Ship Anchor – Complete Information

Introduction

>> The  anchoring  equipment of  a  ship  comprises  the  anchor, anchor  chain  and  the  windlass. The  anchoring  equipment  as required herewith  is  intended for  temporary   mooring  of  a  ship  within  a  harbour  or  sheltered  area when  the  ship  is  awaiting  berth, tide etc.

>> The  modern ship’s anchor  is  called  ‘Stock-less  Bower  anchor’ and  is  developed  from  the  primitive “ stock  Anchor”. Both these  anchors  are  illustrated  as  under.

Ship anchor old type. Stock Anchor. Pic credit : Brittannica

Stockless ship anchor. Pic credit : Brittannica

>> The  modern  ‘Bower  stockless anchor’ is  in  existence  since  the  days  of  steam  propulsion  and  has  been  developed  to  suit  the  stowing arrangement  in  the  hawse  pipe  of  the  ship  and  with  a  lower  holding  strength  than  the  stock  anchor.

>> The modern  stockless anchor  has  a  holding  strength  of   five  times  its  weight  in  Newtons, whereas  the  ancient or  primitive  stock  anchor  has  a  holding  strength of ten  times  its  weight  in Newtons.

>> The  reason  for  this  requirement  of  lightness is  that  the  anchor  has  to  be  hauled  quickly (within  30 minutes maximum ),  when  the  weather  deteriorates   during  anchor  stay  and  the  ship  has  to  haul  up  the  anchor  and  proceed  to  sea. A stock  anchor  will take  longer  time  to  haul  up and  therefore  endanger  the  safety  of  the  ship.

>> In the  worst  condition  it  can  lead  to  capsizing  of  the  ship  especially  if  it  is in  light  condition or  in  the  best  condition  drag  anchor  and  run  aground  ashore. Whereas  in  a sailing ship  the  ship  is  normally  anchored  deep  inside   the  natural  harbour  where the  severity  of  the  sea  condition  does  not  reach. It should  be  understood  that  the    greater  the  holding  force  , the  larger  time  required  to  haul  up  the  anchor.  The  forces acting  on  a  ship riding  anchor  is  described  as under.

Forces  acting on  the  ship  whilst  riding  anchor:-

>> The chain  connecting  the  anchor  to the  ship  is  of  heavy  construction  with  a  breaking  strength  much  higher  than  required .  The  chain  links  are  made  heavy so  that  when  the  anchor  is  dropped  with  the attached  chain  and  the  ship  is  in  stand  still  condition,  the  catenary  so  formed  is  deep  and  almost vertical.

The  shape  of  the  curve  formed by  the  chain  is  called  a  catenary. This  curve  is  like  a  parabola  but  has  a  different  mathematical  formula  when  compared  with  the  parabola.  It  is  a  physical curve  caused  by  a  hanging  chain  supported  at  the  ends and  the  total  depth  of  the  curve  from  the  end supports  is  directly  proportional  to  the  unit  length  of  the  chain.

>> The Bower  anchor  is  made  of  two  parts ,

1. The  head (crown) with  shoulder and the  flukes  folded  inwards

2. The  shank .

>> Both these parts are fitted together by a heavy and large pin fitted tightly on the head but loosely in the shank hole. The head has sufficient internal space to allow free movement of the shank to turn through about 15⁰ on either side of the vertical, so that when it hits the ground it is not stable and can fall on either side of the vertical position.

>> When the anchor is released at the time of anchoring it drops by gravity taking along with it the chain. The anchor head being curved and not flat causes the anchor to fall on either side with the shaft resting on the ground and taking a posture like a plough entering the ground and thus gets fixed in the earth or ground. The details of the forces acting on the ship when riding anchor are given in the sketch shown as under.

T=  the  total  tension  acting at  the  hawse  pipe  exit  and  is  resisted  by  the  ship’s  buoyancy   (has  a  very  small  trimming  effect   at  forward) .

W = the  weight  of  the  length  of  chain  forming  the  catenary ( length  of  chain  from  ground  at point  where  the  chain  makes  contact  with  the  ground  up to  the  hawse pipe  mouth).

T₁ = The  horizontal  tension  at  ground.

