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Ship Manoeuvring

Effect of Wind and Current on Ship Handling

The following figures illustrate the position of the Pivot point as a vessel moves from a position of stop to one moving ahead and astern,

The pressure of the water that acts on the bow or at the stern brings about a shift in the position of the Pivot point.

In this situation no forces are involved and the ship has a pivot point coinciding with its centre of gravity approximately amidships.

Two forces now come into play.  Firstly, the forward momentum of the ship and secondly longitudinal resistance to the forward momentum created by the water ahead of the ship.  These two forces must ultimately strike a balance and the pivot point moves forward.  As a rough guide It can be assumed that at a steady speed the pivot point will be approximately 25% or a 1/4 of the ship's length from forward.

Making Sternway

The situation is now totally reversed.  The momentum of sternway must balance longitudinal resistance this time created by the water astern of the ship.  The pivot point now moves aft and establishes itself approximately 25% or a 1/4 of the ship's length from the stern.

Although not intended some publications may give the impression that the pivot point moves right aft with sternway.  This Is clearly not correct and can sometimes be Misleading.  It should also be stressed that other factors such as acceleration shape of hull and speed may all affect the position of the pivot point. The arbitrary figures quoted here however, are perfectly adequate for a simple and practical working knowledge of the subject.

Position of the pivot point in response to applied forces

Vessel stopped

This is an example of a ship of 160 metres.  It is stopped in the water and two tugs are secured fore and aft on long lines through centre leads.  If the tugs apply the same bollard pull of say 15 tonnes (t) each.  It is to a position 80m fore and aft of the pivot point.  Thus two equal turning levers and moments of 80m x 15t (1200tm) are created resulting in even lateral motion and no rate of turn.

With the ship making steady headway however, the pivot point has shifted to a position 40m from the bow.  The forward tug is now working on a very poor turning lever of 40m x 15t (600tm), whilst the after tug is working on an extremely good turning lever of 120m x 15t (1800t-m). This results in a swing of the stern to port.

Making Sternway

The efficiency of the tugs will change totally when by contrast the ship makes sternway.  Now the pivot point has moved aft to a position 40m from the stern.  The forward tug Is working on an excellent turning lever of 120m x 15t (1800tm) whilst the after tug has lost its efficiency to a reduced turning lever of 40m x 15t (600tm).  This now results in a swing of the bow to port.

Wind

General effects.

In most ships the pivoting point is well forward when moving ahead, so that the pressure on the greater exposed area abaft this point tends to turn the ship into the wind.  When going astern, the pivoting point moves aft and the stern tends to fly into the wind.  The degree to which these effects are felt depends largely on the shape and disposition of the ship’s superstructure.  For example, a ship with a very high forecastle is not affected a great deal when going ahead, but her stern seeks the eye of the wind rapidly as soon as she gathers sternway.

The effect on the ship’s turning circle usually is to expand the curve in the two quadrants in which her bows are turning away from the wind, and to contract it elsewhere.  When turning away from the wind the ship is sluggish in answering her rudder.  She may be carrying lee rudder already to keep her on her course, so that in order to start the turn more wheel than usual must be applied.  When avoiding a danger ahead remember that the advance will be greater’ when turning away from the wind.

Wind effects are felt more strongly when speed is slow, and when she is lightly laden.  As ahead speed is reduced the bow usually falls off the wind more and more rapidly until, when the ship has lost all way, she lies approximately beam-on to the wind.

Effect when turning at rest.  When turning at rest in calm weather a ship pivots about a point somewhere between her centre of gravity and the centre of area of her underwater profile.  This point is normally somewhat forward of amidships, but it moves forward or aft with trim by the bow or stem respectively.  Under the influence of wind the attitude of a ship when stopped depends on the relation between the area exposed to the wind before and abaft the at-rest pivoting point.  Usually a warship lies with the wind within 20 degrees of the beam, and when settled there she requires a greater turning moment than normal to start her turning at rest.

