Anchoring a ship may not be that difficult a task considering the various other responsibilities of Master in respect of shiphandling and tasks other than shiphandling. Anchoring however, is still tricky in an area, considering various factors such as: probable weather (wind direction & gusting); nature of sea bed; direction of ebbying and flooding streams; & sea room available. There should be more factors than usually considered in deciding the number of shackles to use. The purpose of this article is to discuss at length, a few issues in respect of anchoring & heaving.
To understand the modern theory of shiphandling, the theory of pivot point must be understood first, which is able to answer lot of questions on ship’s behavior in various situations.
Pivot Point
Pivot point is a point with a vertical axis about which the vessel will swing or turn if any couple is applied to the ship in transverse plane. Looks like, it is also the virtual position of centroid of ship in motion. Pivot point is close to or nearly at the ship’s centroid when at rest & it moves longitudinally with ahead or astern movement of ship. The force responsible for forward or aft movement of the ship and longitudinal resistance to forward or aft motion respectively created by the water around the ship, balance each other and thereby the pivot point is determined. It changes position depending on the resistance of floatation medium versus the thrust of the advancing vessel. It is seen that in the ships advancing forward at constant speed, the pivot point will be approximately 1/3 L from forward.
Astern Movement and transverse thrust
The transverse thrust when using an astern movement is a great asset to the shiphandler. The helical discharge, or flow, from a right handed propeller working astern splits and passes forward towards either side of the hull. In doing so, applies a different thrust on the parts of hull to port and starboard. On the port quarter it is inclined down and away from the hull whilst on the starboard quarter it is directed up and on to the hull. This flow of water striking the starboard quarter can be a substantial force in tonnes that is capable of swinging the stern to port giving the classic ‘Kick Round’ or ‘Cut’ of the bow to starboard.
Vessel Making Sternway
Transverse thrust is a poor force in comparison to other forces such as wind and tide. With the ship having sternway, a wind acting forward of the pivot point from starboard side may cause strong turning moment to overcome the transverse thrust. On the other hand a port wind may turn the ship very rigorously to starboard.
Transverse Thrust is maximum when starting from rest
The unequal thrust on upper and lower part of rudder (going ahead) and port and starboard side at stern (going astern) is responsible for transverse thrust. When going ahead the lever is large when starting from rest (the entire length L of ship whereas the lever is reduced to 2/3rd L at slow steaming. Astern movement will be given when a slight headway is there (during anchoring, berthing, turning short round, etc.) or when she is at rest. The pivot point is at 1/3rd from forward and at right forward respectively. With sternway the pivot point will shift aft reducing the lever.
Best turn moment is at start
Forward movement is initially resisted because of the inertia of the ship while the turn process starts earlier to this, This results in a pivot point which is initially well forward and approximately 1/8thL from the bow. The importance of this is absolutely vital because at this stage, with the ship just beginning to make headway, the rudder turning moment will be maximum.
Later when the ship begins to build up speed, the final position of the pivot point will be approximately 1/3rdL from the bow. With the turning lever thus reduced, the rudder force becomes progressively less efficient.
The discussion till now was general and similarly applicable to all the ships. In order to understand the subject better, let us take the help of a particular ship. These effects will vary from ship to ship and will have to be applied to a particular ship with some caution.
Let the ship under consideration be a bulk carrier of DWT 60000t. L = 210m, beam = 32m, summer draft = 12m, SHP = 15000 are the assumed parameters. Propulsion thrust in forward direction is 150t (1% of SHP), Stern thrust is likely to be 80 t(50% of ahead thrust). Transverse thrust 4 to 8 t (5% to 10% of stern thrust). Wind thrust from side could be 60 t and wind thrust from forward or aft could be 30t for a nominal wind at 25 kn. In case of the bulk carrier under consideration the accommodation is aft and with the wind roughly on the beam the large area of superstructure and funnel offer a considerable cross section to the wind. Considering the area of freeboard from forward of the bridge to the bow a considerable impact area is obtained. On a VLCC this could be an area as much as 2500m2.
