unit 2 - ship hydro dynamics & hull design

Block 3 Unit 2 Ship Hydro Dynamics

& Hull Design

2. Ship hydro dynamics and Special manoeuvres

2.1 Ship Hydro dynamics

.1 Introduction

.2 Objectives

3. Factors affecting manoeuvring

a. Underwater hull geometry

.4 Theory behind Pivot Point

a. Characters of the ship motion

b. Rotational Movement

c. Rotational resistance

d. Frictional Resistance-

e. Residual Resistance –

f. Lateral Resistance –

5. Position of pivot point

a. The position of the Pivot Point under different conditions:

i. when stationary

ii. Just after ahead movement is given:-

iii. Ship underway with ahead \ astern movement:

iv. Position when turning

v. Vessel stopped and commencing the turn with engines ahead.

vi. Turning at constant speed –

vii Speed loss during the turn

viii. Standing Turns and “Kick ahead”

b. Practical use of the shifting nature of pivot point

6. Directionally stable ship.

(SAQ – Self Assessment Questions)

Answers to (SAQ – Self Assessment Questions)

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2.2. Special Manoeuvres –

.1 Introduction

.2 Objectives

.3 Manoeuvres for man overboard

a. Immediate actions

b. Ship Manoeuvres to effect a rescue

.4 Manoeuvres and process for anchoring

a. Ship Anchoring Procedures

b. Manoeuvring for anchoring

c. Approaching identified anchorage

d. Preparation of anchoring plan

e. Elements of the plan:

f. Preparing to anchor

g. Approaching anchorage

h. Anchorage Position


2.1 Ship Hydrodynamics

1. Introduction

Having learnt the basics of ship manoeuvring, let us now look at actual manoeuvres that you may be a part of when on bridge watches.

2. Objectives

a. To understand the factors that affect manoeuvres

b. To understand ship manoeuvres under varying conditions with or without the use of tugs and or current or winds.

c. To plan for an anchorage and to understand the process of anchoring

3. Factors that affect manoeuvring

As seen in the previous modules; handling characteristics vary from ship type to ship type and from ship to ship. Handling qualities are determined by ship design, which in turn depends on the ship’s intended function. Typically, design ratios, such as a ship’s length to its beam or beam to draft, determine the ships willingness to turn.

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Desirable handling qualities are achieved only when there is a balance between directional stability and directional instability.

a. Underwater hull geometry

The following factors give an indication of how a ship will handle:

Length to beam (L/B) ratio:- High values of L/B are associated with good course directional stability, e.g. the container ships have a L\B of 8 and therefore have a good directional stability while tugs with a L\B ratio of 2.5 to 3 has good turning ability

Beam to draught (B/T): - High values of B/T increase leeway and the tendency for such a ship in a beam wind would be to ‘skate across the sea surface’. Merchant ships have a B/T ratio in the range of 2.75 to 3.75.

Block coefficient and prismatic coefficient (ratios of the ship’s volume of displacement against the volume of a rectangular block or a prism): - Ships with large block and prismatic coefficients have poor course stability and a readiness to turn. When turning, they will do so easily. Large tankers have these characteristics. Ships with a large protruding bulbous bow are likely to have their longitudinal centre of buoyancy far forward. As a result, the ship will show a tendency to turn.

Longitudinal centre of buoyancy: - Is the point around which the ship trims

.4 Pivot Point

A ship rotates about a point situated along its length, called the ‘pivot point’. When a force is applied to a ship, it results in causing the ship to turn (e.g. the rudder). The ship will turn around a vertical axis, which is conveniently referred to as the pivot point. With headway, the pivot point lies between 1/4 and 1/3 of the ship’s length from the bow, and with sternway, it lies a corresponding distance from the stern. In the case of a ship without headway through the water but turning, its position will depend on the magnitude and position of the applied force(s), whether resulting from the rudder, thrusters, tug, wind or other influence. The pivot point traces the path that the ship follows.

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Ship moving along longitudinal axis.

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a. Characters of the ship motion

Longitudinal Motion - Ships in order to fulfil their assigned roles are on the move most of the times, from waypoint to waypoint. This motion of this ship is in a fore and aft direction - in other words Longitudinal Motion. The Longitudinal travel is achieved by ship’s propulsion system through propeller thrust.

