LOW SPEED MANOEUVRING CRITERIA: SOME CONSIDERATIONS
Dr. Ian Dand (BMT Sea Tech Limited, United Kingdom)
Abstract: This paper argues that the present IMO Guidelines for merchant ships do not necessarily guarantee that a ship will manoeuvre satisfactorily at low speeds. This is especially so for waters that are restricted in width and depth. After presenting the IMO Guidelines and discussing some of their shortcomings, it is concluded that criteria are needed for low speed manoeuvring in restricted waters. Some suggestions are made as to the form these might take.
In the mid-1990's the International Maritime Organisation (IMO) produced its Interim Guidelines for the manoeuvrability of ships (Reference I for example). These have been ratified recently and provide a set of criteria against which to judge the manoeuvring performance of a wide range of displacement ships. They cover most of the so-called "standard" manoeuvres, used on ship trials for many years, and are confined to calm water conditions with the vessel at her service speed.
These criteria may produce a ship with satisfactory manoeuvrability at its service speed, but it is not certain that they will in every case. However, there is some concern that the standard manoeuvres upon which they are based do not address the requirements for low speed navigation in shallow or restricted waters (Reference 2 for example), where most merchant ships carry out much of their manoeuvring. Furthermore, they are at their most vulnerable in such situations because low speed manoeuvring in confined waters often occurs in the relatively close proximity of other vessels.
With this in mind, this paper surveys the problem and explores the possibilities of setting a further set of criteria, aimed at low speed manoeuvring in confined waters.
1. THE NEED FOR MANOEUVRING CRITERIA
Any vessel at sea must be controllable. If it is not, it is a danger to others, quite apart from the fact that energy is wasted unnecessarily in trying to keep it under control.
A ship should have good handling qualities and these can sometimes lead to conflicting requirements. For example, an ocean-going vessel should have good course stability at its service speed for long ocean passages, but this should be combined with the ability to turn readily in harbour approaches. The first demands that the vessel be quite directionally stable, while the second demands less directional stability and good turning ability.
It is therefore clear that good handling qualities require the correct balance between directional stability and instability, that balance being determined by the tasks the vessel is required to perform. Such a balance is difficult to define, as indeed is the subjective concept of "good" handling qualities.
Therefore, until the profession is able to "design in" required handling qualities by means of control laws (as in some aircraft) it will be necessary to set manoeuvring criteria. Early attempts at this (Reference 3) used historical data to determine criteria for standard trials manoeuvres in the belief that indices for known "good" ships were those that should be aimed at in design. While the notion of "good" was, as stated above, somewhat subjective, the results, and those of subsequent studies (References 4 and 5 for example) provided goals to aim at. There was a danger that ships with "average" rather than "good" handling qualities would result from this exercise, but, for the service speed at least, some criteria began to emerge.
Much of the motivation and need for these criteria was driven by the desire to eliminate the poor manoeuvrability of some bluff-bodied ships, which carried their cargo in bulk, and the potential environmental and other damage they could (and sometimes did) cause. This was used as a driving force which ultimately culminated in the IMO Manoeuvrability Criteria.
2. THE IMO MANOEUVRABILITY CRITERIA
The IMO Manoeuvrability Criteria may be summarised as follows. They comprise four main, and one optional, criteria and are to be satisfied at the vessel's normal service speed in calm weather, in winds less than 10 knots and in the absence of currents.
1. Turning Ability:
Advance ≤ 4.5 L, Tactical Diameter ≤ 5 L. (where L is the ship length in metres).
2. Initial Turning Ability:
With 10°rudder angle to port or starboard, the ship should not have travelled more than 2.5 L by the time the heading has changed by 10°from its initial value.
3. Yaw Checking and Course Keeping Qualities:
・The value of the first overshoot angle in the 10°/10°Z-manoeuvre should not exceed:
10°if L/V < 10 Seconds
20°if L/V ≥ 30 Seconds
(5+1/2 (L/V))°for 10≤ L/V < 3O seconds
where V is the test speed in m/s
・The value of the second overshoot angle in the 10°/10° Z-manoeuvre should not exceed the above criterion values for the overshoot by more than 15°.
・The value of the first overshoot angle in the 20°/20° Z-manoeuvre should not exceed 25°.
4. Stopping Ability:
The track reach in the full astern stopping test should not exceed 15L. However, this value may be modified by the IMO Administration where ships of large displacement make this criterion impracticable.
5. Dieudonne Spiral
Depending on the behaviour of the vessel in relation to the above criteria, a Dieudonne Spiral test can be performed and the width and height of any hysteresis loop determined.
3. SHORTCOMINGS OF THE IMO CRITERIA
Although the IMO criteria serve a valuable purpose by setting goals for standard and other manoeuvres, they have not been beyond criticism. Some of their shortcomings may be summarised as follows:
・They are for deep water and service/design speed only. This is true and suggests that they may not be valid for low speed manoeuvring in ports. However, their use as a design goal cannot be faulted because, in general, design is carried out for the deep water/design speed case.
・They are for benign conditions only and give no indication of the handling qualities in a wind, current or waves. This is also true, but the criteria must be set for a well-defined condition and the absence of complicated variables such as wind, current and waves is a necessity.
・They cover standard manoeuvres only and do not necessarily cover the type of manoeuvring normally carried out by most merchant ships. The standard manoeuvres are well-known and it may be felt that they convey the overall handling qualities of a ship, even though some of them may seldom occur in the life of some merchant ships. A hard-over turn at service speed with a cruise liner or ferry might result in an unacceptable angle of heel which could cause alarm to passengers or cargo shift. The hard-over turning circle may in fact be an echo of manoeuvring requirements for warships when the ability to turn at speed is valuable in battle (hence the term "tactical diameter"); for a merchant ship a full circle turn may be used to get out of trouble in deep water, but its consequent reduction of speed may be of more benefit. It is true that there are some severe turns that have to be made by merchant ships entering or leaving port, but these are usually not for the full 360° and are generally at low speeds in limited water depths.
・The full astern stopping test on trials puts too much strain on the prime mover and propulsion train. This is heard as a plea from shipbuilders anxious not to over-stress new machinery, but we may also ask whether the test itself covers all practical possibilities for a ship which may have to carry out emergency stops in the shallow and confined waters of a port? Admittedly this should be from an approach speed lower than the service speed, but the ability to stop quickly in confined waters is a vital requirement of many merchant ships.
・The criteria are derived from databases heavily biased toward tankers and bulk carriers. Whereas there may be some truth in this, the criteria have now been in use for some years for a wide variety of ships, from tankers to warships. Whether this has improved the overall safety of operations world-wide remains to be seen.
Whether these criticisms, taken as a whole, detract from the value of the IMO Guidelines as a design goal is difficult to say. For this particular purpose, it could be argued that they are satisfactory and provide sufficient constraints to result in reasonable handling qualities at sea and at service speed.
But what of their relevance to low speed manoeuvres in confined waters, a situation for which they do not appear to have been developed?