SHIPHANDLING (Use of anchor in Special Situations)

Q. What additional precautions will you take in respect of anchoring in an area where range of tide is very large?

After the anchor strikes ground, a very gentle pull to shank will cause the flukes to bite the ground. A strong tidal stream would cause undue stress on flukes due a large momentum. The weight of chain and the caternary will not contribute. It is advisable that the tidal stream rate is about 1 or 1.5 knots and not more than that. This ill be available within an hour of slack water at HW or LW. This can be seen in the diagram, showing the cosine pattern of the tides.

Thus, in above example the light tidal stream rates are from 0500 to 0700. The anchoring however, should be done after the tide has started to reverse, the vessels have just completed swinging around. If the anchoring is done prior slack HW, the ship would swing prior taking weight and thus, a chance of flukes turning upwards and therefore there is a risk of drifting anchor after swing around. In case the anchoring is done after the slack waters before the tidal stream gets strong, the ship will stay on this heading for at least 6 hours. In Bhavnagar, a second anchor is dropped after at every swing around, picked up prior every swing around. Thus the ship is held n position with one cable at long stay and the other at short stay.

Q. What is the equipment number in respect of an anchor?
The equipment of anchors and chain cables for ships of unrestricted services is as given the table provided in class rules and the weight, scantlings, etc. are based on ‘Equipment Number’ (EN), which is calculated as follows:
EN=\Delta ^{\dfrac{2}{3}}+2.0hB+\dfrac{A}{10}
Δ = moulded displacement, in t, to the Summer Load waterline,
B = moulded breadth, in m,
h = effective height, in m, from the Summer Load waterline to the top of the uppermost house;
A = side projected area, in m2, of the hull, superstructures and houses above the Summer Load waterline.

Q. In how any ways other than a conventional anchoring an anchor may be used?
Anchor is used in many ways is ship handling, berthing, unberthing, etc. One of the very commonly used maneuvers being turning to starboard for a wharf on port side. Thus, the wharf is approached at a broad angle say at 60 or 65 degrees. When the stem is about a ship length from the wharf and in line with the midship point of final position, starboard anchor is let go. The anchor is dredged as ship advances with rudder hard starboard and ship turning to starboard. If there is an onshore wind, bows will be quickly attracted by winds. Lines are connected as soon as practicable. An astern movement will cause the vessel to drop astern and stern swing inwards. Astern engines are put before she is parallel to the berth. The starboard swing is controlled by headline. The anchor dredged initially and then when anchor is in correct position the cable is paid out.

A pilot usually knows the sea bed quality and the depth of water in an area. If the cable is not paid out more than an amount that will cause an angle of 45 degrees or less should cause the anchor to dredge. The resistance offered by anchor helps the vessel turn.

The anchor thus used serves three purposes:
1. Assist turning as she berths, particularly where there are no tugs and there is limited place or the berth is occupied aft of final berth position.
2. Secure stem where the wharf is exposed to strong onshore winds and / or the wharf is not strong enough.
3. Assist hauling out of stem at the time of unberthing.

Normally, large vessels which are low-powered and are single-screw are unusually sensitive to the effect of wind and are not easily controlled during berthing, The anchor can keep the  bow from being driven off. It can greatly reduce drift or leeway. By permitting the use of greater power on the propeller, it may be said that in a way the anchor increases the effectiveness of the rudder. Anchor can be used to come starboard side alongside too. Thus, to come starboard side to in calm weathers, approach is made at about 700. Port anchor is let go and the vessel is maneuvered ahead with rudder hard over to port. Astern propulsion however will be just after passing the ‘parallel to quay position’. Both, head as well as stern lines will be connected as soon as possible. Where headline will be used to bring the bows in, the stern line will ensure the stern does not swing out due to transverse thrust.

Q. What factors contribute to the tension in the anchor cable?
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 rate of the tidal stream, there is much higher 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 the rate of tidal stream. The bulk carrier in loaded condition in 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 tidal stream when a ship is at anchor varies. It increases greatly when a ship has the stream 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 the lateral force created can be enormous.

Q. Do you know any formula to calculate the tension?
There is an imperical formula for this.
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.
Thus, the thrust likely to come on the cable of a bulker in ballast condition. (draft 6.5 m). may be  over 85 t if the depth is about 7m & can be about 17t for deep waters.

Q.What may cause an anchor to drag?
High freeboard and the increased flare of hull, may cause a ship to yaw more than normal.  This may increase the probability to yaw. The total tension on cable due tidal stream and wind may exceed the holding thrust provided by anchor causing the vessel to drift. The situation may be eased by veering more cable. More the cable, the heavier the caternary and the greater the tension before the cable is ‘straightened out’. A large scope of cable also exerts a damping effects to counter the tension. Use of engines and the second anchor may get the situation under control if necessary.

Q. In addition to the normal precautions taken prior anchoring, what other factors must be considered by a ship’s Master so that due action is taken if gale force winds are caused?
In addition to the normal precautions prior anchoring a ship’s Master must also consider the following:

  1. Time required to heave 1 shackle or 15 fathom length of cable on 1st gear.
  2. Tidal drift from the moment of short stay till the cable is up and down.
  3. Drift due wind thrust.
  4. Propulsion thrust in terms of speed required to overcome wind thrust.
  5. Propulsion thrust available in ahead & astern directions.
  6. Turning circle in shallow waters.
  7. Turning circle or advance with strong current and or strong wind drift. .
  8. Holding tension by anchor cable at long stay.

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. 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 4 knots or even more in tide favouring conditions. This means that the vessel would drift by about a mile or more if engines are not worked ahead. Thus, heaving up anchor in gale force conditions can be very tricky.

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