SHIP MANCRIVRING
CHARACTERISTICS
J. A. H. Paffett
(National Physical Laboratory)
THE ROYAL INSTITUTE OF NAVIGATION
AT THE ROYAL GEOGRAPHICAL SOCIETY
Ship Manceuvring 'Characteristics
J. A. H. Paffett
(National Physical Laboratory)
IV. INTRODUCTION. Like any other vehicle a ship needs to be started,
stopped and steered' safely; the art of doing this is sometimes called ship
handling. Successful ship handling depends upon three distinct opera-tions: acquiring the right information, making the right decisions and
performing the right manceuvres. In this paper we assume that the
Cap-tain has all the facts and knows the right thing to do, and examine the
physical ability of the ship as a mechanism to respond to his will. Compared with other transport vehicles the ship has a remarkable low
resistance to motion ; the drag of a merchant ship commonly lies in the range
one-hundredth to one-thousandth of its all-up weight force. A very large modern tanker has a drag around 1/2000 of its weight force; if it were a
land vehicle it would run away down-hill on a gradient too small to be perceptible. For comparison, a railway wagon has a drag of around
I/ coo of its weight and a road vehicle moving slowly on a tarmac road a
drag of about 1/7o, rising to a tenth or more of its weight force at top
speed. The ship's low drag accounts in part for its extreme economy as a carrying machine in comparison with land vehicles.
2. STOPPING. Unlike land vehicles shipshave no brakes. The only force
available for stopping other than natural friction is that which can be
generated by the propeller running astern. Now the maximum propeller thrust running ahead is only that needed to overcome the hull resistance, which as we have already seen is only a small fraction of the weight force.
114 VOL. 26
Running astern there is usually less power available from the engines, and
the propeller itself is less effective than when running ahead. Thus for stopping the tanker considered above it will be seen that resistance and the propeller can together only provide less than one-thousandth of the weight force; it follows from Newton's laws that our best deceleration will be less than o-oo ig, or very roughly a knot per minute. Trials with such tankers do in fact show that they take 15 or 20 minutes to come to rest from service speed ; from the reasoning above it can be seen that such sluggish behaviour is inherent in the nature of a massive low-drag
vehicle and is not attributable to any avoidable shortcomings in design.
This sluggishness has received considerable publicity, and braking devices intended to improve matters are invented with great regularity. Most involve extending drag-inducing surfaces into the water flow by such means as hinged flaps or parachutes; some experimental work to
assess such devices has been described.! It will be seen that to produce a
worthwhile deceleration we need to generate a braking force well in excess of the naked hull drag, which calls for auxiliary drag surfaces of great strength and weight. The parachute is probably the most effective device in terms of cost or weight but the operational difficulties of streaming and recovering parachutes big enough to be useful must be
severe. The most effective means of getting way off a ship is in fact to use
the rudder ; the act of turning yaws the ship relative to the water flow with a great increase of drag. The consequent loss of speed is familiar to the mariner, and one ship-owner has developed a standard 'rudder
cycling' mana2uvre which gets rid of headway more quickly than would be
possible with any practical braking device.2 Such a manouvre demands
some sea room, and a Captain who would be prepared to justify it before a Board of Inquiry.
3. STEERING. In stopping or starting a ship we change the magnitude
of the velocity vector, in steering we change the direction of the vector. Stopping or starting demands an accelerating force along the ship's
length, steering demands a sideways accelerating force. The simplest way
to generate such a force is to set the hull at a yaw angle relative to the direction of advance; the hull then acts like a short aerofoil generating the necessary lift. The function of the rudder is not itself to produce the sideways force but to set the main hull at the yaw angle necessary to generate this force. During a turn to starboard a ship's head does not point in the direction in which she is moving but measurably further round to starboard, somewhat in the manner of a racing car cornering
with a controlled skid. The ship therefore sweeps out a path considerably wider than her own beam.
The lateral force needed to direct a moving body round a path of given
curvature is proportional to the square of the speed. It so happens that the hydrodynamic forces due to yaw also vary roughly with speed. It follows (at least for merchant ships of moderate speed) that the path
NO. I MARINE TRAFFIC ENGINEERING 115
at constant revolutions cannot turn a tighter corner than one moving
quickly.
