looses "steerageway" and all headings cannot be held, even with full rudder and all power applied. Publications like  refer to methods to calculate the "control boundaries", the wind velocity at which the rudder angle exceeds 20 or 35 degrees.
When vessels are sailing in wind, and this wind is coming from an oblique direction, the vessel will have to maintain a minimum speed in order to be able to counteract the wind forces. Given a certain speed (say 6 knots) and a certain wind velocity (say 20 knots) from any direction (from bow wind to stern wind), this results in drift angles and rudder angles. These drift and rudder angles can be calculated. The additional resistance is calculated as well. This is demonstrated in Figure 11 , showing the performance of two different ship concepts (a podded ship and a conventional ship) while sailing in wind (see also ). It demonstrates that in stern quartering wind, the required rudder angle to keep control is 25 degrees, while the pod angle of the same concept would be 15 degrees. This means in this case that the rudder configuration would not be acceptable. Only 10 degrees rudder angle left as margin for dynamic effects is not enough. The experience learns that a maximum rudder angle of 20 degrees is just acceptable. A second aspect that could lead to unacceptability is the increase of the added resistance and a possible insufficient engine power. In case the engine power is insufficient to keep speed, an unacceptable situation is found.
Required steering angle to keep course for a pod-driven vessel versus a conventionally propelled vessel
These wind control boundaries can be calculated based on the derivation (calculation or model measurement) of the so-called current forces and wind forces, the rudder forces as function of propeller loading and so on.
This results in three equations (X, Y and N) and three unknowns (drift angle, rudder angle and propulsion power) for each wind speed and ship speed.
10. STEERING IN WAVES
The control in waves can be analysed to boil down to three issues: (1) How much rudder action is needed in moderate waves with a realistic autopilot? (2) Is a ship still controllable in extreme waves or will the ship broach too easily and what are the limiting wave conditions for uncontrollability? (3) What are the limiting wave conditions for adjusting the ships heading (changing from following waves to head waves for example)?
Issue (1) is an aspect that concerns economics. With to many rudder actions, resistance and hence fuel consumption increases. If the rudder angle oscillations become too high, issue (1) becomes automatically issue (2). Issues (2) and (3) are both aspects related to safety. Fortunately, more and more attention is drawn to the subjects, not only from a hydrodynamic point of view (see amongst others  and ), but also from the regulatory bodies (see ).
Within the CRNavies research forum of MARIN, FREDYN is developed (see amongst others  and ) which is used to study the aspects related to steered vessels sailing in extreme waves. The controllability in extreme waves is a subtle combination of very good craftsmanship and difficult hydrodynamics. The study to verify whether heading changes are still possible in certain waves is therefore difficult. Criteria are not set yet. A research effort is, however, made to do so. The FREDYN code (accurate predictions of broaches and capsizes) was developed as a fast time simulation program, is also mounted to a full mission simulator. In this simulator, now extensive possibilities are present to study these phenomena. Even better, if the study comes up with a good guidance, officers can be trained on keeping control in extreme waves, which they will hopefully never encounter in real life. This guidance could be in the form of a two-dimensional chart with ship speed on one axis and wave height on the other. Areas in the chart would then define the combinations of speed and wave height that allow the ship to turn easily, or combinations which could endanger the ship and crew.
Criteria of how good the ship performance should minimally be, are not yet defined; but a criterion could possibly read like this: "The ship must be able to execute a 180 degrees course change with an initial speed of 40% of the maximum speed and at a rudder angle of 2/3 of the maximum rudder angle in waves of say 6 metres height."
This leaves the crew with a margin in ship speed and rudder angle to play with in order to be able to execute this difficult manoeuvre.
11. MANOEUVRES IN CONFINED WATERS
As stated by many authors, manoeuvring in confined waters may be most critical. Interaction effects between banks and other vessels are causing impulses on the own ship and on other vessels. The hindrance causes undesired motions and sometimes uncontrollable motions. Especially since today's economics are pushing us on one hand to perform economy of scale on the ship size and on the other hand to save money on dredging (to increase canal depths) and infrastructure investments (locks and bridges). Ships sailing in these restricted areas require special treatment to assure a minimum level of controllability. Traditional inland vessels on the European inland water system will also comply with this: their rudder designs and rudder areas are advanced compared to seagoing vessels. Dijkhuis et al.  illustrate how the Dutch government is dealing with the admittance policy and with the criteria for this. In the mean time, criteria are developed and acting. These criteria are posed on course keeping ability, turning ability and minimum achievable speed. Criteria are valid for the lowest and highest loading condition and for different water depths.
