Designated Purpose of Anchors
On the basis of generally accepted criteria, a ship is expected to remain at anchor:
(a) In good holding grounds.
(b) In sheltered and semi-sheltered waters.
(c) In winds up to gale force 8.
The master is expected to put to sea if severe storms are forecast.
The existing equipment is not intended by the classification societies to be suitable as:
(a) A last means of defense in case of a machinery breakdown off a lee shore in a storm.
(b) A means of bringing a large moving ship to a stop.
Knowledge of the type of anchor on your ship is necessary because the holding power of each type is different. The holding power of an anchor is normally expressed in tonnes. The maximum holding power is reached when the anchor starts to drag but without a reduction in its stability.
Holding efficiency = Holding power / Anchor deadweight
Maximum Holding Power is reached when the anchor starts to slip but without a reduction in its stability.
The anchor is most efficient when subjected to a horizontal pull by the cable on the seabed. The aim should be to ensure that enough cable is veered to achieve this situation.
If an insufficient cable is veered to exert a horizontal pull at the anchor, its holding power will be reduced in accordance with the following scale:
When the ship drags her anchor, more cable is veered and the anchor holds. This action is correct, but the belief that it is the resistance of the extra cable on the seabed that lets the ship hold is a fallacy; the veering of the cable removed the shank/cable angle and the anchor holds once more.
Amount of Cable to Use
In addition, the amount of cable to veer depends upon a number of factors:
(a) Holding characteristics of the anchor.
(b) Length of stay.
(c) Strength of wind, tidal stream or current, and sea/swell.
(d) Swinging room.
(e) Type of cable.
(f) Depth/draught ratio.
An approximate formula for forged steel cable is:
The amount of cable to veer in shackles is one and a half times the square root of the depth of water in meters.
The Admiralty Manual of Seamanship quotes the amount of cable to veer in shackles is twice the square root of the depth of water in fathoms.
When anchoring in an area where other ships are anchored, the approach course opposed to the resultant of the wind and current can be visually appreciated. This is more difficult to achieve when anchoring in an area devoid of other shipping, especially at night. The larger the ship, the smaller the limiting speed before dropping anchor. In a VLCC, the speed over the ground must be less than 0.5 knots. It is difficult to achieve a situation with the ship stopped over the ground.
A rule of thumb method often used is to reverse the engines until the propeller wash reaches the bridge or manifold (chosen by experience) and then let go the anchor. However, even if the experience of the master is such that he knows the ship is stopped through the water on these occasions, the ship is not necessarily stopped over the ground.
Prior to anchoring, the set and drift of the current and wind should be established as well as possible and the ship placed on a heading opposed to the resultant of these forces. The anchor position should be marked on the chart and the forecastle officer briefed on the bridge.
In large ships, in order to establish that the ship is on the correct heading and the ship is stopped over the ground, the anchor should be lowered to the seabed, brake applied and taken out of gear. The cable should then be paid out slowly until the forecastle officer can use the lay of the cable to ascertain the correct ship's heading. Provided that the length of the cable does not exceed twice the depth of water, the anchor will trip or drag, thus obviating undue stress on the tackle. Some pilots and masters prefer to carry out this operation with the anchor still in gear. When the forecastle officer is satisfied that the ship's heading and headway are correct, then the rest of the cable should be paid out and finally brought up. Excessive sternway, once the anchor has dug into the seabed material, can cause some part of the tackle to fail or carry away. The inertia of large ships is considerable. A VLCC making one knot over the ground has the same kinetic energy as Concorde at 1000 knots.
When the ship is brought up, the position of the bridge should be marked on the chart and swinging circle annotated with clearing bearings.
In the coastal trades of Europe small ships frequently resort to dredging anchors when berthing as an aid in confined areas, often in difficult tidal and wind conditions. On larger tonnage, with the exception in cases of emergencies, it is an art which over the last two or three decades has declined in usage.