Drift.  Any ship drifts to leeward under the influence of wind, the rate increasing progressively with loss of headway or sternway and with an increase in the angle of wind from the fore-and-aft line.  When stopped and beam-on to the wind, the ship, as she drifts to leeward, begins to transmit her motion to the water surrounding her.  The rate of drift increases up to a point at which both the ship and a body of surrounding water are moving bodily to leeward.  Immediately the ship moves ahead or astern she will then enter water that is not drifting and so will reduce her own rate of drift to leeward.

EFFECT OF WIND ON A SHIP

Once a ship has been obliged to reduce to slow speed in a storm the pressure of the wind on her hull will have an increased effect on her handling qualities.  The effect is greater if the ship is lightly laden, or is of shallow draught, or has large superstructures.  When going very slowly or when stopped, most ships tend to lie broadside on to the wind, and in exceptionally strong winds it may be difficult to turn them up into the wind, though it may be possible to turn them away down-wind.  In a typhoon or hurricane it may be impossible to turn certain ships into the wind, which is one good reason why any seaman avoids such conditions with land or dangers to leeward.

Leeway caused by the wind

The amount of leeway a ship makes in a gale depends on her speed, draught and freeboard, and on her course in relation to the direction of the wind and sea.  In winds of gale or hurricane force the leeway with the wind abeam can be very considerable, and may amount to as much as two knots or more, particularly if the ship is steaming at slow speed.

It is a common mistake among inexperienced seamen to make insufficient allowance for leeway, particularly in a prolonged gale when, in addition to the wind, there will be a surface current caused by it.  The amount of leeway made by a ship in various circumstances can only be judged by experience, but it is wise to allow a liberal margin of safety when passing dangers to leeward, because cases abound of ships having gone aground through failure to make sufficient allowance for leeway in the course steered.

Current and tidal stream

Clearly the ship’s handling qualities are not affected in any way if the whole body of water covering the area in which she is manoeuvring is moving at a constant speed.  In narrow waters, allowance must be made for the distance the ship will be moved by the stream during a manoeuvre.  But it frequently occurs in confined waters that the stream differs considerably within a small area, so that the bows and stern may be exposed to quite different currents.

Shallow water

When a ship is moving in shallow water the gap between the ship’s hull and the bottom is restricted, the streamline flow of water past the hull is altered and the result is seen as a greatly increased transverse wave formation at the bows and again at the stern.  In fact, the increased size of the stern wave is a sure indication of the presence of shallow water.  The energy expended in the waves formed by the ship is a loss from the power available to drive her, and therefore in shallow water her speed is reduced.  Furthermore, the restricted flow of water past the stern reduces propeller efficiency, which also tends to reduce her speed.  Usually, the higher the speed the more pronounced is the reduction of speed.  In extreme cases, and particularly in ships of low freeboard aft, the deck aft may be flooded by the stern wave.

The effects of shallow water on the speed of the ship and on the flow of water past the hull when moving ahead have already been described.  These effects may become excessive if the depth of water is less than one-and-a-half times the draught, particularly if the ship enters such water at high speed.  She may become directionally unstable and fail to answer her rudder at all, and the draught aft may increase so greatly as to cause the propellers to touch bottom.

The effects are likely to be particularly pronounced in ships where the propeller slipstream does not play directly on to the rudder.  The effects of shallow water on steering in restricted waters such as canals or rivers are usually worse than in the open sea, and are more likely to have dangerous results.  The only way to regain control is to reduce speed drastically at once.

When manoeuvring at slow speed or turning at rest in a confined space in shallow water, the expected effects from the rudder and the propellers may not appear.  Water cannot flow easily from one side of the ship to the other, so that the sideways force from the propellers may in fact be opposite to what usually occurs.  Eddies may build up that counteract the propeller forces and the expected action of the rudder.  If the attempt to turn at rest in shallow water with ahead revolutions on one shaft and astern on the other fails, or the turn is very sluggish, the situation will almost certainly become worse if the revolutions are increased.  Stopping the engines to allow the eddies to subside, and then starting again with reduced revolutions, is more likely to be successful.

