P&I Loss Prevention Bulletin
Preventing an Anchor from Dragging
The Japan Ship Owners’ Mutual Protection & Indemnity Association
Loss Prevention and Ship Inspection Department
Vol.25
July 2013

INDEX
Preface͐ P 1

i1

Accidents Involving Ships at Anchor ͐ P 2

i2

The Reason Why an Anchor Drags ͐ P 3-4

i3Wind Pressure Force Calculation ͐ P 5

i4

Holding Power Created by Anchor and Anchor Chain ͐ P 5-6

i5

Calculating the Catenary Length of an Anchor Chain͐ P 6

i6

Examples of Anchor and Anchor Chain Stowed on Board ͐ P 6

i7

Excel Spreadsheet Calculation of Wind Pressure Force ͐ P 7

i8

Horsing (Yawing and Swaying) Motion and Impact Force ͐ P 7

i9

Excel Spreadsheet Example of Anchor Holding Power Calculation ͐ P 7

i10

Ship’s Operational Safety Measures for Anchorage and Their Effects ͐ P 7-11

i11

The Critical Wind Speed ͐ P 11

i12

Emergency Measures Taken and Their Effectiveness After Dragging Anchor ͐ P 12

i13

Difficulty in Maintaining Manoeuverability ͐ P 12-13

i14

Preparation for Safe Anchorage ͐ P 13-14

i15

Safe Distance from Other Ships, Shallows and Other Facilities ͐ P 14-16

i16

Dropping Anchor Operation ͐ P 16

i17

The Anchor Cable Veering Rate࣭Scope of Cable To Be Paid Out࣭Brake Force of Windlass͐ P 17

Reference Books ͐ P 18

Preface
According to the investigation by the Japan Marine Accident Tribunal in 2004, 40% of ships anchoring
under typhoon conditions experienced their anchors dragging.
Generally speaking, dragging the anchor happens in rough weather and weighing the anchor may takesome time in such circumstances. Even if the anchor can be successfully raised in rough weather
conditions, further time may be required to restore the ship to full manoeuverable condition. The usual
weighing operation, therefore, may be abruptly transformed into an emergency procedure requiringexperienced skills and cool judgment.
This refers to how and why dragging the anchor occurs and how to deal with a dragging situation.

1

i1

Accidents Involving Ships at Anchor

Accidents involving ships at anchor usually occur when the anchor drags and the vessel drifts without
holding power, leading to collisions and/or groundings or strandings. The following considerations
should be taken into account:-ձ

It can take some time to realise the anchor is dragging, despite the ship drifting. A vigilant bridge
watch is, therefore, essential.

ղIt takes some time to weigh the anchor and restore the ship to full manoeuverable condition, eventhough the ship may be drifting for that period. Contingency plans must be in place to ensure rapid
response times.

ճ During the period beginning with the detection of dragging to the time full control is achieved overthe ship’s manoeuverability, the vessel may run dangerously close aboard, or into another ship or
structure, or into shoal water.

Unless heavy weather causes the vessel to capsize, no serious accident should occur just because a ship
is dragging its anchor, provided there is enough space around it for manoeuverability and enough time
available to restore it to a fully controlled condition.

Pyeongtaek Anchorage and Approach

The considerations outlined above become of even greater importance in the case of a crowded anchorage
where there may be insufficient space between vessels to deal timely with emergencies such as dragging
anchor and drifting out of control. The master of an arriving vessel should satisfy himself first that theanchorage is safe in all respects before committing himself to anchoring.
Masters and deck officers are advised to familiarise themselves with the following concepts in order toprevent as far as possible a ship from dragging its anchor:ձ How and why dragging the anchor occurs.
ղ Difficulties with vessel manoeuverability while dragging an anchor.
ճ The assessment of what constitutes a safe anchorage, including contingency plans involving the timeand space required to regain control of the vessel if the anchor drags.

2

i2

The Reason Why an Anchor Drags

A ship’s anchor drags due to the impact of external forces on
it which exceeds the holding power of the anchor and cable.
Masters and deck officers should be aware of how various
parameters, such as the scope of cable in relation to the depthof water and the effects of wind, wave and tidal forces on thevessel, can in turn exert excessive forces on the anchor andcable system leading to break-out of the anchor from theground and dragging. In the above connection, there still
remain some empirical or “rule of thumb” methods ofassessing the scope of anchor cable required under various
circumstances of water depth and expected weather
conditions, for example:Empirical or Rule of Thumb Methods for Assessing the Minimum Required Length of Anchor Chain
Where:
d: Water depth(m)L:Minimum Required Length of Anchor Chain(m)࣭Japanese Publication Theory of Ship Operation
Fine weather:
L=3d+90m
Rough weather: L=4d+145m
࣭United Kingdom Publication Theory of Ship Operation
L=39 ξ
Traditional Means of Detecting a Dragging Anchor
ձ Checking the ship’s position, to confirm whether it is placed outside of a turning circle.
(The most currently reliable way of checking whether the anchor is dragging is to carefully monitor
the vessel’s position by visual and electronic means to confirm whether it remains within aswinging circle defined by the scope of anchor cable and the distance from the forecastle to thebridge. If it deviates from the circle, the ship is likely to be dragging its anchor. Reliance should
not be placed on a single method of fixing the ship’s position. Cross-check whenever possible withan alternative means.)
ղ The bow cannot stand against the wind.
ճ The ship’s side against the wind hasn’tchanged.
մ Checking to see there is no slacking of chains
just before a ship’s side against the wind turns.
յChecking whether there are extraordinary
vibrations through the anchor chains.
ն Checking the course recorder in case it does
not indicate a “figure-of-eight” motion locus.