By taking moments of areas about keel and aft, the centre of windage area appears approximately at decklevel at 3/8th of length from aft. The centre of the windage area can be much further forward than is sometimes calculated. This when compared with the centre of buoyancy depending on under water profile of the ship and the position of the pivot point (P) will determine the drift and the swing that will be taken by the ship.
To calculate the wind thrust
To calculate the wind thrust many factors must be taken into account. The most important of these, is its windage determined by two factors; (1) the surface area and (2) the frontal shape of the vessel as presented to the wind, i.e. the windage area.
There are many formulae used to find the wind thrust. Some of these are as follows:
The ‘Shape Co-efficient’is like 1.2 , 1, 0.7, etc. For the rounded convex surfaces the shape co-efficient is low whereas for the flat surfaces or the surface offering concavity it is higher.
1. A general formula for wind thrust for 1 sq.m. = ½CdσV2A
Cd being the shape coefficient, σ the density of air, V the velocity air in mps and A the projected area.
2. Another very simple formula used is: Force (tonnes) per 1000m2 = V2 ÷ 18
‘V’ is the velocity of air in metres per second.
This means that for a ‘side elevation area’ of 1000m2 the wind thrust due to a 40 kn (20m/s) beam wind = (V2 ÷ 18) = 22.22 tonnes.
3. Yet another formula to give wind thrust in tones is:
Wind thrust = K x A x V2
K = 0.52 x 10-4 for beam wind.
K = 0.39 x 10-4 for F & A wind.
A = Windage area of ship in side elevation in m2.
V = wind relative velocity in m / sec
Thus, if a 40 kn wind blows from side hitting an area of 1000 sq m then the thrust 0.52 x 10-4 x 1000 x 202 = 16.8 t.If the wind was blowing from forward then the thrust would be 15.6 t.
4. One more formula used by some is:
V2 x 1.2499 x 10-5 x F x A
V = speed in knots F = 1.2 for flat structure & 0.7 for rounded structure. A = area in met sq.
Thus a 40 knot wind hitting a 1000m2 area will cause a thrust = 24 tonnes on a flat surface.
Thus, understanding the size or magnitude of wind thrust on a bulker or a tanker in ballast, it should be noted that kicks ahead of dead slow and slow magnitude will be ineffective at certain wind strengths and more power must be used to achieve turning in moderate wind conditions. It should also be noted that the kicks ahead with full power can be effective against a wide range of wind strengths. Master must also understand the weakness of turning by the transverse thrust in front of a stronger wind thrust turning. The effectiveness of the bow thruster in beam winds must be well understood rather than simply banking on it.
Chain tension due to tidal stream
While anchoring, departing anchorage or anchor dragging situations the rate of tidal stream must be monitored. The force of the tide upon a ship, measured in tones, is directly proportional to the square of the velocity of the tide. This means that for even a small increase in the velocity of the tide, there is an enormous increase in the force exerted upon a ship.
The tension coming on the cable holding a ship in tidal stream depends on the present displacement, the under keel clearance, area of a hull that is exposed to a tide and of course the rate of tidal stream. The bulk carrier in question, in loaded condition. A three knot current may exert a force of 16 tonnes on cable and the windlass. If the under keel clearance is reduced due to change of tide, the tension on a cable may be increased to about 45 tonnes.
The cross sectional area of a hull that is exposed to a tide when a ship is at anchor is relatively small in comparison to the area which is exposed to the tide, when a ship is held with the tide on the beam. If, in addition, the ship has a small under keel clearance, so that the tide is prohibited from flowing underneath the hull and for the full length of the ship is forced to pass around the bow and stern, the lateral force created can be enormous.
T = ‘K’ x L x D x V2
T = thrust in tonnes required to hold a ship in current
‘K’ = 0.033 for deep water.