"Longitudinal Resistance is experienced when the ship is moving in fore and aft direction. When propulsion force and water resistance balances a steady speed or hydrodynamic equilibrium is achieved.

Lateral Motion takes place when a ship is moved bodily sideways (athwartships) either intentionally or unintentionally, for example:

Intentionally - when a ship is being berthed including:

The use of tugs to push or pull ship bodily

Use of bow and stern thrusters,

Use of ship’s mooring ropes to bring ships alongside.

Unintentionally: Ship being (bodily) set by currents, tides or being pushed by winds. Except in the case of a ship being carried sideways by currents or tidal streams, ship’s lateral motion also meets resistance (Lateral Resistance) - when being pushed by tugs or wind. However when pushed by a tug the movement shall be opposed by resistance

No Resistance BecauseX X

X` X`Water Mass Moves

Tide

Y

Y

Diagram: Ships Lateral Motion:

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b. Rotational Movement

This takes place while changing courses at sea or while manoeuvring in ports, ships are required to change direction. This is a turning movement or Rotational Movement.

This Rotational Movement may consist of:

i Along longitudinal axis, when altering course at sea or in ports.

ii Along transverse or lateral axis when caused by tugs or thrusters,

iii Only Rotational Movements: such as a ship being turned around in dock turning basin with use of tugs or shore lines

c. Rotational resistance - rotational movement meets resistance, which is directly propositional to the rate of turn.

It is mainly in the study of rotational movement of ships that the concept of pivot point and the turning levers needs to be studied as an important aspect for understanding ships behaviour. The position of the pivot point, the force applied on the ship, and its direction and the length of the levers determine the effect of the same. Larger the length of the lever or the force increases the effect

On big ships (VLCCs, ULCCs, Bulk Carrier), the distance from the point of impact of force to pivot point can be very large. As you shall see the shift of pivot point due to changes in ahead to astern movement are also very large. Such shift in the position of pivot point by a couple of hundred meters affects the moment of the rotational force (Product of length of Lever x Force expressed as feet tonnes or m/tonnes) - the larger is the lever of that force, the greater its effective leverage.

d. Frictional Resistance

When moving from position of rest in water to ahead or astern, propulsion power overcomes ship’s inertia and overcomes the frictional Resistance. Even though this resistance is felt all along the ship’s sides equally (and though it is taken into account for determining propulsion power), it is not considered for determining the position of Pivot Point.

- e. Residual Resistance

- This is mainly longitudinal resistance. This forms an important component in determining the position of Pivot Point and there is relationship between the position of Pivot Point and the ratio of the

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longitudinal resistance to the propulsion force and the direction of travel.

- f. Lateral Resistance

- However, this does not come into play for straight-line travel as the component of ship’s transverse thrust is small and is overcome as soon as ship gathers momentum. It however, affects the position of the Pivot Point when ship starts turning under the action of rudder and propeller.

+

++

++

(a) Three axis of Movement (b) Rotational Movement

P Rotational axis

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3

(d) Rotational axis

(Vertical Z,Z)

+

+

++

+++

+

(c) Rotational movement

Longitudinal axis(X,X)

X

ZX

Y

Lateral axis Y(Y,Y)

ZRotational axis

(Z,Z)

5 Position of pivot point

As pivot point shift during ship manoeuvres, it is important to have an idea about the possible position of the pivot point under different conditions to anticipate the changes in rotational motion.

Ships rotational moment is about a vertical axis situated along the length of a ship. The position of this axis is influenced by:

Shape of ships hull,

Direction and velocity of ships motion.

Point of impact and

The magnitude of the forces acting on ship.

Broadly, the axis moves with a change of the direction of the motion and its magnitude changes as the magnitude and the distance from

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the pivot point. Pivot Point therefore is not a fixed but a wandering point. (Peripatetic Point).

Let us see how a Pivot Point moves with “circumstances” and its effects on “Turning lever.

a. The position of the Pivot Point under different conditions:

i. when stationary

In the case of a loaded ship stationary in water, on even keel the Pivot Point is very close to the centre of gravity i.e. almost at mid length

.