レコーダー／ Recorder

The above methods remain well-tried but, of course, only confirm that the anchor is dragging. They do
not predict when dragging is likely to commence.
According to one current study, an analysis of anchor dragging has shown that there are two associatedphenomena, or stages, to the process which indicate that dragging may be about to occur before it isdetected by the more usual methods outlined above. (The following considerations exclude those cases
where the initial position of the anchor, rather than the ship, has been fixed and subsequently
monitored.)

3

WIND

࣭The First Stage : Dragging Anchor with Yaw and Sway錨鎖

Yaw and sway motion of a vessel when lying to an anchor is sometimes
referred to as “horsing”. Area [A] in the diagram shows the situation
where the ship is lying at anchor andyawing in a “figure-of-eight” motion.

AnchorChain
A

It has been found that as wind pressure force begins to exceed the anchor’s
holding power, the ship yaws and is pressed to leeward, as shown by area
[B] in the diagram.
It is suggested that, during this period, it should be relatively easy tocontrol the manoeuverability of a ship in such a state and to weigh theanchor.

B
錨の軌跡
Track ofAnchorC

船体
Shipʼs Hull

࣭The Second Stage : Anchor Dragging Causedby Wind Pressure

段階図 /diagram

Where wind pressure force gradually becomes stronger, one side of the ship turns against the wind and isthen pressed and moves to leeward at a certain speed, as shown in area [C] in the diagram.
It is suggested that, during this stage, it is difficult to weigh anchor and, even if possible, this takes aconsiderable amount of time. If weighing the anchor cannot be accomplished, the ship loses its
manoeuverability.
Very large vessels, such as loaded container ships and car carriers have a large windage area. In theircase, full control may be lost even though the ship’s main engine and bow thruster have been utilised
when weighing anchor.
Dragging anchor may not be detected by the usual methods until the vessel has entered the second stage
described above, by which time it may be too late to avoid a dangerous situation from developing.
Personal computers, nowadays, can be used to calculate external forces on the anchor and cable system.
The calculations utilise catenary equations which take into account the water depth as variable, theanchor holding power, the type of anchor, environmental forces and the forces acting on the windlass andcable stopper. The results of the calculations are used to show the minimum required length of anchor
chain and the area needed around the ship for a safe anchorage.
In addition to the usual methods of checking the vessel’s position by reference to fixed points describedabove, early prediction and detection of the dragging of an anchor is also possible using the ship’s wakeindicators in the ECDIS, RADAR and GPS displays. Therefore, counter measures for the safety are
required to be taken as earlier as possible.
The following images are examples of GPS-plotted wakes on ECDIS and RADAR
screens, showing the anchor dragging with yawing:-

ECDIS

RADAR

圧流走錨開始Initiation of dragging anchor by wind pressure force

The display depicts the moment the ship’s starboard bow turns against the wind.
GPS indicates direction and speed of the ship’s movement by vectors.

4

i3

Wind Pressure Force Calculation

Hughes Formula
Ra = 1/2 ×ρ× CRa × Va2

㸦 A cos 2 ȟ㸩 B sin 2 ȟ㸧 / 1000㸦ton㸧ȟ㸸Wind direction from bow 㹙degree㹛㸦Relative Wind Direction㸧

Va

㸸Headwind speed 㹙m/sec㹛Ȩ㸸Air density㹙0.125 kg࣭sec 2/m 4 㹛

A 㸸Ship’s projected area from bow above waterline 㹙੍㹛B㸸Ship’s projected area from side above waterline 㹙੍㹛a㸸Length from bow to wind pressure center㹙m㹛㸦Point of Action㸧

Ra 㸸Resultant wind pressure force㹙kg㹛 Ѝ divided by 1,000 to be “ton”㸦Total Wind FȘ㸸Wind pressure force angle㹙degree㹛㸦Angle of Action㸧

CRa㸸 Wind pressure force coefficient㸸 This varies for different ship types, as follows.

相対風 向角（θ）
Relative Wind Direction
風速：Wind Speed
Va（m/sec）

Passenger㸸1.142 - 0.142cos2 ȟ - 0.367cos4 ȟ - 0.133cos6 ȟ

θ
風圧力角
（α）
Wind Press. Force Angleα

船首から風圧中心までの距離а
Length from Bow toWind Press Center

General Cargo
㸸1.325 - 0.050cos2 ȟ - 0.350cos4 ȟ - 0.175cos6 ȟ
Tanker & Bulk Carrier㸸1.200 - 0.083cos2 ȟ - 0.250cos4 ȟ - 0.117cos6 ȟ

風圧合力RaResultant Wind
Press.