12. LIST OF CRITERIA
Based on the discussion of manoeuvring aspects above, the following list of criteria should be considered:
・The IMO criteria with respect to course keeping and turning should be met, also at any water depth up to Wd/T=1.3.
・The residual rate of turn ratio should be below 0.3.
・The maximum heel angle due to steering should be below 13 degrees.
・The constant heel angle due to steering should be below 8 degrees.
・Ships having special steering devices should also fulfil the requirements. The steering angles are equal to the steering angles of rudders. That means that the criteria for a 10/10 zigzag test are valid for 10 degrees pod angle, waterjet or thruster angle as well.
・Depending on the mission of the vessel: leaving the quay should be possible at wind speeds below 20 knots, 30 knots or sometimes 40 knots.
・Necessary rudder angle in wind of 40 knots at 8 knots ship speed should be less than 20 degrees.
・The ship must be able to execute a 180 degrees course change with an initial speed of 40% of the maximum speed and at a rudder angle of 2/3 of the maximum rudder angle in waves of say 6 metres height.
13. CONCLUSIONS AND RECOMMENDATIONS
When examining the requirements regarding the manoeuvring behaviour of the ship during its full operational life, the following conclusions and recommendations are made regarding applying criteria to the manoeuvrability of ships:
・The "IMO A751 requirements" only relate to a small portion of what should be considered as "safe manoeuvring".
・A set of other criteria is proposed that cover a larger area of ship operations than the aspects covered by IMO resolution A751(18).
・Despite that designers can design ships fulfilling minimum manoeuvring requirements, it does not eliminate the need to provide better guidance to the operators of the vessel. The pilot card, wheelhouse poster and manoeuvring booklet should be provided with better information and assist the operators in their daily work.
 Rhee, K.P.; On the yaw-checking and course keeping ability of IMO standards. Group discussion on IMO standards and ITTC, September 10, 2003.
 Arai, Y., Kobayashi, H., Fukuto, J. and Endo, M.; A simulation study on the valuation of the ship control difficulty in restricted waters. Proceedings MARSIM 1993.
 Biancardi, C.G; Integrating ship manoeuvrability with safety. Proceedings MARSIM 1993.
 Hoekstra, M.; Numerical simulation of ship stern flows with a space-marching Navier Stokes method. Thesis, Delft University of Technology, October, 1999.
 Proceedings of ITTC Manoeuvring Committee, September 2002, Venice, Italy, Appendix A. Manoeuvring in shallow and confined waters.
 Toxopeus, S.L. and Loeff, GB.; Steering fast vessels with pods. HIPER 2002, Bergen, Norway.
 Jong, J.H. de, Jagannathan, S., Gray, D. and Mathai, T.; Tanker Escort Requirements, Assessments and Validation. Annual Meeting of The Society of Naval Architects and Marine Engineers, Washington, U.S.A., October 1995.
 Quadvlieg, F.H.H.A. and Toxopeus, S.L.; Prediction of crabbing in early design stage. PRADS 1998, The Hague, Netherlands.
 Martin, L.L.; Ship Manoeuvring and Control in Wind. SNAME Transactions Vol. 88, 1980.
 Hooft, J.P. and Pieffers, J.B.M.; Manoeuvrability of Frigates in Waves. Marine Technology, Vol. 25, No. 4, Oct 1988, pp. 262-271.
 Tuite, A.J. and Renilson, M.R.; The controllability of a small vessel operating in severe following seas. Int. Shipbuilding Progress, Vol. 46, No. 446 (1999) pp. 129-140.
 Guidance to the master for avoiding dangerous situations in following and quartering seas. MSC/Circ. 707, October 1995.
 Kat, JO. de; Extreme rolling, broaching and capsizing - Model tests and simulations of a steered ship in waves. Naval Hydrodynamics Symposium, Washington, D.C., August 1998.
 Dijkhuis, T., Toorenburg, J.C.K. van and Verkerk, F.; New manoeuvring criteria for the river Rhine. Proceedings MARSIM 1993.
Ir. Frans Quadvlieg is manager of the department Ships-Manoeuvring at MARIN. He achieved his MSc in Naval Architecture/Ship Hydrodynamics in 1992. Since 1993 he has worked at MARIN in several positions, from Project Manager Software to department manager being responsible for all projects related to the steering and manoeuvring of ships.
Ing. Pieter van Coevorden works in the Hydrodynamic Department for the Royal Netherlands Navy. As such he deals with the manoeuvrability of naval vessels, ranging from frigates to landing craft and submarines.