Merchant ships have to carry out some of their most complex manoeuvres at speeds lower than that for which they were designed, often in shallow and confined waters. Furthermore they are often in close quarters situations or comparatively near to fixed structures or land. Examples are transits of the Suez and Panama Canals, access to and from many of the shallow water ports of Europe, transits of the Ma Wan Bend in Hong Kong or the Thorn Channel at Southampton and many of the confined harbours on cruise ship routes.

　

 In addition, the desire for some types of merchant vessels (from comparatively small coastal vessels to cruise ships) to be able to self-berth, has added a new emphasis on low speed manoeuvrability and handling qualities.

　

 Is it time, therefore, to consider a set of guidelines for low speed manoeuvring (including shallow water) or can we rely on the present IMO guidelines?

　

 We have seen that the IMO guidelines are open to criticism, some of which applies to low speed manoeuvring. But, do the standard manoeuvres of IMO, carried out in deep water at service speed, have any relevance to low speed manoeuvring in shallow and confined waters? To demonstrate the difference shallow water can make to a standard manoeuvre, Figure I shows the measured characteristic curves of a, by now obsolete, container ship model in deep and shallow water. The vessel itself had twin screws and a single rudder and had very poor handling characteristics in confined and shallow waters. Figure I shows that, had the vessel achieved the IMO criterion for turning ability (at its high service speed in deep water) it would almost certainly not have done so in shallow water.

　

　

　

 Low speed manoeuvres also differ, in their demands, from those associated with the design case at service speed in deep water. Typical manoeuvres expected of a ship at low speed, in shallow water, are as follows:

　

 It is usually vital in port approaches that speed can be reduced under control without excessive sheer, especially when stopping,

　

 The ability to stop quickly without excessive transfer is also vital, Such crash stops would usually be from a speed lower than service seed.

　

 Some high-powered diesel vessels may not possess a "Dead Slow Ahead" setting low enough for the speeds required in port approaches. In some cases an auxiliary electric motor is used for shaft speeds below the diesel engine stall speed, specifically for this eventuality. Control at such speeds may also be maintained by the use of rudder angles greater than +35°, usually set by an interlock with speed.

　

 Self-berthing vessels need to be able to sideslip under control, often against a beam wind.

　

 The kick ahead is invaluable in low speed manoeuvring; its purpose is to cause the vessel to change heading without moving ahead appreciably and is very useful when positioning a vessel at very low speeds. The manoeuvre is most effective when the vessel has a high virtual mass and a rudder (or rudders) located abaft the propeller(s) to gain the benefit of the propeller slipstream, Used properly, the screw bias of a single screw ship also plays its part in a kick ahead.

　

 Vessels moving at low speed are often required to swing - in response to a kick ahead for example. The ability to check the swing is, of course, of equal importance.

　

 Some merchant vessels may be required to make stern boards, sometimes of significant distance, Examples are found in some container terminals when large container vessels must move astern from the swinging area to a berth.

　

 Interaction in canals, rivers and fairways can cause a vessel to sheer away from the bank. Sufficient control must be available to allow such situations to be recovered satisfactorily.

　

 Transfers of cargo under-way from large to small tankers occurs in various parts of the world and is often done at low speeds in water whose depth may or may not be restricted. Good control in this situation, in which ship -ship interaction may occur, is essential.

　

 Vessels can sideslip over the ground in strong currents using propellers, rudders and bow thrust units. By correct orientation to the flow, the "lift" generated by the current can be utilised to sideslip into a berth (see Figure 2). Such manoeuvres are used in regularly on the Humber Estuary in the UK when the current is acting in many ways as a surrogate tug.

　

　

 At low speeds, wind tends to have more of an effect because the apparent wind is more likely to move to the beam. The ability to handle this without drifting significant]y off course is essential in confined waters.

　

 While not a manoeuvre, the field of view from the wheelhouse can have an effect on the way a ship behaves in restricted waters. No matter how good its handling qualities may be, if its handlers have no clear idea of its immediate environment, the ship will be handled differently to one with good all-round, and vertical, fields of view.

　

 In order to perform these and other manoeuvres at low speed, a ship must have good low speed handling qualities. Engine and rudder response must be reliable and positive and the response of the vessel must have no hidden and unexpected vices. There is therefore a need to define good low speed handling qualities.

　

 This leads to the concept of separate low speed manoeuvring criteria and the need for them is perhaps emphasised when the existing IMO manoeuvres and criteria are judged against the low speed manoeuvres listed above. It is clear that they give no positive information about ship behaviour at low speed in confined waters. They were not designed to do this. However, the inference is that good manoeuvring Performance at service speed in deep water will result in good low speed performance in restricted waters. It would be unwise to rely on this inference.

　

 Of the IMO Criteria, turning circle characteristics have little relevance to low speed manoeuvring in port, although advance and transfer to 90°heading change are useful for some of the bends which have to be navigated. They would be more useful if the way they changed with water depth was known. The Course Changing manoeuvre gives an indication of the ship's response to the rudder. It is a useful measure, but it is doubtful if deep water behaviour reads across to that in shallow water. Z-manoeuvre results showing course-checking ability are open to the same criticisms, whereas for stopping ability, of great value in confined waters, the IMO Criterion gives no indication of the ability to stop, under control, from, say 10 knots, rather than service speed.

　

 There is no indication of backing or sideslipping performance, or the ability of a ship to counter a strong wind, especially one on the beam. Kick ahead performance is not dealt with and neither is the ability to manoeuvre in a current or in a narrow channel.

　

 Before speculating on what form low speed criteria might take, one important point should be made. At low speeds in confined waters, ships can make use of tugs. However, these are generally used with large or unwieldy ships, especially in severe conditions; smaller ships may not use them at all. They are almost universally used for very low speed manoeuvres as the vessel berths and unberths, often for comparatively brief periods, leaving the vessel to fend for itself in shallow and confined waters, once its speed is above about 5 or 6 knots. Furthermore, the increasing trend toward self-berthing with some ship types, in order to avoid tug charges, makes good low speed handling all the more desirable.