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.
Making Headway
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.
Making Headway
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
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
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.