Resultant wind pressure force is proportional to thesquare of wind speed.
i4

Holding Power Created by Anchor and Anchor ChainS 㸸Catenary length against the external force 㸦m㸧y 㸸Water depth + Hawsepipe height from sea surface 㸦m㸧l 㸸 Minimum Required Contacted length of the chain㸦m㸧
L 㸸Minimum Required Length of Anchor Chain (m) 㸦S 㸩 l 㸧
Tx 㸸External force 㸦kgf㸧
H㸦Holding Power Created by Anchor and Anchor
Chain㸧= Ha + Hc = Ȣ a  Wa̓ + Ȣ c  Wc̓  l

B: ベルマウス（Bell Mouth）

T ：外力（External Force）

T ：外力（External Force）
水面
Sea Surface

L= S +

錨鎖の係駐部
Cable Contact5S

錨鎖の懸垂部
垂部部（カテナリー部）
Catenaryy

海底
Sea BottomH 㸸 Holding Power Created by Anchor andAnchor Chain㸦kgs㸧
Ha

Wa' 㸸 Anchor Weight in Water 㸦kgs㸧
= 0.87Wa 㸦kgs㸧

㸸 Holding Power by Anchor 㸦kgs㸧

Wc' 㸸Anchor Chain Weight per m in Water 㸦kgs㸧
= 0.87 Wc 㸦kgs㸧

Hc 㸸 Holding Power by Anchor Chain 㸦kgs㸧
㸦Resistance of Cable㸧l 㸸 Minimum Required Length of Anchor Chain (m)Wa 㸸 Anchor Weight in Air 㸦kgs㸧Ȣ a 㸸 Anchor Holding Factor

Wc 㸸 Anchor Chain Weight per m in Air 㸦kgs㸧 Ȣ c 㸸Anchor Chain Holding FactorȢ a 㸸 Anchor Holding Factor
Type of Anchor
Sand
JIS
AC14

3.5
7.0

Mud

Dragging Anchor

3.2
10.6

1.5
2.0

Ȣ c 㸸 Anchor Chain Holding Factor
HoldingȢ c0.75 㹼 1.0
i5

Dragging
Sand
0.75

Mud
0.60

JIS

AC14

Calculating the Catenary Length of an Anchor Chain

The following formula provides the catenary length of an anchor chain.
external force becomes greater as the catenary length increases.

It should be noted that theS = ට‫ܡ‬t ൅ ૛㸦‫ܠ܂‬Ȁ‫̓܋܅‬㸧‫ܡ‬S
㸸 Catenary length against the external force 㸦m㸧y㸸 Water depth + Hawsepipe height from sea surface (m)Wc' 㸸 Anchor chain weight per m in water 㸦kgs㸧
= 0.87Wc 㸦kgs㸧
Tx 㸸 External force㸦kgf㸧
Under the condition that L [Minimum Required Length of Anchor Chain (S + l ) ] is fixed at a certain
level, if Tx [External force (kgf) ] increases, S [Catenary length against the external force(m) ] will also
increase. On the contrary, however, l [Minimum Required Contacted length of the chain (m) ] decreases
so that H[Holding power created by Anchor and Anchor Chain (kgs) ] will be diminished.
i6

Examples of Anchor and Anchor Chain Stowed on Board

Examples of anchors and chains stowed on board typical kinds of ships are shown below. The anchor
and cable outfit is determined in accordance with the Equipment Number of each ship which is set by theClassification Society rules. The minimum weight of anchor chain per meter length may be estimatedby the formula 0.0219d2. The actual as fitted details of a vessel’s anchor and cable outfit should be
included in the ship’s documents.
Examples of Anchor and Anchor Chain Outfits
Type of ship

Weight of anchor

230,000DWTVLCC
6,000units PCC
8,000TEUCTNR
25,000GTCTNR
5,000GTG.CARGO

18.7tons
10.5tons
15.0tons
8.3tons
2.7tons

ᚄ
102mm
87mm
98mm
81mm
56mm

6

㘌㙐
1m ࠶ࡓࡾࡢ᭱ᑠ㔜㔞
228kgs
166kgs210kgs144kgs
69kgs

i7 Excel Spreadsheet Calculation of Wind Pressure Force(Please refer to attached excel file)i8

Horsing (Yawing and Swaying) Motion and Impact Force
The vessel is oscillating about the anchor with a yawing andswaying motion from side to side, as indicated by the movementfrom ձ Ѝ ղ in the diagram. At this stage, the anchor cable isunder tension.
At position in the diagram additional weight comes on the system as
the vessel is brought up sharply and “snubs” the cable. As tension
then relaxes, the ship’s bow tends to pay-off in the opposite direction(position ճ to մ in the diagram).
The pattern of oscillations continues through positions յ , ն ,շ and ո in the diagram and is then repeated.
In this way, the ship’s center of gravity is moving in a
“figure-of-eight” pattern as illustrated by the green track in the
diagram.

i9 Excel Spreadsheet Example of Anchor Holding Power Calculation(Please refer to attached excel file)
i10

Ship’s Operational Safety Measures for Anchorage and Their Effects

Counter measures

Effectiveness

Remarks

Increase draught bytaking in ballast water

Ship’s weight is increased so thatvessel’s motions (Horsing) ared
ecreased.