PASSING THROUGH A NARROW ENTRANCE

The difficulties in passing through a narrow entrance when entering or leaving harbour arise from the desirability of approaching on a steady course at right angles to the line joining the two piers, or from the presence of a cross wind or stream.  If there is no room to approach at right-angles the use of tugs or warps is essential.

If there is a cross wind or stream the approach should be made from a point up stream or up wind to allow for leeway during the approach.  If the approach is made crabwise to allow for the cross current, a sharp swing using full wheel may be necessary as the ship reaches the entrance, to keep her stern and bows clear of the piers.  This need will almost certainly be reinforced by the fact that the bows, on reaching the point between the pier-heads, will probably enter slack water, or be sheltered from the wind, so that the stern will be carried rapidly down stream or down wind if no remedial action is taken.  The advantage of keeping up a reasonable speed if possible during the manoeuvre is obvious.

Dredging an anchor, may be a helpful expedient when passing through a narrow entrance.

PASSAGE THROUGH CANALS, RIVERS AND NARROW CHANNELS

The effects of shallow water on the speed and steering of a ship, are intensified in a canal or similar narrow shallow passage, because the movement of water around the ship is confined.  A ship moving along a canal pushes ahead of her a volume of water proportionate to her size and speed.  A lateral wave is formed just ahead of the ship, constituting a zone of increased pressure, just astern a similar but smaller wave travels along with the ship.  Between these two waves there is a trough along the length of the ship constituting a suction zone.  Anything floating is repelled by the wave at the bows, and similarly the bows of the ship itself are repelled from anything solid such as the canal bank.  The suction zone tends to attract any floating thing towards the sides and quarters of the ship, and also to cause the after part of the ship to be attracted towards the bank.  The water level in the canal ahead of the ship is raised; while astern of her it is lowered.  If speed is increased and the depth and width of the canal are little more than the draught and beam of the ship, the effects are noticeable a long way ahead and stern of the ship.

Effect of canal on ship’s speed

To maintain the level of water in the canal an opposing current is set up that flows rapidly past the sides of the ship.  This current is strongest close to the ship and near the surface, and weakest at the bottom of the canal and near its sides.  Combined with the shallow-water effect, this opposing stream retards the ship’s progress.  For example, a heavy ship passing through the narrow sections of the Suez Canal may make good only 5 knots at revolutions for 7 knots, while passage through the Gaillard Pass of the Panama Canal may reduce the ship’s speed by as much as 40 per cent.

To prevent damage to the banks and to craft moored, a speed limit is imposed in canals and in many rivers, and this must be rigidly obeyed.  If the draught is such that there is only a little water under the keel, the ship’s speed should be kept well down, and a careful watch kept on the state of the wave formation caused by the ship’s passage.  An increase in the bow and stern waves indicates that the ship is going too fast.  She tends to settle deeper in the trough, and her speed may drop suddenly, causing the stem wave to overtake the ship and render the steering uncontrollable.  The same effect may occur when the revolutions are reduced rapidly, so it is all the more important not to go too fast, and if obliged to reduce speed, to do so gradually if possible.

To sum up, a ship when in a canal has a critical speed above which her steering becomes increasingly erratic because of the shallow-water effects.  This is known as the canal speed, which cannot be exceeded with safety.

Effect of canal on ship’s steering

So long as the ship remains in the centre of the canal the pressure distribution is equal on either side of the ship; the steering will not be affected and little wheel should be required to keep her on course, provided that the canal is of symmetrical cross-section.  The fact that little wheel is being used indicates that the ship is following the best track in a canal or narrow passage.  Conversely, the need to apply a large amount of wheel to keep on course shows clearly that the pressure distribution is unequal.  This may occur either because of the configuration of the bottom, or simply because the ship has approached too close to one bank.  The danger is that the pressure from this bank against the bows, combined with the attraction of the after part of the ship to that bank, will throw the bows off this near bank and cause the ship to sheer violently over towards the opposite bank.