‘K’ = 0.033 x f for shallows water.
f = 5 for Dp/Dr = 1.1
= 4 for Dp/Dr = 1.2
= 3 for Dp/Dr = 1.5
= 2 for Dp/Dr = 2.4
V = current velocity in m/sec. Let us calculate the thrust likely to come on the cable of the bulker in question (in ballast condition. (draft 6.5 m). Tension can be 85.8 t if the depth is about 7m & can be 17.2 t for deep waters. This is a very important point which must be understood by a Master.
Following are the general considerations in respect of anchoring:
Amount of cable required:
The amount of cable used must be adequate and long enough to ensure that a considerable length of cable near the anchor hugs the sea bed. An approximate formula from a Japanese convention is L=3d+90m for fine weather and L=4d+145m for rough weather. A more common formula being L=39 ×√ d. Thus, the number of shackles to use = 1.5×√ d.
Nature of bottom
Earlier anchors were of low holding capability and held satisfactorily in firm sea beds such as clay, sand, shingle, etc but will drag in softer sea beds such as soft mud and shell. Improved designs of Admiralty anchors are much better and holds in any kind of sea bed, because the improved anchors embed themselves deeper in the softer grounds during the final period of drag before they hold. No anchor however will hold on rock, except by fluke. Anchor will also not hold if fouled by some material, picked up on the sea bed, which prevents the anchor from biting the ground.
Sometimes, a disused cable may be picked up during a walk-back. Different actions may be necessary to deal with this situation, some time the local help. If the flukes are caught in a rock it may be necessary to part the cable and buoy the end for later recovery.
Anchor design and holding capability (tension) of an anchor:
The holding tension of an anchor is expressed as a ratio of holding tension and anchor weight and varies, depending on type of anchor, from 3:1 to 10:1. Admiralty Standard: Maximum holding tension is about the 3 to 3 ½ time own weight. The AC 14 has a ratio of holding tension to anchor weight of 10:1.
The holding tension of AC 16A and 17 anchors. is approximately the same as for the AC 14 anchor. The AC 17 anchor’s holding tension in fact is about 7 times its own weight.
Type of chain:
There are basically three types or grades of ship’s chain cable. Their composition and application in the service is as follows.
Grade1 chain is non-ferrous, manufactured from copper based material usually referred to as aluminum bronze. It is supplied to mine sweeper vessels. The grade 2 of forged steel is supplied to the majority of ships. The grade 3 forget steel is of higher grade steel than grade 2 and consequently stronger than it.
Yawing
A ship, especially one with a high freeboard and the flare of hull, usually yaws considerably. At the end of each yaw the ship is likely to drag her anchor, because she first surges ahead and then falls back on her cable, thereby imparting a jerk to the anchor. Typically, similar is the pattern in dragging. The situation may be eased by veering more cable. More the cable, the heavier the centenary and the greater the tension before the cable is ‘straightened out’. If the motion is stopped before the cable is straightened out, there will be no jerk. A large scope of cable also exerts a damping effects on the yawing by its resistance to being dragged sideways. If the yaw becomes excessive the situation can be relieved by letting go a second anchor at the end of the yaw away from the first anchor. Both cables are then veered so that the ship rides with one anchor at long stay and the other at short stay.
Local Knowledge
The type of sea bed is of paramount importance. The anchor may be used to dredge, to turn, when confident of the type of sea bed.. An advantage of paying out extra cable in ‘anchor dragging’ is a well known phenomenon.
Increase of draft
Master must note that a ship may experience an increase in draft due to list when turning, Whether the under keel clearance is reduced due turning or due the draging anchor taking the ship to shallows, squatting (due relative flow of water wrt hull even if propulsion is not there), will further reduce UKC.
Dredging v/s. Holding
If the amount of cable in the water exceeds 1½ to two times the depth of water, the flukes will bite the ground. Thus, for simply dredging, the ratio should not exceed this figure. If this figure is exceeded the anchor is likely to dig in and commence holding.
In addition to the normal precautions prior anchoring a ship’s Master must also consider the following to rule out the standard ways in which most ships ground after dragging:
- Time required to heave 1 shackle or 15 fathom length of cable on 1st gear.