Y

X

X

Y

P

G

ii. Just after ahead movement is given:-

A ship underway under the effect of the propeller (after inertia has been overcome and before the longitudinal resistance is felt) has its pivot point pushed in the direction of travel-forward or aft and this new position is temporarily 1/8 L from bow or stern as the case may be depending on the direction of ships movement (or direction of propeller rotation). This temporary position of pivot point well ahead, gives a good turning lever and is used for kick-start manoeuvres

iii. Ship underway with ahead \ astern movement:

When ship starts moving through water and as soon as the longitudinal resistance is felt at the fore (or after) part of the ship, the Pivot Point moves in the direction of the force (resistance) i.e aft when ship is moving ahead or ahead when ship is moving astern. At constant speed, pivot point settles about ¼ L from the bow when ship is making headway or ¼ L from the stern when ship is making sternway.

If at the same time, the ship also has lateral (side ways) moment, this can affect the position at the pivot point – due to lateral forces and

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lateral resistance. This is important particularly when the ship is turning

iv. Position when turning

The centre of the rotational motion of turning ship depends upon length to beam ratio of the vessel. It is generally assumed that the pivot point on a ship under headway and turning under rudder lies about 1/3 L from forward due to Lateral pushing back the pivot point further (from ¼ L at constant speed).

Table 1

L/B ratio 9 8 7 6 5 Pos. Pivot Point from bow 0.33L 0.34L 0.35L 0.37L 0.4L

Ship moves aheadpivot point moves to a point1/8 from bow initially(Position 2)

P

P+ +

1/8

Position 1

Position 2

v. Vessel stopped and commencing the turn with engines ahead. (No environmental factor affecting).

Ship starts turn with rudder hard over and with engines ahead (either slow, halt or full).

This rudder and engine action will attempt to turn the ship as well as propel it ahead and in doing so:

i) Forward moment is resisted because of inertia.

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ii) Pivot point moves ahead about 1/8 L from the bow because of the propeller thrust (force) –

This gives good lever for turning movement to start before the ship gathers forward momentum or just as the ship starts making headway.

As the ship moves ahead after overcoming inertia, the water resistance on the bow eventually balances the forward propulsion force at a steady speed and the pivot point shifts aft to a position ¼ from the bow.

At a steady speed, while turning, the lateral resistance (at the bow on the side which the ship is turning and the stern in opposite direction) also comes into play pushing the pivot point further aft to about 1/3 L from the bow. Because of this, turning lever is reduced and rudder force becomes less efficient.

As the ship starts turning she slides sideways through the water, both initially and during the turn and meets water resistance all along the shipside towards which the stern is turning. This also reduces rudder force. This is the lateral resistance when turning.

(c) Position of pivot point

X X SHIP ON EVEN KEEL

pSHIPSTOPPED c

c

p

1/4L

MAKINGSHIP

HEADWAYOF ORIGIONAL

PIVOT POINTSHIFTS AHEAD+ +

+

1/4L

X

SHIP MAKING STERN WAY

SHIFTS AFTPIVOT POINT

pp

ship making stren way at steady speed

(b) Position of pivot pointship making head way at steady speed

(a) Position of pivot pointstationary ship

X

X

X

POSITION

++

Position of pivot point when turning with propeller and rudder

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vi. Turning at constant speed –

Rudder force and lateral resistance achieve balance in a turn at constant RPM. Thus, turning circle areas at slow, half or full RPM are comparable. Only thing that differs is time taken to complete the turn and therefore the rate of turn.

posn.1

posn.2

posn.3

+

+

+

+

+

+

+

+

+

++

+

3. As ships start turningpivot point moves to about

2. At steady speed ahead steady courseP 1/4 L from bow

1/3 L from the bow

(0.3 to 0.4L)

posn.4

1. Intilal position of pivot point mid ship (near c of g)

vii Speed loss during the turn

Speed during a turn always suffers a marked reduction because during a turn, ship is moving ahead and sideways so she experiences resistance on the side, which acts as a brake. Speed reduction may be as much as 30% to 50%. This fact may be used for speed reduction (by rudder cycling or turning full circle) if sufficient manoeuvring area is available.

viii. Standing Turns and "Kick ahead"

Minimising lateral resistance and maximising rudder force achieves standing turn. Initially the turn is started with the vessel stopped or

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moving at very slow speed. Engines are then ‘kicked ahead’. The rudder force would now be at its maximum, turning lever the best and therefore faster turn.