Consider stability issues.

Trim by the head

Veer
cable

more

anchor

The point of action shifts afterward
and tends to decrease the horsing
motion.
Increases anchor chain holding factor.
Extended catenary length absorbs
more external force on anchor.

Drop the other anchor

Can reduce yawing and horsing
motion by half, and reduce force onanchor by 30%㹼40%.

From the outset ofanchoring, to deploy
both anchors

Riding to two anchors is said toincrease holding power and to
decrease horsing motion.

Useofbowthrusters

By stemming the wind, this caneffectively reduce the horsing motionand ease cable tension. If the power ofthe bow thruster is 80% of the windforce on the bow, it is said that widthof oscillating motion and impact force
are diminishe dy about 40%.
This can be an effective deterrent to
the horsing motion and will relieve
the tension on the anchor and cablesystem.

Use of the main engine
in combination withsteering

7

Considerstability
propeller immersion.

issues.

Maintain

Consider that weighing anchor is difficultin rough sea conditions and more time will
be required to weigh the anchor.
Consider amount of second cable required
is one and a half times the depth of water.
Consider the possibility of fouling thecables, particularly when pitching heavily.
Danger of fouling an anchor if the vessel
isturned under the influence of wind and/or tide.
The possibility that extended use of thebow thrusters may not be possible fortechnical reasons. Ensure that the bow
thrusters are kept submerged when theship is pitching and rolling.

Do not allow the vessel to pay-off suddenly
when the tension on the anchor cable has
been eased as a sudden increase in tension
may break-out the anchor. Do not allow
the vessel to override the anchor,
particularly in shallow water where thevessel could impact on the anchor if
pitching.

(1)Example Calculation of the Increase in Holding Power When Cable is Veered

The critical wind speed under the following preconditions
when one shackle of anchor cable is veered from a PCC ladenwith 6,000 units is calculated as follows:-

㸦Preconditions㸧
㸦Wa㸧㸭 Anchor Weightin Air㸦Wa㸧
㸸10.5ton 㸦Ȣ a㸧 7.0
э㸦Wa'㸧㸭 Anchor Weight in Water

9.135 ton㸦Wa'㸧

㸦Wc㸧㸭 Anchor Chain Weight per min Air(Wc)㸸0.166 ton/mAnchor Chain Holding Factor 㸦Ȣ c㸧 1.0
эAnchor Chain Weight per m in Water 0.144 ton/m㸦Wc'㸧㸦y㸧㸭 Water Depth + Hawsepipe Height from Sea Surface㸦y㸧㸸 25.0m
Length of One Shackle of Anchor Cable㸸 27.5m
Ship’s Projected Area from Bow Above Waterline㸦A㸧㸸 800 sqm
Wind Pressure Force Coefficient㸦CRa㸧㸸 0.75
Air Density㸦Ȩ㸧㸸0.125kg/sec 2 /m 4
(Situation Prior to Veering Additional Cable)
Before any additional cable is veered, the ship remains in a stable position by virtue of the anchor
holding power, which is equal to impact force on the anchor and cable system. The anchor cable isassumed to have formed a catenary with no cable lying on the ground. Before determining the criticalwind speed, the wind force on from ahead must be calculated.
It can be demonstrated that the impact force (which, in this case, is the same as the anchor holding
power) should be divided by 6.

B:ベルマウス(Bell Mouth)

T : 外力 63.9t
External Force

水面
Sea Surface

S′=150.9m

y=25m

海底
Sea Bottom

錨額の懸垂部（カテナリー部）
Catenary8Anchor Holding Power Being Equivalent to Impact Force (external force)㸸63.9 tonfэ the Wind Force from Ahead

10.65 tonf

Catenary Length㸦S'㸧
㸸150.9 m㸦5.5ss㸧
The critical wind speed can be calculated from the Hughes Formula as 16.90m/sec.
The average wind speed can then be calculated as 11.3m/sec – 13.5m/sec.
The critical wind speed can be converted into the average wind speed which is solved that the criticalwind speed be divided by 1.5 or 1.25.
These average wind speeds are the maximum limits under which the vessel in this example can lay
safely at anchor without exerting more force on the anchor and cable system than it can withstand.