>> This  horizontal  tension T₁ is  the  vector  sum  of  the  current  and  wind  force. This  is  how  the  ship  is  moored  and  drift  prevented. Hence  the  anchor  resists  the wind and  tide.  The  ship  is  always  facing  the  combined  wind  and  tide  in   normal  times.

>> When a medium sized ship anchors about 5 shackles are paid out. Assuming that the ship has anchored at slack water, there may be only 2 shackles forming the catenary.

A shackle is a unit of length and equal to 15 fathoms or 90 feet and is the standard length of a chain .

>> In this condition the weight of 2 shackles is able to moor the ship. As the tide builds up the ship drifts aft picking up about two more shackles making the length of the catenary now to 4 shackles. In this condition the weight of the anchor together with the weight of one shackle of chain is greater than the horizontal component of the weight of the 4 shackles of chain forming the catenary . This is explained mathematically as follows;

          T₁  =   W Tan

>> Where W is the vertical angle . For all angles below 45^{0  ‘}W’ will be greater than T₁ and hence the ship has a reserve force or strength of the weight of the anchor and one chain shackle in this condition.

>> Now if for some reason the weather deteriorates and both the current and wind force increase the remaining shackle will be lifted out of the water and further deterioration in the weather may make the catenary degenerate into a straight line . This is the limiting condition when the vertical angle exceeds 45⁰ and the horizontal tension becomes larger than the weight of the anchor leading to the condition of anchor dragging.

>> During anchor stations some captains ask their C/O’s “How is the anchor leading.”. the C/O’s reply is normally 5 to 10 degrees to the vertical at slack water gives confidence to the captain that there is sufficient chain in the water to take care of stronger currents.

Anchor  material  and  testing

>> Most  of  the  modern  anchors  for  large  ships  are  now  made  of  cast  steel  so  that they  are  tough and  can  resist shock  loading  and  breakage  especially  if  the  anchor  strikes   a  rock  or  rocky  surface  whilst  anchoring.

>> About  100 years  back  all  anchors  were  made  of  cast iron  and  the   method  of  testing  them were  by  dropping  them  on  hard  prepared  ground  from  a  height  of  75  feet. If it  survives  this  test  without  breaking  or  cracking, it  was  sent  to  the  ship  and  if  it  cracked   or  broke  it  went  back  to  the  foundry  for  recasting. This  is  destructive  testing   and  not  used  now.

>> Modern  anchors  are  subjected  to  non-destructive  testing  and  the  procedure  is  as  follows.

Non-destructive  testing of ship anchor

>> The   anchor  is  placed  on  the  test  bed  of  a  chain  testing  machine. The  shank  end  is  connected  to either  the  fixed  end  of  the  machine  and  the head  is  connected  to  the  ram  end of  the  machine  which  moves  inwards  by  hydraulic  pressure .

>> The  pressure  is  monitored  on   the  pressure  gauge  fitted to  the  hydraulic  cylinder.  The  required  proof load  is  given  by  P* machine  constant  in  tons  or  KN as  required.

>> The  arrangement is  sketched  below. The  proof  load  is  kept  for  30-60 minutes  as  per  class  regulations and  any  deformity  in  shape  of flukes  or any  crack  formation  is  to  be  observed  before  approving  the  anchor.

ANCHOR TESTING METHOD

Can a old anchor from scrap yard be used as replacement anchor?

>> An  old  anchor  obtained  from  a  ship  scrap  yard can  be  used  as  a  replacement  anchor  provided the  following  conditions  are  met:

• The selected  anchor conforms  to  the  mass requirement  as  per  the  equipment  letter
• The general  inspection  reveals  no
• The  anchor  is  to  be  subjected  to  a proof  load  test  as  per  the  proof  load  given  in  the  tables  and  witnessed   by  the  class    The testing  machine  should  be  approved  by  class.

Anchor Proof load test table

Anchor Proof Load Table

Classification  rules  on Anchors and Chains

>> The Anchor & Anchoring equipment is therefore not designed to hold a ship off fully exposed coasts in rough weather or to stop a ship which is moving or drifting. In this condition the loads on the anchoring equipment increase to such an extent that its components may be damaged or lost due to the high energy forces generated particularly in large ships.