As the ship turns 90º, the lateral forces tends to slow the turn and rudder force will be less effective because the pivot point would move from1/8 L to ¼ -1/

3 L. In order to maintain the leverage, it would be then necessary to slow down and again give a kick ahead to get the maximum rudder force and reduced lateral resistance.

b. Practical use of the shifting nature of pivot point

Shifting nature of pivot point affects turning forces and therefore the process of handling of a ship. E.g.

i) When the ship has headway, the pivot point is forward at about ¼ L from the bow. If a ship under this condition is either pushed or pulled by tugs laterally or by use of bow thrusters, the distance between the pivot point and force applied (turning lever) is reduced and the turning effect shall be reduced. In such a case, it would be more effective to use rudder than the bow thrusters.

ii) Whenever a ship has headway, the pivot point is well forward and the distance between Pivot point and Rudder force is at its maximum. Rudder is therefore most effective. This fact is made use of in kick-start manoeuvres

Ships propulsion Power (Thrust) is expressed as SHP. (Shaft Horse Power) from which this force can be converted in tonnes. Similarly, Tugs propulsion power is expressed as SHP and Bow/Stern Thrust power is expressed as SHP

6. Directionally stable ship.

Ship is directionally stable if deviation from a set course increases only when an external force or moment is acting to cause the deviation. On the other hand, ship is said to be directionally unstable if a deviation in course begins or continues even in the absence of an external cause.

Directionally unstable ship is not easy to manoeuvre. Stable ship requires less energy expenditure in maintaining its set course.

Broadly, directional stability or instability can be determined by examination of ship’s under water profile. If the area of the hull and its appendages is concentrated towards the after end and if they are disposed equally on either side of the centre line - then the ship is

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likely to be directionally stable. Hydrodynamic stability is related to the following hull parameters.

If ship's length is increased, her directional stability would be increased

Increase in beam and block coefficient will reduce yawing,

Deepening draft improves directional stability and trim by stern improves it still further

A ship with poor directional stability can bring about some improvement by enlarging rudder area to a maximum of 2% of the lateral underwater area.

Ships with finer lines are more easily handled. Large ships such as super tankers tend to be directionally unstable.

(c) Pivot point when ship has stren away.

FORD TUG FARTHEST

p

p

in center

P 1/4 L FROM STERN STERN WAYSHIP MAKING

AFT TUG EXERTS LESS EXERTS MORE PRESSURE

STERN SWINGS LESSBOW SWINGS FARTHEST

LEVER FORAFT TUG LESS

LEVER FOR FORWARDTUG

GREATER THAN AFT TUG

>

(b) Pivot point when ship is moving ahead

LEVER FORAFT TUG

LEVER FOR

(LESSER)<

(GREATER)FORD TUG

STERN SWINGS FARTHESTBOW SWINGS LESS

FROM P, EXERTS MORE LEVERAGE - PRESSURE

AFT TUG AWAY 3/4 LP 1/4 L FROM BOWSHIP MAKING HEADWAY

SHIP STOPPED TUGS EXERT EQUAL

& PARALLEL

(a) Pivot point when ship is stopped in water

1/2 L 1/2 L

TugTug Pivot point

FROM MIDSHIPSEQUAL DISTANCE

PULLING)TUGS PUSHING (OR

(C) - P

STERNWILL

MORESWING

AWAY FROM P

PRESSURE

( headway )

AND SHIP MOVES

+++

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Table 2

Relationship between the length and other Manoeuvring parameters

L/B ratio

Pivot Point from bow

Turning circle Diameter

Turning circle circumference

Drift angle Degrees

9 1/3 L 4 L 12.6 L 14 8 21/32 L 3.8 L 12 L 15 7 5/14 L 3.6 L 11.3 L 16 6 3/8 L 3.3 L 10.5 L 17 5 2/5 L 3 L 9.4 L 19


a. What are the factors that affect manoeuvring?