Correction of a sheer in a canal

In a canal the use of the wheel alone may be quite insufficient to correct a sheer, hence the ship handler should be ready to use the engines on the instant, or to let go an anchor immediately, if the need arises.

Experiments have further shown that it may be less effective to reverse the engine or propeller pitch on the side away from the sheer than merely to stop it.  There is also the danger of damaging the propellers by swinging the stem too close two the bank.  Meanwhile the rudder may be entirely ineffective in checking the sheer, and, if so, the anchor opposite the direction of sheer should be let go and dragged at short stay.

In a large ship, if prompt action with the engines and rudder as described has failed to have any effect on the sheer, it is probably best to apply full astern power in order to take the way off the ship, and if necessary also to let go both anchors.  If this is not done by the time the sheer has carried the bows past the centre of the channel it is unlikely that the ship can be prevented from striking the opposite bank.

In smaller single-screw ships a sheer is best checked by full ahead revolutions (or full pitch) and full rudder, but on occasions the sideways force of the propeller when going astern may be used to prevent the stern swinging on to the starboard bank.

In any ship quick judgment is necessary when correcting a sheer, to ensure that the correcting action is removed and possibly countered as soon as it begins to take effect; otherwise it is quite easy to produce a sheer in the opposite direction and ground the ship on the bank from which she was originally swinging away.

Smelling the ground

The effect of water pressure against the bows from the presence of shelving water on one side, causing the bows to swing away into deeper water, is the phenomenon known as smelling the ground.  In a narrow passage or canal it can produce a dangerous sheer towards the opposite shore or bank, but it can be beneficial if the water opposite the shoal is deep add safe.  The effect is most marked if the bottom shelves steeply.

Rounding a bend where there is little current

As the ship approaches a bend in a canal or river there will be a tendency for the bows to smell the ground on the outer bank and so to be swung round the bend.  In negotiating a bend it may be found that it is unnecessary to use any wheel towards the direction of the bend, because the water pressure on the outer bow will be just sufficient to carry the ship round.  In fact, if the ship approaches the bend on the outer side of the channel it may be necessary to use opposite wheel to keep her safely in the channel as she rounds the bend.  If she approaches the bend too close to the inner bank there is a danger that she may take an uncontrollable sheer towards the outer bank.  Nice judgement is therefore required in selecting the best course to follow and if there is little current it is generally advisable to keep to the centre of the channel, but inclining slightly to the outside of the bend, when it will often be found that very little rudder is required to negotiate the bend.

Negotiating a bend in a strong current

If there is a strong current or tidal stream running round a bend, as is often the case in rivers and estuaries, its effects may be quite opposite to those caused by smelling the ground.  On the straight reaches in a river the current usually runs more strongly in mid-channel than at the sides; but on bends the current normally runs strongest and deepest along the outer bank of the bend, and there may be slack water, or even a reverse current, along the inner bank.

When a ship is moving upstream round a bend the current may tend to throw the bows outwards, thus counteracting any tendency of the water pressure to push the bows off the outer bank.  The ship handler must be prepared in such a case to use wheel boldly in the direction of the bend and must avoid approaching too close to the inner bank.

When the ship is moving downstream, particularly if the approach has been made somewhat on the inner side of the bend, there may come a time when the current is tending to push the stern strongly towards the outer bank.  It may therefore be necessary to use opposite wheel to forestall this, or at least to apply early the opposite wheel needed for steadying the ship on the next straight reach.  Again any tendency to smell the inner bank may be overcome by the outward pressure of the current on the stern.  The degree to which these effects of current are felt depends on the length of the ship in relation to the width of the navigable channel.

A pilot’s knowledge of local effects in rivers and canals is often invaluable.  As a fairly general rule, ships should not pass each other on sharp and narrow bends.

