Anchoring is one of the most important and critical operations in shipping and every deck officer must be thoroughly familiar with the procedures and favourable factors that are required for anchoring.
As a junior officer, many of you must have been on Bridge with the Master during Anchoring and must have noticed many particular things that Masters do in the process of Anchoring and might have wondered the reasons behind such actions, here we will try to figure out the answers for you.
In this article we will discuss what all factors are considered and how to proceed for anchoring. We will try to keep it as simple as possible.
HOLDING POWER OF ANCHOR
- The holding power of an anchor is around 4 to 5 times the weight of anchor and it is 100% only if the anchor chain is lying horizontal on the seabed away from the anchor.
- As the chain starts lifting from the seabed, the holding power starts reducing.
- Holding power of an anchor depends on the area of the fluke, bigger the fluke, more is the holding power.
CHOICE OF ANCHORAGE
- Type of Seabed – The preferred seabed should be soft and cohesive. For example- Clay, mud, firm sand, grave etc.
- The area should be clear of cables and pipelines.
- The area should be as far as possible be an authorised anchorage in sheltered or semi sheltered area.
- Enough sea room should be available taking into account the swinging circle of the vessel.
- Position checking methods should be considered, for example- presence of shore objects etc.
- Depth of water– UKC should be maintained as per the company’s policy but in general should not be less than 25% of the depth and if possible, the depth of water should be less than 50 metres, because more the depth, more the cable to be paid out, which will result in a bigger swinging circle of the ship, thus requiring more sea room.
- Direction and Speed of Approach
- Approach should be made against wind and/or current, whichever is stronger, vessel can be guided by other vessel already at the anchorage.
- Speed should be adjusted in such a way that when the vessel is about to reach anchoring position, vessel can be stopped with an astern kick before letting go and walking back the anchor.
- Anchoring position must be marked on the chart and swinging circle should be marked according to the amount of cable to be paid out.
- Amount of Cable To Be Used
Following should be considered:
- Nature of Seabed- Shortest cable can be paid out in clay and long cable to be paid out in sand.
- Available swinging room.
- Expected weather conditions at anchorage and whether the anchorage area is sheltered, semi-sheltered or open.
- Strength of wind, current and tide.
- Duration of stay at anchorage.
- Depth to draft ratio.
- ADDITIONAL IMPORTANT INFORMATION
- Anchor usually holds when cable paid out is more than twice the depth of water.
- When “dredging” the anchor, cable paid out must be less than twice the depth of water.
- For anchoring purpose, minimum cable to be paid out should be at least 4 times the depth of water; ideally it is 6-8 times. The number of shackles to be used can be calculated by the formula:
Cable in shackles = 2√depth in fathoms
- Normally, the windlass is designed with such power that anchor and 3 lengths of shackles can be lifted up vertically suspended in the water.
- Approach against the current or against the wind, whichever is stronger while slowly reducing the speed.
- Anchor should be lowered outside the hawse pipe, ready to either let go or walk back.
- When the anchoring position is reached, astern kick should be given to stop the vessel on the ground, in simple words, Speed over Ground / Speed made good should be ZERO.
- Watching the propeller wash gives a good indication whether vessel is stopped in water. When the propeller wash is just abaft the midships, vessel is supposed to be stopped in water.
- Let go or walk back the anchor on the windward side so that anchor cable keeps clear of bow plating. Initially the cable paid out should not be more than twice the depth of water; this is to avoid anchor holding while the vessel is moving and cable tending to ship from the windlass.
- The OOW at the anchor station must inform the Bridge how the cable is leading and kick on the engine has to be given accordingly to stop the vessel on the ground till the cable becomes up and down.
WHEN TO LET GO OR WALK BACK THE ANCHOR
- It is recommended that in depths up to 20 metres anchor is allowed to let go from water level.
- In depths of 20-50 metres anchor should be walked back till it is about half a shackle up from the seabed and then let go from that position.
- In depths above 50 metres anchor can be walked back all the way up to the seabed but it should be ensured that ship is sopped over ground.
HOW TO JUDGE THAT THE VESSEL IS BROUGHT UP
Brought up means that the anchor is down and anchor is holding. After paying out the final scope of the cable, the vessel starts swinging till her heading becomes stationary. Then she starts falling back resulting in a long stay, then slowly the weight will ease off to a medium or short stay. Thereafter, the stay becomes more or less constant, vessel is then said to be brought up.
HOW TO JUDGE ANCHOR IS DRAGGING
- If the anchor cable is consistently at long stay, it indicates vessel is dragging very fast.
- If the stay becomes long and short alternatively with the jerk, it means vessel is dragging slowly.
- GPS position has changed considerably.
- If there are no regular sine waves on the course recorder.
- By fixing the position from shore objects.
Hope this covers all the important topics and the practical aspect of Anchoring.
If you have any more questions or doubts, feel free to comment.
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