b. What are the various forces acting on the ship just as the ship gets underway?

c. How does the pivot point move as a force is applied? Is the amount of shift dependent on the aspect and the quantum of the applied force?

d. Under what circumstances are the bow thrusters most effective to start a rotational motion?

e. Two tugs are positioned at the forward and after stations. If the ship gives a kick astern which of the tugs shall have the more effect?


a. Underwater hull geometry is the major factor which include

Length to beam ration

Beam to draught ration

Block coefficient and prismatic coefficient

Longitude centre of buoyancy

b. As the ship gets underway, resistance is experienced in the forward part and the thrust given by the propeller is experienced aft. Its only when the two balances that the ship moves at a steady motion.

c. The pivot point moves in the direction of the force applied The direction depends on the direction but not on the magnitude of the force

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d. Bow thruster shall be effective when the ship is moving astern as the pivot point is abaft and the bow thrusters have a large lever.

e. As the pivot point is aft the forward tug shall be more effective

2.2 Special manoeuvres

.1. Introduction

This module aims at understanding and planning the manoeuvres and the procedures, such as man overboard and anchoring which may have to be initiated by a OOW on his own till the master arrives on the bridge. Cadets should take into account that these types of manoeuvres cannot possibly be practised in real life situations, for obvious reasons. It is therefore essential that the procedures to be followed be thoroughly understood.

2. Objectives

a. to understand the process to be followed where a man is lost overboard (MOB)

b. to manoeuvre the ship to rescue the MOB

c. to understand the process for preparing for anchoring at the anchor stations

d. to understand the process for anchoring

3. Man overboard

a. Immediate actions

Watchkeeping officers are required to take certain actions immediately on being informed of, or actually seeing, a man falling overboard. However, on such occasions any other consideration only delays the effectiveness of the actions. The initial actions provide the most effective measures and should not cause damage to the ship. On being informed of man overboard, following immediate actions should be taken:

Put the wheel hard over to the side on which man has fallen. This will reduce the hazard of the person getting caught into the suction current of propeller. It will also help in reducing the speed and keep the ship close to the MOB position.

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Throw the man-overboard (MOB) life buoy secured with the MOB light and the smoke float. This shall identify the position


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of the incident with a fair accuracy. If the life buoy is released quickly, it may even allow the person to get hold of it.

Many GPS systems have the provision to “Mark” the man overboard position manually. (Does your ship have one?)

Inform Master and the engine room and put the engines on stand by.

Call out the rescue boat crew and prepare for lowering. Preferably lower the boat to the embarkation deck. Do not lower further, till the Master orders the same. Preparing the rescue boat to recover the person overboard may be done in the mean time. Engine room must be ready for immediate manoeuvring.

Ascertain the following Information that the Master would have to take into account:

Identity of the person if possible,

Was the person a swimmer?

Was the person wearing life jacket or warm clothing’s etc.?

State of the wind and swell. Wind & Swell may be strong making recovery by a lifeboat difficult.

The position of the MOB, if ascertained immediately after the incident. Was a life buoy thrown to mark the position?

If the time and position where the man may have fallen overboard is not known, when was he last seen?

Visibility

b. Ship Manoeuvres to effect a rescue

It is not possible to state a perfect single manoeuvre, which will be suitable in all situations. What is important is that:

The action taken should have regard to the prevailing conditions.

It should be aimed to save the life or lives as far as practicable

The procedure or manoeuvre adopted should not endanger more lives.

International Aeronautical Merchant Ship Search and Rescue Manual (IAMSAR) gives details of the manoeuvres which allows the ship to return as near as possible to the position where the man was assumed to have fallen overboard. These manoeuvres are explained

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in very simple terms and we would like you to read them and work on the following task.

4. Anchoring.

a. Ship Anchoring Procedures

After a long sea voyage, a ship arrives in a port or near a port area and may have to anchor the ship. This may be because of customs, immigration and cargo formalities or due to non-availability of a berth.