The ordinary single-screw merchant ship carries her rudder
im-mediately abaft the propeller, where it enjoys the full benefit of the slip
stream. As ship speed is reduced the slip stream falls also, and rudder forces decrease in proportion to the square of the speed. In the absence of extraneous forces this still enables a course of constant curvature to be followed, but if wind and weather act on the ship, these forces
in-crease relative to the rudder forces until, as speed approaches zero, they take complete charge ; the rudder becomes useless and the ship drifts.
At service speed the single-shaft merchant ship has excellent steering
control. Full helm will send the ship ultimately into a circular course which commonly has a diameter of about four ship lengths. The main limitation of the conventional rudder appears during stopping; if the propeller is stopped or turning astern, the rudder generates little or no force, and so that Captain has no control over the ship's heading. There is some similarity with a vehicle with locked brakes skidding on ice.
The skilled seaman can retain some kind of control by using occasional bursts of ahead power, correcting his heading while rudder force persists but cutting shaft revolutions again before headway builds up.
If stopping is achieved simply by stopping engines, or running them
astern, a considerable distance may be covered before the ship comes
finally to rest, in a direction determined largely by sea and wind. In comparing the steering behaviour of one ship with another frequent
reference is made to directional stability. A ship which has excessive
directional stability is one which demands a large rudder force to achieve
a given rate of turn. A ship with negative stability will insist on turning
one way or the other when the rudder is locked amidships. Comfortable
ship handling calls for a moderate amount of positive stability and this can be provided without difficulty in merchant ships of conventional
form. The very full forms adopted in the VLCC's are, however, less easily
endowed with positive directional stability; some of these ships have
pronounced negative stability.
Directional stability is conveniently illustrated by a plot of rate of turn against helm angle. A stable ship has a single near-straight plot as in Fig. i, which shows that a steady course can be held with zero helm angle. An unstable ship, however, has a plot which bends into a reversed slope near
the origin, the pecked curve in the figure. In this case a large port helm angle will give a turn to port and a large starboard angle a turn to board, but zero helm will give a turn indeterminately to port or to
star-board. In such a ship a steady course can be held only by using repeated
excursions of the rudder towards the determinate part of the curve, a tedious exercise for the helmsman and a drag on the ship. An unstable
ship can be satisfactorily 'ridden' by an auto-pilot, but the drag remains.
The provision of twin shafts does not necessarily make a ship more manceuvrable over-all. Twin shafts do however offer the important
VOL. 26
advantage of enabling a ship to turn in azimuth without gaining headway,
by running one screw ahead and the other astern. The turning action is only in part due to the couple formed by the shaft thrusts; some un-published work at the Admiralty Experiment Works suggests that the greater part of the turning action arises from the propeller-induced
pressure field on the ship's bottom plating. Ship's steering is a
complica-ted subject, but an excellent account of modern steering theory has
recently been published by Burcher.3
4. SHALLOW WATER EFFECTS. In shallow water hydrodynamic
inter-actions occur between the ship's bottom and the seabed and these affect ship handling in several ways: the resistance to ahead motion is increased,
FIG. 1. Relation between helm angle and rate of change of heading plot. Plot illustrating directional stability.
NO. I MARINE TRAFFIC ENGINEERING I 17
the steering is degraded and the hull experiences a bodily sinkage com-bined with a change in trim. Shallow water effects have been known for many years but ships of moderate size have seldom been inconvenienced;
seamanlike precautions (and perhaps the use of tugs) have sufficed to
secure safe handling. In recent years, however, draughts have become so
large that shallow water effects have to be allowed for in sea areas pre-viously regarded as 'deep' and indeed they may be significant many
hours' steaming from port and out of sight of land. The Master of a large ship must therefore take note not only of the sea room around him on the surface but also of the amount of sea-water under his keel.
The increase in drag cuts the speed for a given engine power, or revo-lutions setting, and so could be regarded as making for safety. However it
cannot be assumed that the increased drag will enable the ship to stop more quickly in shallow water than in deep, because the drag increase
can be more than offset by an increase in the hydrodynamic 'virtual mass' of the moving vessel. The result is that the deceleration in shallow water
is actually less than that in deep, even by as much as one-third, with a
consequent increase in the head-reach or distance covered in stopping.
The effect of shallows upon steering is complex. If we consider the
sequence of events as water gets progressively shallower we find first that, in some ships, there is a slight decrease in directional stability, resulting
in an increased rate of turning for a given rudder angle when the water
depth is about 21 to 3 times the draught. This is followed in all ships by a sharp increase in directional stability, so that the turning performance for
a given helm angle is very much worse than in deep water.