(Situation After One Additional Shackle of Cable is Veered)
After a further shackle of cable is veered, the critical wind speed will be increased.
Only part of the longer cable system will lay along the ground with the remainder forming part of a newcatenary, which will now be longer as it will be extended by the increased external force caused, in turn,by the increased critical wind speed.
There is a minimum required contacted length of chain, that is, the minimum length of chain in contact
with the ground to ensure the anchor is properly embedded. This length㸦l 㸧 can be determined by thefollowing formula:‫͛܉܅‬ൈૃ‫܉‬㸩‫͛܋܅‬ൈૃ‫܋‬ൈ‫ܔ‬S̓+ 㸦27.5m ̽ l 㸧 =ට‫ܡ‬૛ ൅ ૛ ൈ 㹙ሿ  ൈ ‫ܡ‬‫܋܅‬ᇱ

B:ベルマウス(Bell Mouth)

T : 外力 67.3t
External Force

水面
Sea Surface

L = S + =178.4m

S=154.8m

y=25m=23.6m
海底
Sea Bottom

錨鎖の
係駐部
Cable
Contact

錨鎖の懸垂部（カテナリー部）
CatenaryS̓㸦Catenary Length before One Shackle is Veered㸧㸸 150.9m㸦5.5 shackles㸧
l 㸸 Contacted Length of the Chain (laid over the bottom) One shackle of cable (27.5 m) veered consists of͆ l ͇ for 23.6m and a part of catenary for 3.9 m.
㸦S㸧 Catenary Length㸸 154.8m
㸦 l 㸧 Contacted Length of the Chain 㸦laid over the bottom㸧
㸸 23.6m
㸦L㸧 Length of Anchor Chain 㸦after one shackle of cable is veered㸧㸸 178.4m 㸦6.5 shackles㸧

9The holding power created by the anchor and cable system in this case becomes 67.3 tonf and isequivalent to the impact force.
Because of the extension for 23.6m, the impact force has been increased by 3.4 tonf 㸦 = 67.3 tonf - 63.9
tonf㸧
The wind force from ahead is, therefore, solved as 11.23 tonf.
The critical wind speed can be calculated as 17.3 m/sec.
[This is 0.4 m/sec more than before the additional cable was veered 㸦17.3m/sec -16.9m/sec㸧.]The average wind speed can then be calculated as 11.5m/sec 㹼 13.8m/sec.
In comparison with the average wind speed before one shackle of cable is veered, there are
increases of 0.2 m/sec ~0.3m/sec to the critical wind speed.

(Example considering the case when a full length of cable (12shackles) is veered)If twelve shackles are veered, the critical wind speed is computed as follows;㸦S㸧 Catenary Length㸸175.0m
㸦l 㸧 Contacted Length of the Chain 㸦laid over the bottom㸧
㸸155.0m 㸦+22.4 tonf㸧㸦L㸧 Length of Anchor Chain
㸸330.0m 㸦12 shackles㸧
Holding power created by anchor and anchor chain in this case becomes 86.3 tonf being equivalent toimpact force. The wind force on front is 14.38 tonf.
The critical wind speed in this case is 19.6m 㸦+2.7m/sec㸧.
The average wind speed becomes 13.1m/sec 㹼 15.7m/sec.
This is an increase of 1.8m/sec 㹼 2.2m/sec.
Importantly for those on the bridge, the critical wind speed is not increased as much as might be
expected even if the anchor cable is veered considerably.

(2) Example of Reducing Horsing Motion by Using the Bow Thruster
The wind force from ahead (a PCC laden with 6,000 units), in an example where the wind speed is
16m/sec, is 22 tonf (gusting in storm conditions could result in a maximum instantaneous wind velocity
of about 24m/sec, say, one and half times as strong as in the example ). Generally speaking, in order tocope with the above wind force, the following horsepower (assuming 1ton=100hp or PS and 80% of thiscapacity is required), would be necessary:22 × 0.8 × 100=1,760PS
From the practical view point, so as to enable the bridge personnel to reduce horsing motion, the mostefficient way of using the thruster, and its prolonged use, should be discussed with the engineering
department so as to avoid damage to the equipment.

(3) Reducing Horsing Motions by Using the Main Engine and Steering
In theory, an effective method of reducing horsing motions is to stem the wind by working the main
engine in conjunction with use of the rudder. In practice, however, when this is done, it should be
remembered that the wind speed is approaching the critical limit. Care must be taken, therefore, toavoid a situation where the anchor cable repeatedly slackens and then becomes taught as this may
impose excessive impact forces on the anchor.

10(4) Using the Second Anchor As a Snubber࣭Riding to Two Anchors
To drop the second anchor to act as a check, or snubber, is said to effectively reduce the so-called horsing
motion. A suitable scope for the snubber is one and a half times the depth of water.
Nevertheless, it is impossible to completely stop the horsing motion. Care should be exercised when
deploying a second anchor to avoid entangling the cables and creating a foul hawse, particularly whenthe vessel is pitching heavily.
When riding to two anchors, there is a possibility that the cables may become entangled resulting in afoul hawse. This method is not recommended for large ships because of the practical difficulties
involved in disentangling large and heavy anchor cables and it is likely that outside assistance will be
required. Smaller vessels may be able to clear a foul hawse themselves.