A ship may be required to anchor for receiving stores, provisions, bunkers, crew change, surveys, and repairs, awaiting port clearance. A ship may also be forced to take shelter at an anchorage due to unfavourable weather in the open sea.

Anchoring involves the following processes:

• Removing cement from the spurling pipes and clearing the hawse pipe covers.

• Lashing and unlashing of the anchor securing system.

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• Engaging and disengaging system for the windlass gears.

• Anchoring terminology.

• Anchoring and heaving up operation at forward station.

b. Manoeuvring for anchoring involves:

• Deciding anchorage position

• Bringing the ship to a predetermined position for anchorage

• Letting got anchor

c. Approaching identified anchorage

Appraisal

In order to select the position to anchor in given area, study should be made regarding the anchorage choice with the help of appropriate Nautical Publication at passage planning stage itself. Publications such as sailing Direction/’pilots’, guide to port entry, port hand books – circulars, appropriate largest scale charts, tide tables, meteorological data etc may indicate the areas in approaches to port which should be earmarked as anchorage for ships before entering port / pilotage waters. In addition, Ship-Port exchange (Master-pilot Exchange) - Port Passage plan - would also result in Port Authorities suggesting anchorage position.

However, the Master must consider the following factors when choosing anchorage for his ship.

a. Depth: - At least 1.5 times the deepest draft of the ship with due allowance for range of tides, Sea and Swell Condition. This depth reduces due to shallow water effect to some extent. Adequate clearance at low water spring needs to be taken into account to avoid excessive current force when the tide drops.

b. Small UKC at spring tides (low water) would require paying out too much of chain leaving inadequate length for further use. (Depth/Draft ration not more than 2). This will depend upon how much of cable is on each anchor and size of the ship).

c. Holding qualities of seabed: mud, soft mud and clay Seabed is considered to have good holding qualities. Rocky and sandy beds are not.

d. Adequate Swinging and Manoeuvring area

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e. Approaches to and exit from anchorage should have adequate depths and widths – with enough sea room to


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manoeuvre in case the anchorage plan is to be changed / aborted.

f. Underwater obstructions (Pipelines, cables)

g. Distance from passing traffic to avoid interaction,

h. Facilities for position fixing should be available to detect dragging

i. Ease of communication with shore (including launches, etc).

j. Likely weather during the stay at anchorage,

Additional factors

i) Purpose of stay (laid up, Repairs, orders, Bunkers, Stores

ii) Duration of stay (Crew change berth not vacant)

iii) Season (fair-weather /foul)

iv) Need for physical/personal contact (Provision, F.W., quarantine, medical, cargo work repairs survey etc).

Caution

k. Avoid choosing anchorage spot close to prohibited anchorage (In case of vessel dragging etc).

l. Prepare an alternate anchorage plan.

m. Prepare a Contingency plan. (Sudden change in any of the above may need one)

d. Preparation of anchoring plan

Anchoring plan along with manoeuvring plan will have to be drawn up along with a checklist for preparation to anchor after studying relevant publications and deciding on the spot (or alternate spot) for anchoring the ship.

Be completely familiar with ships manoeuvre data (Booklet)/characteristics especially as appropriate for current condition of load, draft, trim and likely depths and UKC through which ship will have to be handled

Direction and strength of wind and tide. Allowance must be made in the plan for weather conditions, tides and sea/swell conditions expected when approaching the anchorage.

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e. Elements of the plan:

If possible, plan to arrive at end of ebb tide (low water slack) with vessel on hand steering.

Approach courses (tracks) and speed

Waypoints (with bearings / distance from objects ashore) at which to reduce speeds to stop / reverse engines in order to check headway

a. Position to drop anchor. Depth of Water

b. Determine the required length of chain.

f. Preparing to anchor

Time to start preparation – 2 hours / 1 hour before arrival – Factors to take into account - Weather, Traffic Density, Day or Night

Echo sounder to be put “ON”.

People to Call – Master, Bridge Team. C/O, Bosun, ER personnel.