It goes without saying, that the type of sea bed is of paramount importance to dredging, soft mud being the most obvious choice. The bottom must also be free of obstructions such as power lines, pipes, and rock. As the anchors normally dredge just inside the lines of the ship, there must also be adequate under keel clearance to avoid damaging the hull.
Research has shown that whilst the anchors and the cable are relatively reliable, the windlass is not. This is partly due to the disproportionate size of a windlass on the much larger ship of today. It should be remembered that the Kinetic Energy created by a ship of > 50,000 dwt moving over the ground in any direction, at more than say 0.3 knots, is enormous. By comparison, windlasses may only be capable of lifting a minimum design weight, which is specified by the Classification Societies. This is the weight of the anchor and four shackles of cable, hanging dead in the water and therefore nothing in comparison to the full weight of the ship. If all of this energy is imparted into the anchor it has to go somewhere and it is usually to the windlass. The dissipation of such energy results in as brake or gear failure!
Extensive research by the Classification Societies has established some safety parameters within which to operate when dredging anchors.
a) The amount of cable in the water should not exceed 1½ x depth of water. (Some sources quote 2 x depth). If this figure is exceeded the anchor is likely to dig in and commence holding.
b) The design speed of a windlass gypsy in gear is about 30 feet/minute which is approximately 3 minutes a shackle. This is equal to a ship speed of 0.3 knots over the ground.
c) The windlass is only designed to lift the dead weight of the anchor and four shackles.
If therefore, the amount of cable in the water does not exceed 1½ x depth, we have a safety factor which guards against speeds in excess of 0.3 knots, because the anchors will not dig in and hold, but drag.
When the amount of cable exceeds 1½ x depth, the speed must be below 0.3 knots, especially if the windlass is in gear or the brake is screwed up. The anchor will most certainly dig in and attempt to hold the full weight of the ship.
On smaller ships, when pilots are putting out anchors for dredging purposes, it is customary to 'let the anchor go' on the brake. On large ships, however, with unknown equipment and crews of mixed nationality and capability, it may be appropriate to walk out the anchors in gear rather than let them go on the brake. This affords the pilot much more control over the operation, guarding against the crew 'screwing up' the brake with the wrong amount of cable out, or worse still, letting the cable run away, leaving the pilot with 12 shackles out on both anchors!
Each tanker company has its own specific Standing Orders for anchoring large ships; their individual methods invariably work around these recommendations and should be adhered to at all times.
DREDGING TWO ANCHORS
Dredging is remarkably effective at eliminating the two major problems that occur when berthing without tugs, namely control of heading and speed. The effect of dredging can be seen in figure 3.
The ship is conducting a dredge with two anchors, working into a port side berthing position. By dredging the two anchors the pivot point is brought right forward between the two windlasses. This gives the ship an excellent steering lever. The small weight of the two anchors is also surprisingly effective in controlling the speed of the ship. Even whilst maintaining dead slow revolutions, the ship may gradually slow down and stop. This, in turn, ensures that the pivot point remains forward and that lateral resistance.
The net result is an ability to keep the speed down, but at the same time use main power more efficiently when controlling heading with kicks ahead. The bow can virtually be driven on the anchors, to the desired position. At the same time, the speed is kept well under control.
Some may express concern as to whether two anchors being dredged will foul each other while the ship is turning. The answer to this is no because, if the ratio of cable to a depth of water is correct, one anchor simply cannot reach the cable of the other; they should also be easy to recover once alongside.
There is a very fine line between the success or failure of a dredging operation. The following points are crucial in order to achieve the desired results:-
a) Maintain the normal slow speed of approach prior to letting go, or walking out, the anchors. If the ship is stopped the anchors will dig in and it is then difficult to get underway again. The ship may also drift badly out of position.
b) Walk the anchors out as early as possible. This gives sufficient time to get the feel of the ship before the berth is reached.
c) Do not let the ship stop too early. This lets the anchor flukes drop and dig in and it then takes substantial power to get the ship underway again.
d) Do not let the speed build-up. This pushes the pivot point back to its normal position. Consequently, the anchors cease to be effective and are of no use.
e) Do not exceed 1 1/2 x depth. It is possible, however, especially on small ships, to 'feel' the anchors and adjust the amount of cable accordingly.
f) Keep the weight on the anchors. Going astern the ship will take the weight off the cables and they become ineffective.
g) Avoid rushing the operation, particularly swinging, on large ships. There should only be a gentle and steady strain on the cable(s); the maneuver being slow but very effective.
h) Once in position on the berth, slack back the cables as the tension in them is sufficient to pull the ship off the berth.