This matter of turning performance needs closer examination. As a criterion of turning performance we can use the ship's behaviour when she has settled down on a steady circular course under constant helm
angle. The most convenient quantity to measure is the rate of change of
heading in degrees per second; this demands only the compass and a stop-watch. The radius of the actual turning circle is also important but
its measurement demands special instrumentation, and so radii are not so
often quoted. Sea-going observations in most ships will probably show that shallows only reduce the rate of swing marginally, and it might be
assumed that the radius of turn is only increased in a similar proportion. Such an assumption could however be dangerously misleading because it leaves out of account the change in speed. The loss of speed in turning at constant revolutions in shallow water is less than that during a similar turn in deep water; this can be explained in terms of a virtual mass effect, similar to that already referred to in connection with stopping decelera-tions. Since speed drops away less sharply in shallow water the ship may
sweep out a much larger arc, although the compass repeater may show little departure from its deep-water behaviour.
To illustrate this point we quote some figures from model experiments carried out at the National Physical Laboratory. These were intended to
8
shallow water, the latter representing a depth 1.2o times the draught.
The results were as follows.:,
Rate of change of heading (degrees per second)
Steady speed on circle, as fraction of approach speed
Deep water Shallow water
O54 0.48
0-49 0. 8 5
From these figures it can be deduced that the radius of turn in shallow
water was nearly double that in deep; this ratio was in fact confirmed by a
photographic record of the model's behaviour. It is of interest that a
doubling of the radius has been observed in a full-scale tanIcer2 (not of the
same form as that used for the model tests referred to above) in water with a depth/draught ratio of about 1.4.
In fairness it should be noted that the effect of shallows, in extending the radius, only becomes fully developed when the ship has settled to her final speed and rate-of-turn, a fairly extreme manoeuvre which is probably
not often called for. If such a turn is needed, to avoid obstacles or other
ships, then the effect of shallows must clearly be taken into account. The more moderate manoeuvres used in normal course adjustment are likely
to be less sensitive to shallows.
The third effect of shallow water is the production of bodily sinkage and trim, colloquially referred to as 'squat'. Like the steering effects, squat has been known for many years but has not caused any particular alarm, because conventional ships spend little time in water shallow enough to make squat significant. In any case squat can be reduced by
slowing down and a prudent Master would expect to reduce speed when
approaching shoal water. Today's large ships may, however, still be in
hydrodynamically shallow water when they are far out at sea; the Master
may have to allow for squat as a normal passage routine rather than an occasional harbour hazard. It becomes important to know something of
the nature and magnitude of the squat effect.
Theoretical examination confirmed by experiments with models shows
that a typical modern full-form tanker, initially at level trim, suffers a
bodily sinkage combined with a change of trim in the bow-down direction when moving in shallow water. The effects at the bow are additive. In the case of a 300 metre long tanker moving at 14 knots, an initial under-keel
clearance of 4 metres at the fore-foot would be reduced by squat to 2 metres. An initial clearance of 2 metres would be reduced to zero and the forefoot would strike bottom. The loss of clearance depends upon
the square of the speed, so that squat can always be reduced by slowing
down. The magnitude of the effect is not particularly sensitive to ship form, but it does appear to be worse with the very full forms found in
modern tankers. The figures for squat commonly used at present by tanker
NO. MARINE TRAFFIC ENGINEERING 119 ago ; recent work by N.P.L. suggests that the SOGREAH figures somewhat
underestimate the amount of squat suffered by modern tankers in
un-restricted shallow water.4 Squat may not be a `manceuvring characteristic' in the conventional sense but nevertheless it is a phenomenon which has
to be taken account in handling large ships in such areas as the North Sea and Thames Estuary. It has to be allowed for too in planning new harbour works, dredging schemes and hydrographic surveying
pro-grammes.
It might be argued that this interest in squat is rather academic since underkeel clearance can always be read on the spot from the echo sounder; indeed some tankers have a plurality of sounders for this pur-poseone on each corner. The echo sounder undoubtedly has its uses but it has one major drawback: it indicates what has happened already, when the time for avoiding action has passed. What the tanker Master wants to know is if he is in danger of going aground, anywhere on his
planned route, while he still has time to change his plans ; this means that he must be able to predict soundings, tide and squat in advance, as well as survey their effects in retrospect.
