3. MOTIVATION FOR A QUEST FOR MANOEUVRING CRITERIA
One of MARIN's core businesses is to perform an adequate judgement and if necessary improvement of a ship in the design stage. The request towards the ship's performance in terms of manoeuvrability is in general: "the ship shall be good manoeuvrable". This means that IMO criteria have to be fulfilled.
However, the "IMO A751 requirements" only relate to a small portion of the ship's "safe manoeuvring".
The following highly relevant operational aspects are important as well and there are questions that need to be answered in MARIN's day-to-day business:
・Adequate manoeuvrability in shallow water.
・Maximum achievable wind forces for harbour manoeuvring.
・Low-speed manoeuvring capabilities.
・Steering in wind and waves at relatively high speeds. Ability to execute a 180 degrees. Turn in waves.
・Limited heel angles.
・Adequate straight-line stability.
・Low speed manoeuvring.
When judging the capabilities of each vessel on these items, one should make a distinction between the norm needed to achieve safe vessels and the norm to achieve that the ship is "adequate to the mission it has to perform".
In some cases, these questions can be answered using statistics: comparison to similar vessels. MARIN is in the lucky position that an enormous amount of information has been compiled, either as full-scale measurements, basin model-scale measurements and bridge or fast-time simulations. This enabled us to build up a statistical database containing the behaviour of a large amount of vessels. Each new vessel can be rated against existing vessels of the same type and the same mission. The qualifications "superior to", "comparable to" and "worse than" are related to this database.
However, databases alone are not enough. For vessels with special missions, or special designs, comparison is difficult. This means that in this case a more detailed study towards "which manoeuvring behaviour is acceptable" should be carried out. A bridge simulator is the best tool for this.
Having this in mind, a questionnaire has been put together. Using interviews, ship operators are asked their opinion on the minimum required steering behaviour of vessels. Students of the Dutch merchant academy have performed these interviews on a series of conning officers of Dutch-flag vessels. The inter-views were especially focussed on obtaining information on both IMO types of manoeuvres and non-IMO manoeuvres. This gave us the lead to the importance of heel angles and the performance in shallow waters. This led to important conclusions:
・Ballast load is often the easiest condition, but especially the heel angle is of importance.
・Shallow water at moderate speeds (for example sailing in the Strait of Malacca).
・Heel angles during manoeuvres are of significant importance.
・Low speed manoeuvring is depending on the ship type. There is a class of vessels which always uses tugs (bulk carriers, large tankers). Another class preferably uses no tugs.
Having observed this, it is the work of the hydrodynamicists to translate this information in "objective" limiting values. The target should not be to obtain a method to express the manoeuvring qualities in one number (such as propagated amongst others in  and ). It is preferred to have individual criteria for each quality. With individual criteria, it can be recognised which qualities of the ship are fulfilled and which qualities need to be improved. In that way the hydrodynamic optimisation can focus on the criteria that are not fulfilled. Furthermore, the criteria should be such that they can be checked by objective tests. In this respect, free sailing model tests are the best methods, giving high quality results within a short time.
While performing the research, it became apparent that the quest for criteria is answered in a different way for ship designers and for ship operators.
Operators tend to require answers to "knowing how the ship reacts". In a way, a much more enhanced "wheelhouse poster" is required. From the operator's side, amongst other, the following questions were rated important:
・How different does my ship react in shallow water?
・Up to which speed is my ship reacting on its rudder?
・How much bow thruster force can I expect as function of the forward speed?
・At which wind speed do I need tugs?
The designers (with the hydrodynamicists on their side) are, however, trying to derive criteria from that in order to build ships adequate for sailing and fit for purpose:
・What is the required size and type of my rudder?
・How many bow and stern thrusters are required and which power is necessary?
・Which heel angles are acceptable?
Having recognised these aspects, it became clear that two aspects are important: guaranteeing a minimum level of manoeuvrability (depending on minimum safety level and on mission requirements) and supplying more information with respect to the manoeuvrability to the crew.
In the following sections, several criteria for non-IMO conditions are treated. Each of the individual hydrodynamic aspects are indicated as points of attention indicated by the interviews. The knowledge on the subject is fed from experience in the basins and from full-scale observations. We are focussing on the safety issues, and not on individual requirements for the mission. Subsequently, the following aspects are discussed:
・Heel angles due to steering
・Steering with special devices
・Steering (controllability) in wind
・Steering (controllability) in waves
4. INFLUENCE OF SHALLOW WATER
Manoeuvring properties are of particular importance in shallow water. The following three aspects play an important role:
・Due to squat, caused by increased velocities under the vessel, the ship will trim and sink, causing a different submerged portion of the vessel and causing (in case of bow down trim) a more course unstable vessel.
・The flow of water around the aft ship will become much more difficult, especially for a beamy vessel. This could cause flow separation and therefore undesired loss of effectiveness of the rudder. The different wake fields in deep and shallow water are illustrated in Figure 4. This figure illustrates the wake field at the location of the propeller for deep water (left-hand side) and for shallow water (right-hand side). This has been calculated using MARIN's RANS calculation software PARNASSOS .
Fig. 4 Wake field in deep and shallow water (Courtesy of IHC Holland N.V. Dredgers)
・Due to the larger overflow velocities under the vessel, the forces due to drifting and rotating are changing. This results in increasing lift and drag coefficients (see the excellent work of the ITTC in ).
A well-known example is the Esso Osaka, which demonstrated an increase of the advance and tactical diameter with decreasing water depth. Most cases are indicating the increase in course keeping stability in shallow water. However, many ships also encounter decrease of course keeping ability in shallow water. As an illustration, we refer to Figures 5 and 6, in which it is demonstrated with a computer simulation model based on cross flow drag theory and adaptation of the hydrodynamic coefficient based on the shallow water coefficients, that the course instability grows. While for the directionally stable vessel, the overshoot angles decrease from 6 degrees to 4 degrees, for the course unstable vessel, the second overshoot angle increases from 29 degrees to 38 degrees (the criterion for the second overshoot angle was 34 degrees).
Fig. 5 Overshoot angles for a directionally stable vessel
Influence of water depth
for marginally directionally stable vessel
Fig. 6 Overshoot angles for a marginally directionally unstable vessel
Influence of water depth
for marginally directionally instable vessel
This means that (without attempting to generalise the problem) that course unstable ships will become more course unstable and course stable vessels will become more course stable. This means that course unstable vessels will have more difficulty fulfilling the IMO criteria towards overshoot angles in shallow water. Course stable vessels will, however, have more difficulty in achieving the manoeuvring towards advance and tactical diameter.
Vessels that will have to sail in shallow water, which are 90% of the vessels, should therefore fulfil not just the IMO criteria, but should remain significantly under the criterion to have enough margin to fulfil the criteria in shallow water as well. The target is that the vessel should have enough course stability and turning ability to fulfil the IMO requirements with respect to overshoot angles and turning circle dimensions in shallow water. The criteria should be fulfilled in water depths larger than 1.3 times the draught of the vessel (Wd/T < 1.3). At this water depth, the speed is obviously lower than the speed used for the tests at deep water. With this decreasing speed, the overshoot angles decrease as well.
A set of free sailing model tests or captive (PMM or rotating arm) tests at both water depths (deep water and the lowest keel clearance) will give these results.