Planning for Anchoring
1. Identify a suitable anchoring position before entering the anchorage area
2. Conduct a planned approach including speed reduction in ample time and orienting the ships head prior anchoring to
(a) Same as similar sized vessels around or
(b) Stem the tide or wind whichever is stronger
Decide on which method of anchoring to be used and the number of shackles depending on the depth of water, expected weather, and holding ground. A simple rule in determining the length of cable to use:
Length of cable = [(Depth of water in meters * 2) + 90 ] / 27.5
When good holding power cannot be expected:
(e.g. Strong Wind, Strong Current, Harder Sea bottom)
Length of cable = [(Depth of water in meters * 3) + 140 ] / 27.5
Radar parallel indexing technique can be used, as it is an effective tool in maneuvering approach to anchoring position.
A fixed reference point is necessary for establishing the intended anchoring position relative to this fixpoint.
Preparation for Anchoring
The Chief Officer must supervise letting go or weighing the anchors and should only assign experienced crew members to anchor work.
Prior to Anchoring, the Chief Officer should be aware of:
a. Approximate anchoring position
b. Method of approach
c. Which anchor to use
d. Depth of water
e. Method of Anchoring
f. The final amount of Cables
Procedure of the Introduction to Anchoring
At the Forecastle:
Check brakes are on and clear the securing devices (Anchor Lashings, Bow Compressed Bar, etc.)
a. Start Hydraulic (Source of) Power of Windlasses
b. Check Anchor Shape / Light
c. Check Communication with the Bridged. Check Lighting on Forecastle including a torch, at night time
e. Ensure all personnel is wearing Safety Helmets, Safety Shoes, and Goggles.
Before Letting Go Anchor:
The Chief Officer shall confirm that there is no craft or any obstacle under the bow and inform to the Bridge.
The Master shall ensure that the vessel's GPS speed at the time of anchoring is near-zero or indicates a slight sternway.
The speed should be verified by visual transits and/or Radar ranges of Landmarks if available, or other fix conspicuous targets.
Where means of communication between Bridge and the Anchoring party is by Portable radio, the identification of the ship should be clear to avoid misinterpretation of instructions from another user of such equipment in the vicinity.
Routine Anchoring Operation
There are 2 methods for Anchoring according to a depth of the water:
Method 1 (Preferable for Depths up to 50m )
a. Walk out the anchor to Half a shackle above the sea bottom
b. Hold the cable on the brake and take the windlass out of gear
c. Stop the vessel over ground
d. Drop the anchor
e. Control the speed of cable flow by the brake, while not allowing pile-up
f. Bring anchor cable direction forward and confirmed anchor holds its position
If the brake fails, or there is too much speed over the ground, the cable will run out to the bitter end with consequent damage. The brake lining could also be damaged due to this Dynamic load.
Method 2 (Suggested for Depths over 50m )
a. Stop the vessel over ground
b. Walk out the anchor under power until the complete length of required cable is paid out and anchor holds it position on the seabed.
c. Bring anchor cable direction forward and confirmed anchor holds its position.
The vessel must be completely stopped to avoid major damage to Windlass.
Particular Caution for VLCCs / big ships
VLCCs, because of their inertia requires great caution while anchoring.
They can suffer equipment failure if attempting to anchor whilst moving at speeds as low as half a knot over the ground.
Hence, the vessel must be nearly stopped not only in the linear direction but axial too, meaning the bow should not be swinging much either while anchoring.
The depth at which the vessel can safely anchor is about 110m or less, beyond which the windlass may have extreme difficulty in recovering the anchor.