Current work at N.P.L. should enable tanker operators to predict squat magnitudes with some confidence to within a small fraction of a
metre. This facility will however only be of value where tidal predictions
and chart soundings down to 30 metres are available to a like degree of accuracy. Charts and tides however are beyond the scope of this paper. A hydrodynamic phenomenon related to squat is the well-known proximity effect, or 'bank suction', experienced by ships moving in
restricted channels. As the hull approaches the bank, forces arise which drive the ship yet further towards the bank. An inherent disadvantage of
the conventional stern rudder as a steering device is that its use may
actually make the situation worse. In steering away from the bank the
stern clearance is reduced for a while. The successful ship handler puts his helm over soon enough and gently enough to keep his stern from going aground. Awkward though proximity effects are, seamen have developed
the skills to cope with them in canals and harbours. Bank suction also
operates in dredged channels, which nowadays may be many miles from
shore. Here the demands upon the ship handler may be more severe because he cannot see the bank which is 'sucking' him, and a sideways
drift will not show up on the compass. These circumstances demand a
navigational aid which will tell the Master not only that he is somewhere
in the dredged channel, but also his position relative to the two sides of
the channel to within a few metres.
Proximity forces arise too between ship and ship. If ships pass on
opposing courses the forces act only briefly and matter little. The over-taking situation is more serious since the more prolonged proximitymay
enable a dangerous drift to build up. Proximity effects are pronounced when ships steam close together on parallel courses at equal speed, as in
I 20
manceuvre by tankers transferring cargo under way will demand a high
degree of ship handling skill. Proximity forces depend upon speed and so can be reduced by slowing down. This is not necessarily the best
course, however, since rudder forces are reduced by slowing in the same
proportion; slowing down may give the Master more time to think but not necessarily more control over the path of his shipindeed if wind forces are important he may have less control when moving slowly.
s. IMPROVEMENT OF THE SHIP'S MANCEUVRABILITY. We now consider
whether and to what extent it is possible for the ship designer to im-prove the manoeuvrability of the ordinary merchant ship. There is not
much scope for change in the main hull. Admittedly features such as the stern deadwood, the skeg in a twin-shaft ship and the shape of the fore-foot have some bearing on directional stability and there is some room for
adjustment; but generally the hull shape has to be fixed by design considerations, stability, resistance and sea-keeping, which must
over-ride manceuvrability. No great improvement is to be sought here.
The propulsive device offers interesting possibilities. If the full pro-pulsive thrust can be exerted in directions other than along the ship's axis a tremendous improvement in turning performance becomes
pos-sible. Unfortunately there are severe mechanical difficulties in the way, but notwithstanding these a number of commercial vectorable propulsors
have been produced. In the Schottel propeller the complete propeller
and drive assembly can be slewed about a vertical axis like an outboard motor. The VoithSchneider propeller employs feathering blades rotating about a vertical axis. Both devices confer extreme manceuvrability and a
craft with one at each end can execute the most complex gyrations.
However, the vertical axis propeller is mechanically complex and
hydro-dynamically less efficient than the conventional screw. Moreover the
power which can be handled by any of these devices is verylimited; the largest current Schottel unit can handle about i 700 kW and the largest VoithSchneider about 1300 kW of mechanical power.
Another approach is to install auxiliary thrusting devices, separately
powered from the main propulsion system. The lateral thrust unit is the
most familiar example, usually consisting of a propeller operating in a
transverse tunnel near the bows. The ability to crab sideways into a berth is most valuable, but the price includes not only the capital cost but the carriage of a heavy prime mover, which lies idle for most of the time, and an increase in hull resistance of perhaps 2 or 3 per cent, due to the tunnel. The transverse force, moreover, is reduced sharply if there is
even a small fore-and-aft water movement along the ship's side. A variant on the auxiliary thrust is the Pleuger active rudder which carries a small
propeller driven by a submerged electric motor. The rudder can be put over to 9o° and, with the main engine stopped, the auxiliary propeller
will spin the ship about a vertical axis.