Machinery to be ready: Windlass, (Power “ON” – operate), gears, steering to be tested, Anchor lashing to be removed, gypsy brakes, anchor signals, communications equipment to be in readiness

Tools to be handy

Instructions to be given to person on forecastle including:

Which anchor,

Walking back/how much,

How much to pay out initially, how much total length –

Is turning short with anchor contemplated.

g. Approaching anchorage

Before anchoring, it is necessary to judge that the ship is stopped (over the ground). Taking visual Bearing of objects abeam or a near the beam are preferable such as Transits, Land objects or lighthouses or Beacons is preferable. Bearings of Buoys and ships nearby are not reliable and hence not recommended. Bridge watch shall monitor ships way through water and keep the master informed. If the bearing is steady, (does not change) – ship is stopped. A check is kept on the compass heading to determine if the ship is swinging. If the ship is not heading the current, the ship shall swing to align itself to it.

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At the same time, remember:

That the ship swings to starboard if you are going astern

Flow of water past the ship indicates that the ship is moving forward.

In shallow water when the engines are put astern in muddy waters the muddy waters seem to move forward

A sure method is to drop a lead line (Hope you have one on board)

Radar bearings also can give you indications whether the ship has stopped.

Modern ships have Doppler logs to quantify the ship’s motion over ground or over water in a Ahead – Astern direction, additionally they also give a athwartship motion of the bow and stern. See module on “Speed Log”)

Remember that if the ship is swinging at the same time it may be difficult to ascertain whether the ship is dead stopped.

h. Anchorage Position

Position obtained on anchoring will be the position of the bridge. Position of the bow or of the anchor can be drawn from this to know the position of anchor (in case this is lost) and to prove that you did not anchor in prohibited area. Such fixing of position shall also allow you:

To draw the ship’s swinging circle using the amount of chain brought up. This is also a requirement at certain ports and must be practised after every anchoring.

• To know if vessel has dragged.

• To assure the watch keeper that sufficient distance is available from nearby objects on chart.

• Many GPS systems have the provision to “Mark” the anchor position and setting up of a “anchor watch” feature. ( has your ship’s GPS been provided this feature?)

i. Use of anchor buoys.

Anchor buoy / buoys is a device that is available on board or can also be made on board the ship, to indicate the position of the anchor / anchors when they are dropped.

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i) The buoy: can be made of hardwood /Hard piece of wood 2” thick and say 3' x 3' approximately shaped as a circle or cone and with a hole in a corner big enough to take a small shackle pin through it. The buoy can be painted suitably so that it can be identified as an anchor readily.

ii) A wire/Nylon rope with an eye can be attached to the buoy with the shackle. This rope must be sufficiently long (Depth of water and tidal rise and swell height and allowance for tidal current etc. Attachment to the anchor can be through the ring or shackle of the anchor by means of an eye or loop. First, the connection to the anchor is made with the rope and adequate length joined to the buoy.

iii) The buoy with adequate slack is taken up on the rail / bulwark on forecastle and lightly lashed.

iv) When order is given to let go the anchor the buoy is also thrown over the side

(SAQ – Self Assessment Questions) Special manoeuvres

f. Describe the immediate actions that should be taken when a man is seen falling overboard

g. what is the main intention in the special manoeuvres for MOB described?

h. How is the position of the anchor marked and why?

i. Why are anchor watches necessary? What routine is to be followed on such watches?


f. Raise an alarm. Call the master, throw the MOB lifebuoy overboard. Use the “mark” feature on the GPS if available. Keep a look out to keep the man in sight, follow one of the manoeuvres mentioned, inform engine room to prepare for manoeuvring. The other actions that could be taken but not immediately are increase lookouts, hoist signal “O” and send urgency message on radio to ships in the vicinity etc

g. Main intention of the special manoeuvre for MOB is to return to the spot where the man fell overboard

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& Hull Design

h. The position of the anchor should be marked with an anchor buoy. It can also approximately be marked by fixing the ships position and by calculating the length paid out on the cable in the direction the ship is heading. The anchor buoy is accurate and in case the cable parts the anchor can be recovered by spotting the anchor buoy.

i. Anchor watches are necessary to ensure principally that a). the ship is not dragging, b) no unauthorised persons approaches and / or boards the ship. c) No other ship is dragging on to own ship

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unit 2 - ship hydro dynamics & hull design

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