Anchors should be ready for letting go on arrival and departure port, when in anchoring depths. At least, any wire lashings are to be removed and the anchors held on the brake.
In critical situations, to arrest the movement of the vessel, after stopping/reversing the main engine, it is preferable to let go both anchors simultaneously instead of one.
For weighing in the anchors, to reduce the load on the windlass, and keep the cable near vertical, as required, short movements to be given on the main engine (and Bow Thruster used, where is applicable)
The stay and direction of the cable and the residual shackles are to be continuously reported to the bridge.
Anchor Wash to be run to clean the chain and the anchor.
When the anchor is fully hove, the brake is to be applied and the windlass taken out of gear.
The bow stopper is to be put when it is deemed safe to do so.
If it does not engage properly on the chain, then it is to be lowered across the chain as far as possible and lashed down in this position in such a manner, that if the cable does slip, the bar will fall into place across the chain.
Walk Back speed
On the standard, the weighing chain speed is 15cm / second (= one shackle 27.5 meters / 3 minutes). It's corresponding to 0.3 kts of ship's speed.
It means ships speed with combining turning rate and drifting to the wind and the current lee side should be controlled less than 0.3 kts against direction anchor chain is going.
Do not use propelling beyond maneuvering Half Ahead / Astern Engine, except in emergency cases, once walk back is commenced.
Communication between bridge and forward
For the good image as macro-wise to actual ship's pose and condition of anchoring progress, Bow watchmen must report following to the Commander on the Bridge.
a) Approaching speed should follow a gradual decrease plan
b) Reducing speed for anchoring
c) Anchoring speed (Going walk-back)
d) Status chain number has been drawn on each shackle interval
It is preferable to report as "XX shackles in the water" in order for the commander to easily visualize the Length of chains being in the water and can estimate the timing of touching anchor with a bottom.
e) Anchor chain direction and tightness after bottom touch
The report shall be a combination of the following factor
For Horizontal direction:
"clockwise" and "straight down" (or upright)
For Vertical direction :
"as Long hand (of the clock)"
"as Short hand (of the clock) "
For Tightness :
"Strong tight", "Moderate tight, "Easy tight" and "Slack"
Ex. Chain - 2 o'clock - as short hand - with moderate tight
Anchor watch check item - deck officers guideline
Following are the basic check item that should be taken into account by deck officer while performing anchor watch
1. Instruction from the Master or Chief Officer
2. Ships position w.r.t swinging circle as marked on the chart
3. Length of anchor chain in use
4. Signals, lights, and shapes now in use
5. Visitors identity, number, and business
6. Onboard work
7. Carrying out Master's and Chief Officer's instruction?
8. Is the deviation of the vessel's current position from a measured position within the value given by master?
9. Is any oil floating on the sea around the vessel?
10. Is under keel clearance being monitored and any change in UKC on similar heading investigated?
11. Are regulation signals, lights, and shapes being displayed?
12. Are VHF receivers set to the correct working/watch channels?
13. Is there any ship (anchored or underway) that is likely to collide with own vessel?
14. Reporting and record of necessary matters?
15. Is the accommodation ladder raised to deck level when not in use?
16. Is the safety net fitted properly?
17. Is lighting sufficient?
18. Are any small crafts approaching vessel (ISPS vigilance maintained)?
19. Is the anchor chain monitored for excess weight?
20. Have OOW confirmed matters to be turned over to successor?
21. Necessary items entered in Logbook?
22. Master call during excessive Wind Velocity, reduced visibility, or in the case of Whirling (Yawing) angle :
The factors to be considered in choosing an anchorage are:
Holding quality of the bottom:
The nature and the slope (irregular) of the bottom should be considered poor holding ground
Adequacy of room for a swing
Protection from wind land sea
Strength of tidal stream and wind
Duration of stay at anchor
Type of anchors and cable
Nature of bottom and anchor design
Older types of anchors will hold satisfactorily in firm seabeds such as:
But will drag in softer sea beds such as :
Recent improvements in anchor design have resulted in obtaining satisfactory hold in any kind of seabed
No anchor, no matter how well designed, will hold on rock, except by a fluke of the anchor.