None of the manceuvring devices referred to so far in this section are available in sizes large enough to be of any real use in the large ships of
NO. I MARINE TRAFFIC ENGINEERING 121
today. The most useful contribution which could be made to improving the manceuvring of large ships would be the development of a device to
generate really large forces at low and zero ship speeds. No complete solution is in prospect but it is likely that there is still scope for con-siderably improving the effectiveness of the stern rudder. For example
the conventional steering gear stops at 35° of helm; admittedly the rate of increase of lift has fallen off at this angle but a useful increase of force can
still be obtained by increasing the helm angle to 45° or even more. The
drag of course goes up too, but this is a positive advantage in achieving a
tight turn; it enables the engine revolutions to be pushed up for a given
speed, thus increasing the slipstream over the rudder. In one model of a
2 so,000-ton tanker tested at N.P.L. an increase of rudder angle from 350 to so° reduced the final turning circle radius in the ratio i : o.7.
Beyond 450 or so° any conventional rudder will stall completely and lift will fall away. Boundary-layer control techniques are however avail-able which will enavail-able stall to be deferred to higher angles. For instance the replacement of the rudder leading edge by a rotating cylinder results
in attached (i.e., un-stalled) flow right up to the extraordinary helm
angle of 900. A model equipped with such a rudder has a turning circle in
deep water of virtually zero radiusthe ship pivots about a point in its own lengthand even in very shallow water the turning is superior to
that of a conventionally equipped model in deep water.5 Needless to say
there are mechanical difficulties in translating this concept to the full scale. The 25-metre N.P.L. coaster Vic 62 is now being fitted with a rotating cylinder rudder for sea trials off Southampton to enable the practical aspects of such an installation to be explored. The high-lift
rudder is an attractive device because it operates upon the whole available power of the main engines, possibly tens of thousands of kW as against the few hundred available in a lateral thrust unit. The force available from deflecting the slipstream from the main propeller is very large and could well justify the complication of installing a circulation control device on
the rudder.
6. MANCEUVRING CHARACTERISTICS OF THE SHIP SYSTEM. If we
con-sider the ship and her crew together as a 'system' we see that the
execu-tion of a manceuvre involves not only the `hardware'the ship and her machinerybut also the officer-of-the-watch and the helmsman, whose
responses affect the promptness and accuracy of the manceuvre. This paper is concerned with the contribution which the naval architect can make to manceuvrability and it might be thought that crew responses were outside his field, depending as they do on selection and training
rather than material things. Brief mention must however be made of the
ship simulator. Training involves practice, and practice in handling a
large ship is slow, expensive and, conceivably, even dangerous to acquire.
The designer's contribution in this area can be a machine to replace the ship, a shore-based simulator embodying a computer programmed to
depths and weather conditions. On such a machine ship handling ex-perience can be obtained quickly, cheaply and safely. Two such
simu-lators are already in operation in Holland, and proposals for another in the United Kingdom are now under discussion between D.T.I. and interested
bodies in British industry.6
The ship-crew system contains an interface where man and mechanism
inter-act. This lies on the bridge where we might expect some study by the ergonomists, to improve bridge layouts by easing the reading of
instruments and facilitating the transmission of orders.
7. CONCLUSIONS. Merchant ship manceuvring characteristics, which
are very different from those of land vehicles, are well understood. In small ships means exist for achieving extremely high manceuvrability where it is needed. In modern very large ships manceuvrability is less easily improved but there may be some scope for the development of special rudders. In these ships shallow water can seriously influence
handling characteristics and the effects may need to be allowed for in the open sea. The technology exists for conducting ship-handling training on a shore-based machine as an alternative to training on board.
REFERENCES
Clarke, D. and Wellman, F. (1971). The stopping of large tankers and the feasibility of using auxiliary braking devices. The Naval Architect, April.
2 Clarke, D., Patterson, D. R. and Wooderson, R. K. (i972). Manceuvring trials with the 593,0oo ton deadweight tanker Esso Bernicia. A paper presented at the Royal Institution
of Naval Architects, London.
3 Burcher, R. K. (1972). Developments in ship manceuvrability. The Naval Architect, January.
4 Dand, I. W. (5972). Full form ships in shallow water: some methods for the pre-diction of squat in subcritical flows. National Physical Laboratory Ship Division Report 160, January.
5 Steele, B. N. and Harding, M. H. (1970). The application of rotating cylinders to
ship manceuvring, National Physical Laboratory Ship Division Report 148, December. 6 Vickers Shipbuilding Group (1972). The U.K. ship manoeuvring simulator. A paper presented at Vickers House, London, April.
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