>> The anchoring equipment presently required herewith is designed to hold a ship in good holding ground in conditions such as to avoid dragging of the anchor. In poor holding ground the holding power of the anchors will be significantly reduced.

>> A good  holding  ground  as  defined  by  the  above  stated requirements  are;

• A current  speed  of not more than  5  m/sec (max tide current  speed)
• A  wind  speed  of not more than  25m/sec
• Depth of  water ranging  between  85M and 100M .
• The ground should  be  sandy  or  muddy and  not  rocky

>> A  good  anchoring  ground is  normally  marked  on  the  chart  by  an  anchor  symbol  on hydro-graphic  charts giving  details  of  port  anchorages.

>> The  length  of  chain  provided  varies  between 86   M  to 770  M  depending  on  the  size  and  dimensions  of  the  ship.

>> To  find  the  equipment  details  for anchors  and  chains as  given  in  the  rigging  tables  of  the  classification  rules,  the  following  procedure  is  adopted.

Equipment number

The equipment number, EN, on which the requirements of equipment are based is to be calculated as follows :-

EN = K * ENc

where,

ENc = Δ^2/3 + 2BH + 0.1A

Δ = moulded displacement, [t], to the summer load water line

A = area [m2] in profile view of the hull, superstructures and houses above the summer load waterline, which is within the Rule length of the vessel. Houses of breadth less than B/4 are to be disregarded.

H = effective height, [m], from the summer load waterline to the top of the uppermost deckhouse, to be measured as follows:

H = a + Σhi

a = distance [m] from summer load waterline amidships to the upper deck at side

hi = height [m] on the centre line of each tier of houses having a breadth greater than B/4. For lowest tier, hi is to be measured at centre line from upper deck, or from a notional deck line where there is a local discontinuity in the uppers deck.

Calculation notes

>> In the calculation of H and A, sheer and trim are to be ignored.

>> Parts of windscreens or bulwarks which are more than 1.5[m] in height are to be regarded as parts of houses when determining H and A. The height of the hatch coamings and that of any deck cargo, such as containers, may be disregarded.

>> ‘K’ is a factor depending upon the type of vessel and service notation as given below: For fishing vessels,

K = 1.00

For other vessels,

K = 1.00 for vessels of Unrestricted Service.

K = 0.85 for vessels of Coastal Service

>> Having found the equipment numeral,  the details regarding sizes and quality may be obtained from the rigging tables provided in these regulations.

>> The equipment numeral is also given in the class certificate and hence there is no need to refer to this formula to find the equipment numeral whenever there is a need as in the case of ordering new chain lengths when old chain lengths have been worn beyond the rule requirement.

>> For information and guidance a specimen of the Rigging table is reproduced as under.

Rigging table

Rigging table

How to use rigging table?

>> As an example let us consider the group EN 2380 to EN2530. The corresponding equipment letter is J+, each anchor weighs 7.35 tons and the total length of chains provided is 605 meters.

>> The chains always come in lengths of a unit “shackle” , where 1 shackle is 15 fathoms or 90 feet. The shackle is an English unit and all chain manufacturers make chains in one shackle length. The rule length is therefore increased to the nearest full shackle length to meet the rule requirement .

>> For ease of operation the total length of 605 meters will be divided into two equal halves and the nearest minimum full chain length in shackles per chain is given by 605/2*3.7878/90=12.73 shackles. The nearest full shackle length is 13 shackles. Hence each anchor will be connected to 13 shackle lengths of chain to make up for the rule length of chain.

>> As per the classification rules the ship has to be provided one spare shackle of chain. This spare shackle is therefore connected to one of the chains as that is the most convenient way of stowing the spare chain length. If it is kept in the forecastle stores, it will occupy all the space and make it difficult to stow spare mooring ropes and wires.

>> Hence on most ships either the port or starboard chain is longer by one shackle and that extra length is accounted for the spare shackle length. The shackle length is connected to the next shackle length by a ‘Kenter “ shackle which is specially designed to fit in the cable lifter link slots, and also be capable of being opened and joined.

Anchor chain links

The link is called a stud link and is different from an ordinary chain link.