振れ止め錨Sunbber
Anchor

2 Anchor Mooring

二錨泊
i11

双錨泊

The Critical Wind Speed

While looking into various reference books, there is no concrete indication.
Reasons
࣭Within a single anchorage, the holding power of each vessel̓s anchor is dependent upon the conditionof the ground in the immediate vicinity, and this may vary from location to location within theanchorage.
࣭Holding power is created, not only by the anchor, the flukes of which must bite into the ground firmly
and remain buried at their designed angle, but also by the resistance of the cable laid over the ground.
Bearing in mind the possible variations in the composition of the sea bed, the actual holding power
may not always conform to the theoretical value obtained by calculation.
࣭Further, complex combinations of ship motions, such as heel due to wind pressure, pitching, heaving,surging, swaying and yawing in the wave pattern, together with restraints imposed by the stress on
the anchor cable, cause continuing changes in the direction of the anchor cable and the angle of action
on the mooring system. The result is that the anchor cable may be subjected to shock stresses as thecable sags and then tightens.
࣭In practice wind direction and speed are likely to change so that the horsing motion may not be
constant and the motion may even be accelerated.
It should be remembered, therefore, that the value of wind pressure force and impact force derived fromthe formula may not reflect the actual state. This may be misleading and ship’s staff are advised totreat with prudence the computations set out on §9, which are offered as guidance, only.
After taking into consideration all the factors set out above the safe and prudent decision may well
be not to anchor.

11

i12

Emergency Measures Taken and Their Effectiveness After Dragging Anchor

Once an anchor starts to drag, immediate counter-measures should be taken. These include weighing
the anchor in order to restore a maneuvoerable condition and then re-anchoring, seeking sheltering in anarea where drifting is safe, or returning to the open sea.
Should there be space around the ship with no other vessels lying at anchor, there may be time to restore
a controlled condition by stemming the wind and weighing anchor. However, analysis of cases of anchor
dragging reveals that, major accidents such as collisions with other ships and groundings are almost
always the result of a ship dragging its anchor because control was not restored in the way describedabove.
(1) Veering an Additional Cable and Use of the Second Anchor
Once a ship starts to be pressed to leeward, inertia increases and more power is required to overcome it.
At the very early stage of dragging, when the horsing motion becomes apparent, and before the ship ispressed to leeward with increasing speed, it may be beneficial to veer more cable, or to deploy a second
anchor. Nevertheless, as discussed above, the addition of more cable is not expected to increasesignificantly the holding power of the system.
Adding cable to the first anchor is not seen as an effective means of stopping a ship from beingpressed and drifting to leeward.
(2) Use of Bow Thruster
While dragging anchor and being forced to leeward, the use of bow thrusters to make the ship come up
into the wind may be effective. However, to be successful, the minimum thruster power must be equal
to the wind force on the bow.
For example, in the case of a pure car carrier, and assuming that the downwind movement is initiatedwith an average wind speed of 18m/sec, the required thruster power is about 28ton (2,800hp or PS).
(3) Use of the Main Engine and Steering
In order to maintain a ship’s head into the wind, in conjunction with large rudder angles, the required
power of the main engine is approximately as follows:-

Steering :Hard OverWind speed
20m/sec
25m/sec
30m/sec

Engine Order
Slow Ahead
Half Ahead
Full Ahead

In rough weather with the vessel rolling, pitching and yawing, etc., different engine settings may beneeded, always bearing in mind the need to avoid propeller racing.

i13 Difficulty in Maintaining Manoeuverability
If the ship swings broad on to the wind and begins to move to leeward, considerable time and space may
be required before effective control can be restored by using the main engine, rudder and bow thruster.
It should be remembered that when the propeller is working the effect of the bow
thruster will be decreased by about 20% per 1knot of ahead speed.
In other words, at about 5 knots, the effect of the bow thruster is negated.
Limitation of Manoeuvering by Rudder
Considering a pure car carrier laden with 4,500 units sailing across the wind using 15°of rudder,
diagram1 illustrates the limitations on manoeuverability imposed by various wind strengths comparedto the vessel’s speed.

12Numbers entered in the vertical axis are wind speed per ship’s speed and the wind force angle is enteredalong the horizontal axis. The yellow zone shows the area under the curve in which the effect of therudder is lost.
In an example where the car carrier is, say, dragging her anchor and the wind is on the beam (relative
angle 90 degrees) and she tries to turn into the wind, it is likely that this will be unsuccessful if the windspeed is eight times the ship’s speed. In other words, should the ship be successful in weighing anchor
and obtaining some speed, stemming the wind would remain extremely difficult. With a wind speed of
20m/sec, the ship’s speed would have to be more than 5knots (2.5m/sec), in order to stand a chance ofstemming the wind.
Diagram2 also indicates that, when dragging anchor under the influence of a wind speed of 20~25m/sec
the ship will move to leeward at a speed of some 3~4knots as shown in area [C] in the diagram.
It is estimated that, in the case of the car carrier, the maximum ratio of wind speed to ship’s speed which
will permit the vessel to maintain a given course is 2.8~3.8. This should not be confused with limitationof manoeuvering by rudder.
WIND