Effect of wind when rounding a bend

The swing of a ship as she turns round a bend tends to be increased or decreased by a wind, depending on the direction in which it is blowing.  Probably the most difficult situation is that of a lightly laden ship proceeding down river and before the wind, when negotiating a sharp bend to starboard . As the ship turns round the bend both the current and the wind tend to swing her stern towards the outer bank; if this swing gets beyond control, to reverse the screw (or pitch) will be useless because the sideways force of the propeller will only accentuate this swing.  But all should be well if the rate of swing is kept firmly under control at the start.

Two ships Meeting in restricted channels and canals

When two ships meet in a restricted channel, rather than incline much towards their respective sides they should steer to pass close to each other.  With care there is little possibility of accident because, as they close, the water pressure between them will force their bows apart; on passing they will tend to parallel each other; and when separating, their sterns will be drawn together.  These influences will thus counteract the effect of the nearer bank, and the two ships should have no difficulty in regaining the centre of the channel.

In broad and deep canals ships going in opposite directions may be able to pass each other while both are under way, but usually one of the ships must make fast to the bank to allow the other to pass her.  In some canals - the Suez Canal, for example - the bank is cut away at intervals to form sidings in which a ship can make fast.

Makingfast to the bank.  A ship is made fast to a canal bank in much the same way as she is secured alongside a jetty, except that the way of the ship is reduced very gradually to stop her abreast the berth with as little astern movement of the engines as possible.  Plenty of time should be allowed for the manoeuvre, and it is usual to begin reducing speed when the ship is about a mile from the berth.  An unhandy ship is usually stopped in the centre of the canal and then warped alongside the bank by means of her hawsers.  Easily handled ships can be manoeuvred closer to the bank before the hawsers are sent ashore; but great care must be taken, particularly in twin-screw ships, to avoid getting the stern too near the bank and so endangering the rudder and inshore propeller.  Except in a current or strong head wind, two breastropes should be sufficient to hold the ship in her berth.

Effect on the berthed ship when another ship passes.  A berthed ship will surge considerably in the wash of a passing ship, and no hawsers will be able to hold her steady.  The berthed ship should therefore be free to move as the other passes her, and her hawsers should accordingly be slackened right off as the passing ship approaches.  Given that the berthed ship lies with her bows towards the approaching one, the effects of the passing ship’s bow wave, trough and stem wave on the berthed ship as they reach and pass her are as follows.  First, the berthed ship’s bows are repelled towards the bank as the bow wave reaches her;’she then surges ahead in the current of the trough and her ‘ bows are drawn away from the bank by its suction while her stern is pushed towards the bank by the bow wave; as the stern wave reaches her bows her movement ahead is stopped, and at the same time her stern is sucked out from the bank by the trough; finally, as the stern wave passes her stern, the ship surges astern and tries to follow in the wake of the passing ship.

This surging must be controlled by the engines and rudder to prevent the stern from being drawn too far out from the bank and colliding with the quarter of the passing ship, and also to prevent the berthed ship from surging too far astern.  The surging can be controlled quite easily by a kick astern and then a kick ahead on the engines, with the wheel put over away from the bank.  In twin-screw ships only the offshore propeller should be used.

Course of the passing ship.  The passing ship should approach along the centre of the canal at slow speed, and should endeavour to keep along the centre of the canal as she passes the other ship, even though it entails passing close aboard of her.  Even if the course of the passing ship would appear to take her too near the berthed ship, the ships will in fact sheer away from each other; whereas if the passing ship attempts to give the other a wide berth, she may then come under the influence of the far bank of the canal and take a sheer towards the berthed ship, with dangerous results.

Dredging an anchor

Towing an anchor at short stay in order to improve manoeuvrability is sometimes called dredging an anchor.  For example, when going alongside in an offshore wind, or when passing through a narrow entrance where there is a strong cross-stream, the ship lets go the anchor at slow speed as she approaches, and drags it along the bottom.  This has the dual effect of enhancing the control of the bow and of holding it upwind or upstream.  Care must be taken to ensure that no underwater cables or submerged equipment are in the vicinity.