[FAQ] How An Anchor Holds The Ship?

Credit: Matthew Wheeler/Unsplash

Anchoring is as frequent an operation on board as loading and unloading cargo. But in spite of being a frequent operation, the number of incidents related to anchoring never seems to reduce. That is when many minor incidents never come to light to a wider audience.

The truth is that even after being a routine operation, an effective way of anchoring is not child’s play. 

How It Holds The Ship?

When an anchor is dropped, The crown of the anchor is the first to hit the sea bottom. As the ship moves back, the flukes take their position and embed themselves into the seabed. It does not matter from what height the anchor is dropped, the crown will always hit the bottom first. 

The flukes will only dig into the seabed once the ship moves aft and chances are facing downwards into the sea bottom. As the ship moves back, the fluke takes its position and embeds itself into the seabed.

The opposite happens when we pick up the anchor. When the chain is all picked up, the flukes face upwards and get uprooted from the bottom.

Holding Power Of Anchors

All other factors being common, there are three things that affect the holding power of anchors. First is the construction of the anchor, second nature of the seabed and third is the scope of the cable.

  • Holding power due to anchor construction

A ship’s Equipment number decides the weight of the anchor and the length of the chain. The fluke area determines the holding power of the anchor.  

The International Association of Classification Societies (IACS) governs the rules for anchors.IACS enlist three types of anchors.

  • Normal holding power anchors,
  • high holding power anchors
  • Super high holding power anchors.

Because of the high and super high holding powers, these anchors can have a lesser weight than normal anchors. This is because the high holding design (Larger fluke area) compensates for the loss of holding power due to lesser weight. This is because the high holding design (Larger fluke area) compensates for the loss of holding power due to lesser weight.

Even though anchor weight has nothing to do with holding power per se, it contributes to some extent to holding a ship in its position. More weight of the anchor would need more force to push the ship from its position. For this reason, anchor weight is more often used as a function of holding power.

  • Holding Power Due To Nature Of Seabed

Another factor that affects the holding power of the anchors is the nature of the seabed. Sand is considered to be the strongest holding ground. Soft mud is the least holding ground. This is because of obvious reasons.

Anchors embedded into soft mud would leave the bottom easily compared to the harder surface like sand. Mariners must consider the nature of the seabed to determine the possibility of dragging the anchor.

  • Holding Power Due To Scope Of The Anchor Cable

The correct scope is essential for safe anchoring and better holding of the anchor. The scope is the ratio of the depth of the water to the length of the cable deployed. The more the scope, the better an anchor will hold the ship. The idea of having more scope is that the chain angle with respect to the sea bottom should be minimum. More the angle, the lesser the holding power. OCIMF has published a graphical relation between this angle and the holding power of the anchor.

As a thumb rule, a Scope of 6 is advisable for anchoring. That is when anchoring into a depth of 20 meters, we should pay at least 120 meters of cable. When anchoring in strong wind or current areas, we should have a scope of more than 6, up to 10 sometimes.

There are two situations where the scope of 6 or more may not be always possible.

  • In congested anchorages like Singapore and
  • In deep water anchorages like Fujairah.

In congested anchorages, this is due to insufficient sea room and In deep water anchorages due to insufficient cable length. In calm weather, the lesser scope in these areas should not be a problem. But if you expect wind force to increase, increased possibility of anchor dragging should be part of the risk assessment.

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Source: myseatime