変針操船の限界
風 速 /船速 比

1210Limitation of Maneuvering by Rudder

変針不可能域

錨鎖

Unabale maneuvering

Anchor
Chain

area

8
A

6
Wind Speed/Shipʼs Speed

変針可能域
Maneuvering Area

4

B

2
0

錨の軌跡

30

60

Track ofAnchor90 120 150 180

図１／ diagram1

相対風向角
（度）

C

Relative Wind Direction (deg)4,500Unit Load PCC, h/d=1.3h：水深（Water depth） d：喫水（Draft）

船体
Shipʼs Hull
図 2 ／ diagram 2

i14

Preparation for Safe Anchorage

Accidents involving ships lying at anchor usually occur in the form of dragging anchor and drifting
without anchor holding power followed by collision and/or grounding/stranding. The basic way of
preventing such accidents is as follows:(1) When Anchoring is Anticipated, the Following Considerations Must Be Taken into Account:To select a sheltered good anchorage
࣭Land configuration
࣭The bottom configuration
࣭Holding grounding condition13࣭An appropriate depth࣭Sufficient room
࣭Sheltered from such an external force as wind and sea
࣭Degree of congestion of other ships at anchorage
To prevent an accident in the event that the anchor drags
࣭Keep a safe distance from other ships
࣭Keep a safe distance from shallows/other facilities(2) Technical Measures While Lying at Anchor
࣭External forces associated with wind speeds, directions, wave height, wave period, flow direction andflow velocity
࣭Ship type
࣭Hull dimensions
࣭Draught
࣭Trim
࣭Understanding the holding power of the anchor system
࣭Quantitative assessment of wind pressure forces
࣭Management of the main propulsion systems(3) Prediction and Early Detection of Dragging Anchor
࣭Understand fully the relationship between holding power and external forces࣭To detect dragging anchor by observing the horsing motion
࣭To use track display function of ECDIS 㸭 RADAR 㸭GPS(4) The Counter-measures To Be Taken
After Dragging Anchor is Detected࣭To weigh anchor and establish manoeuverability
as soon as possible
࣭To weigh anchor during the period of the swinging motion

i15

Safe Distance from Other Ships, Shallows and Other Facilities

Although it is essential to decide on the anchor position so as to avoid an accident, even after dragging
anchor, regrettably, there are no definite criteria to gauge the safe distance from other vessels, shoals
and other obstructions.
Mariners should take the following items into consideration when assessing the area which may beneeded while restoring manoeuverability by using the main engine/rudder/bow thrusters if the anchordrags.

1. A radius of swinging circle
A circle with a radius of minimum required length of anchor chain + the ship’s LOA

2. The speed of dragging anchor under wind pressure force
This is approximately 3 㹼 4 knots.

3. Required time to weigh anchor
In general, an anchor cable will be retrieved at a rate of about 9m/min
To retrieve 1 shackle takes about 3minutes.
It may not be possible to heave in a taught cable continuously, thus prolonging the operation ofweighing anchor.

14

4. Required time to prepare the main engine
for use.
To have the main engine on standby beforehand, if dragging anchor is predicted.

5. The required time to attain sufficient propelling speed when restoring manoeuverability after thevessel has been forced to leeward with the wind on the beam.

Case study
Case 1
Weighing anchor after dragging anchor is
detected㸦diagram 3ࠊgreen vessel㸧

Turning Circle

࣭The ship̓s LOA is 200m.
࣭She is lying at anchor with 8 shackles.
࣭Time to weigh anchor is 1.5 times longer thanusually required.
࣭During that period, the ship is being brought toleeward at 4 knots.
࣭After weighing anchor, it takes 15minutes until
the ship̓s speed reaches 5 knots.
࣭In the interval, the ship̓s side has been pressedby the wind.
࣭The main engine and bow thruster are readyprior to dragging anchor.
࣭Whilst weighing anchor, both are available.
࣭The wind speed is 20m/sec.

舵が効き始める
Start Turning

0.23N.Miles

1.0 N.Miles
揚錨 (Up Anchor)
5kts
2.4N.Miles

揚錨 (Up Anchor)

1㸬The radius of the swinging circle is:8 shackles (220m) + 200m =420m=0.23 miles
1.0N.Miles

5kts
2㸬Required time for weighing anchor:Whilst weighing anchor, it is deemed that the
0.3N.Miles
0.625N.Miles
ship’ manoeuverability is uncontrollable.
The anchor cable recovery rate in moderate（3L）
weather is known to be 9m per minute.
図 3 ／ diagram 3
Therefore it would take about 24 minutes toweigh anchor in moderate weather. However,in this situation, it would take 1.5 times as long or about 36 minutes. In this interval, the distancewhich the ship is pressed to leeward is about 2.4 miles (distance covered in 36 minutes at 4 knots).