- Tidal drift from the moment of short stay till the cable is up and down.
- Drift due wind thrust.
- Propulsion thrust in terms of speed required to overcome wind thrust.
- Propulsion thrust available in ahead & astern directions.
- Turning circle in shallow waters.
- Turning circle or advance with strong current and or strong wind drift. .
- Holding tension by anchor cable at long stay.
The limitation of windlass and anchor
The limitation of windlass and anchor, in respect of using them on big ships in the severe weather conditions must be well understood by the ship’s Master. When trying to heave the cable in deep waters and the situations where the ship is moving adversely, there is the possibility of parting of cable or the windlass breaking down. Ships have grown big but the windlass and anchor power have not grown in similar proportion. The time taken to heave up a shackle length at the speed of first gear must be accurately known. The design speed of a windlass gypsy in gear may be 30 feet / minute which is approximately 3 minutes a shackle.
This means if the depth is 20m, draft 7m, 5 shackles in a water; it will take 15 minutes or so to make the cable ‘up and down’ or the flukes leave the sea bed. In anchor dragging conditions, with flukes not biting because of insufficient cable length, the vessel is likely to drag at a good 5 knots or even more in tide favouring conditions. This means that the vessel would drift by 1.25 miles or more if engines are not worked ahead. Thus, heaving up anchor in gale force conditions can be very tricky. A mathematical calculation of force vectors or an experienced approach is necessary. By dredging the two anchors the pivot point is brought right forward between the two windlasses.
Bulker, grounds on beach and is lost later
An almost brand new bulk carrier, of 40,042 GT, in light ballast condition and ready to load coal, had anchored 2.5 miles offshore, south of the harbour entrance. It was May 2007. The vessel was to wait for berth for three weeks. The Master anchored with nine shackles in 35 metres of water in a designated anchorage where many vessels were at anchor.
Bulker grounded during the passing of a severe storm on 8 June 2007. The bulk carrier, grounded on beach. The wind thrust carried her onto rock ledges on the beach and her hull breached. During the initial stay vessel had witnessed good weather, the wind strength increased from the south-east. The Master, on receiving a gale warning, paid out an additional shackle of cable. During that evening, a number of vessels left the anchorage and put to sea due to the worsening weather. A few hours later, one of the vessels started to drag anchor. The local Vessel Traffic Information Centre advised the vessels that were dragging anchor and, within 24 hours, more vessels had also put to sea, but 41 ships remained at anchor, enduring 40 knot winds and eight-metre high seas.
Early the following morning, the bulker began dragging its anchor and the anchor was weighed. However, with the rapid rate of drift caused by the ship dragging its anchor, the vessel was only 1.2 miles from the shore. The Master attempted to turn the vessel away from the coast, but with such strong winds and the substantial windage area due to the ship being in ballast condition, the vessel was driven onto the beach, grounding onto the rocky shoreline and breaching the hull. At the time of the grounding, the average wind speed was recorded as approximately 40 knots. The windlasses of three vessels in a similar situation all failed when attempting to weigh anchor, with one vessel cutting its cable when under considerable tension, and the other two managing to slip their cables. These vessels were fortunately able to pull away from the coast. Two weeks later, the Pasha Bulker was refloated and later towed to a repair yard.
A reefer vessel at anchor waiting for orders, in ballast condition close to the Spanish coast, dragged anchor in 50 knot winds. She eventually ran aground on rocks and was declared a total constructive loss. Here, even with local knowledge, the vessel anchored only six cables from the shore on the pilot’s ‘instructions’ in a designated anchorage. There was less than one hour from the time the officer on watch (OOW) confirmed that the anchor was dragging to the vessel being aground. The Master had even taken the precaution of having the main engine on immediate notice and on bridge control two hours before the vessel started to drag. However, due to the fact that the vessel was in ballast, the rate that the vessel dragged its anchor and the close proximity of the shore allowed no time to react and redeem the situation. In this incident, the windlass hydraulics failed when trying to raise the anchor and the vessel grounded on rocks, with one shackle still on deck.
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