Amount of cable required
The cable must be long enough to ensure that a part of it near the anchor always remains in the seabed.
The rest of the cable acts as a spring in preventing the anchor from being jerked when the ship is yawing from side to side or pitching.
ANCHOR WORK TERMS-
Terms associated with anchoring:
The bitter end of the cable
An anchor is said to be dragging when, instead of holding the ship, the ship drags it along the bottom; this may occur in heavy weather, in a strong current, or when an insufficient cable has been paid out. A small amount of dragging on anchoring is necessary, in order to bury the anchor in the seabed.
The scope is the ratio of the length of the anchor cable to the vertical distance from the hawse pipe to the seabed. (Depth plus height of hawse pipe above the seabed).
The scope used depends on several factors:
·The nature of the holding ground. Stiff clay, rock, shells, very soft mud, and stones are considered poor holding ground.
The amount of swinging room available as the wind or stream changes in direction.
The degree of exposure to bad weather at the anchorage.
The strength of wind and stream.
The duration of stay at anchorage.
Is lowering the anchor or paying out the anchor cable on power (windlass clutched).
The vessel is said to brought up when the vessel starts to ride the anchor after anchor has been let go and the required amount of cable is paid out. This is determined when the vessel is heading into the tide, current or wind and anchor cable has come to a short stay.
The bitter end was originally the turn of a cable’s end around the bitts. It now refers to the end of the anchor chain cable secured in the chain locker or the loose end of a line. In all cases, the inboard end is referred to as the bitter end.
The bitter end is secured to the pipe using a pin so that it can be easily let go in an emergency.
When vessel sails into deep waters for a long voyage then the anchors are secured by housing them right in the hawse pipe, engaging bow stopper and lashing the cable using devils claw or wire and bottle screws. The Spurling pipe is covered and cemented to prevent the ingress of seawater during passage on shipping seas.
Is a deck fitting used to take the load of the anchor cable, anchor, and the dragging forces acting on the vessel after the vessel is brought up.
A ship has a foul hawse if the cables are crossed or otherwise foul of each other when she is riding to two anchors. If she swings 180 degrees she will have a foul hawse, and the cables are then said to have a Cross in them: another 180 degrees more in the same direction would cause an Elbow in the cable; a further 180 degrees would cause an elbow and a cross, and yet another 180 degrees would cause a round turn.
Clear or foul Anchor
The anchor is reported to clear or foul as soon as it is entirely sighted. To be clear the anchor must be hanging from is its ring and clear of its own cable and of any obstruction such as a bight of rope or chain picked up from the bottom.
When the anchor cable is leading close to vertical then the anchor is said to be at short stay.
When the anchor cable is leading at acute angles then the anchor is said to be at long stay.
To decide whether the vessel is brought up or not
After the anchor is let go and the required cable has been paid out. The vessel will swing into tide and or current. At first, the cable will stretch out on anchor biting the seabed and will be leading almost right ahead at long stay, and then she will come to a short stay. When the vessel is riding the anchor with the bow into tide/current at short stay the vessel is said to be brought up to the anchor. This can also be checked by plotting the position of the vessel on a chart or by bearings.
To check whether the anchor is dragging
By watching the bearing of two fixed lights or objects in line. Beam bearings are the best. If they change, the ship is dragging.
By dropping the deep-sea lead on to the bottom, and noting if it trails ahead of the ship.
By putting the hand on the cable before the windlass. Listen for any sound of the anchor dragging by bringing the ear close to the cable. Both these methods, however, may be deceptive because vibration and sound are often caused by the cable moving on the hard ground even though the anchor is holding well. Also, in soft mud, the anchor might drag without causing any vibration or sound.
By observing the anchor cable if she comes to long stay then a short stay and then long stay repeatedly means she is dragging anchor.
By plotting position of the vessel and confirming that the position is within the swinging circle.