3㸬After the anchor is aweigh, further 15 minutes elapse before the vessel̓s speed reaches 5 knots.
During this period, the vessel will drift about 1 mile to leeward (distance covered in 15 minutes at 4
knots).
The distance sliding horizontally(0knot +5knots)¹2  0.25 hours =0.625miles
Required distance to turn ship̓s head is 3 times as long as the ship̓s LOA (=200m)
200m  3 = 600m = 0.30miles
Total : 0.925 miles
Required distance from the anchor position is illustrated in diagram3.
Vertical direction (to leeward): 3.63 miles 㸦㸻 0.23miles +2.4 miles+1 mile㸧Horizontal direction: 0.93 miles 㸦㸻 0.625miles +0.3 miles㸧

15

Most accidents such as collisions and groundings involving ships occur when anchor drags because
insufficient sea room has been allowed for.

Case 2
㸦diagram 3ࠊblue vessel㸧Dragging is detected during the horsing phase and (Blue ship) weighing anchor is commenced
immediately.
At the initial stage of swing motion, the ship can weigh anchor and even though the ship’s side stands
against the wind, the required area for attaining manoeuverability is 1 mile to leeward and 0.625miles
horizontally outside of the initial swinging circle.
Vertical direction (to leeward): 1.23 miles (=0.23 miles +1 mile)
Horizontal direction: 0.93 miles (=0.625 miles +0.3 miles)
In practice, the distance to leeward could be shorter than illustrated in diagram3 since the main engine,
rudder and bow thruster may be used to advantage. It should be remembered, however, that the use oflarge rudder angles will decrease the ship’s speed.

Detect dragging anchor within the earlier time frame of the swinging motion
and decide whether to weigh anchor immediately and leave the anchorage
or to re-drop the anchor at another location.

i16

Dropping Anchor Operation

Recently there has been an increase in the number of accidents involving anchor cables becoming
entangled or anchors and cables being lost. These accidents have mostly been caused by mistakes being
made in the operation of letting go the anchor. In particular, most accidents have been caused by notcontrolling the running-out speed of the anchor cable, that is, without braking when the anchor is let go.
Test results show that the speed of anchor free fall reaches 10m/sec after 50m when an anchor is let gowithout braking. So to say, 12 shackles (=330m) could totally run out at 33seconds.
According to investigation results, although most mariners involved in anchor-related accidents statedthat the brake did not work well, thorough investigations on site have established that a bent brakeshaft and / or lack of maintenance were the cause. The crew were unable to properly apply the brakeforce.
To ensure safe anchoring, the veering rate must be limited to 5 to 6 m/sec using brake force.
If the depth at an anchorage exceeds 20m, the possibility of damage to or loss of the anchor and its cable
becomes greater due to excessive running out speed if the anchor is allowed to free fall. To avoid thishazard, the anchor should be lowered by walking back into the water until the anchor reaches about 5m
above the bottom.
When letting go, the brake should be applied in order to slow the veering rate until the length veered isabout 2m 㹼 3m more than the water depth. This should prevent the cable from piling onto the anchor,
as shown in the picture.
After the anchor touches the bottom, the ship's sternway should be limited to about
0.5knot 㹼 1knot in order to avoid imposing excessive strain on the cable and also to further
avoid piling. The aim is to lay the cable across the ground in an orderly fashion and without imposing
any excessive stress on the system.

16

i17
i17

The Anchor Cable Veering Rate࣭Scope of Cable To Be Paid Out࣭Brake Force of Windlass

Graph1 shows the relationship between brake force, scope of cable and veering rate determined during
trials on board a 230,000dwt VLCC when anchor and cable are paid out using the brake.
During the trial, the cable was first released with half brake applied. The brake was applied 3 seconds
after letting the anchor and was fully applied again after another 5 seconds in order to stop veering
completely. As can be seen, the length of cable veered in this time about 21m.
If the anchor is let go by free fall and the veering rate exceeds 10m/sec, it becomes difficult to arrest thecable and the brake lining may be damaged.
If, however, the veering rate is limited to about 5~6m/sec by the timely application of half brake, suchdamage will be avoided.

（ｍ/sec）
8
7
（ton）
20
15
10

Running Out Speed
繰出速度
Total Length
繰出量

6
5

（ｍ）
25

Break Power
ブレーキ力

4
3

20
15
10

2

5

1

０

０
０

5
1

2

3

4

５

０
6 （sec)

ＶＬＣＣ 投錨実験結果／ Result of VLCC Anchoring
グラフ１／ graph 1

17

Reference Books
࣭Theory and Practice of Ship Handling, by Dr. Kinzo Inoue
࣭Navigation Handbook
࣭Japan Captains̓ Association [A Guide to Ship Handling]࣭Ship Handling, by Dr. Keinosuke Honda
࣭Ship Handling, by Dr. Satoshi Iwai
࣭Study of Ship Anchoring Supporting system (Kobe University)
࣭Various reference pictures provided by Japan Captains’ Association and Nippon Yusen Kaisha--Capt. Takuzo Okada
Master Mariner
General Manager
Loss Prevention and Ship Inspection Dept.
The Japan Ship Owners’ Mutual Protection & Indemnity Association---

Japan P&I Club
Website http:www.piclub.or.jp‫ە‬Principal Office㸦Tokyo㸧
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