Anchor Chain links

>> The sketch(Anchor chain links) of two stud links , one a common link and the other an enlarged link, a kenter shackle and an ordinary shackle are reproduced here to understand their uses.

>> The ordinary shackle is used to join the anchor at the anchor end and the chain in the chain locker connecting it to the bulkhead stiffener. The kenter shackle connects one chain length to another chain length and the stud links are of two standards one common size and the other enlarged link for larger vessels..

Grades of chain link

>> The chain links are graded according to the carbon content in the steel and corresponding heat treatment and the three standards of link material are graded as CC1, CC2, and CC3. The three standards are graded as follows :
CC1———— low strength.
CC2———–medium strength.
CC3———–high strength.
>> Owners are free to choose any grade of steel from the given three qualities. Having selected a grade the same grade is to be used throughout the chain length for the full life span of the ship, since the cable lifter slots are made as per the diameter according to the grade selected.

Anchor  chain  stowage  and  connections

>> The chains are stowed in two separate chain wells placed in the chain locker forming part of the forepeak tank. The chain is led out of the locker through a pipe with a bell mouth at the inner end of the spurling pipe.

>> This bell mouthing is provided to enable the chain to move in a rotating fashion as it is being stowed while heaving up the anchor. This rotating motion enables the chain to be stowed properly without forming a heap.

>> The chain passes out of the spurling pipe wraps on the cable lifter and enters the hawse pipe where it is connected to the anchor.

>> The reason why stud link is used is it reduces free movement of the links within the chain locker and thereby prevents “ kinking” of the links. When kinking happens the free flow of the chain is prevented by a lump of chain getting stuck at the bell mouthing preventing free flow of the chain.

Anchor end of the chain

At  the  anchor  end  a  turning  pendent  is  connected  between  the  anchor  and  the  connecting  shackle. This  turning  pendent is  provided  to  allow  free  rotation  of  the  anchor  when  it  is  lifted  out  of  the  water  without  allowing  the  chain  to  twist.

This  rotation  occurs  after  long  anchorage  stay. In  a  day  ,  whilst  at  anchor the  ship  drifts  through  one  circle  around  the  anchor  caused  by  high  and  low  water. The  anchor  chain  gets  twisted  by  one  turn. If  the  ship  is  at  anchor  for  10  days it gets twisted  by  10 turns. when  the  anchor  gets  lifted  out  of  the  water,  the  stored  twist  energy  in  the  chain  is  released causing  the  chain  to  unwind  along  with  the  anchor. The  anchor  being  heavy  will  gain  high  rotational  momentum  and  hence  will continue turning  even  after  the  chain  has  straightened and  that  will hinder  hauling the  chain  through  the  hawse pipe. To  free   the  chain  from  this  rotational  movement  this  pendent  is  fitted. The  sketch  of  the  pendent  is  given  under.

Bitter end of the anchor chain

>> The inboard end of a ship anchoring cable which is secured in the chain locker by the clench pin is called the bitter end of anchor cable.

>> In an emergency especially when the weather turns bad and operation of the windlass is not possible, either because the windlass is defective or the entry to forecastle deck is not possible, the chain can be made to run out by dismantling the joining shackle in the chain locker, and releasing the windlass brake.. A marker buoy is attached to the chain so that the chain and anchor can be recovered later.

>> Provision is to be made for securing the bitter end of the chain cable to the ship structure. The fastening for securing the bitter end is to be capable of withstanding a force of not less than 15% and not greater than 30% of the minimum breaking strength of the as fitted chain cable.

>> It is to be provided with suitable means such that, in case of emergency, the chain cable may be easily slipped to sea from an accessible position outside the chain cable locker. Where the mechanism for slipping the chain cable to sea penetrates the chain locker bulkhead, this penetration is to be made watertight.

>> Bitter end should be capable of holding the chain weight, but it should be constructed so that, it will break incase of emergency so it doesn’t damage the ship structure. As mentioned above, there has to be a provision outside the chain locker which will release the bitter end of the chain in case of any emergency and the anchor and chain is to be released to the sea.

>> Alternatively the cable end connection may be accepted where it has been designed and constructed to a recognised National or International Standard.

>>  The cable clench supporting structure is to be adequately stiffened in accordance with the breaking strength of the fastening provided.