Why Must the Anchor of a Merchant Ship Be Slack to Keep Her in Position?

The anchor of a merchant ship needs to be kept slack to effectively hold the vessel in position due to several critical factors that enhance its holding power and stability. Understanding these factors can help enhance the safety and efficiency of maritime operations.

Holding Power

A key reason for keeping the anchor chain slack is the anchor's holding power. An anchor needs to dig into the seabed to provide substantial holding power. If the anchor chain is too taut, it can hinder the anchor's ability to set properly. By allowing the chain to be slack, the anchor can drop down and embed itself more effectively into the seabed, thereby increasing its overall holding capacity.

Movement and Forces

Ships are subjected to various external forces such as wind, currents, and waves. When the anchor chain is too tight, any movement of the ship can cause the anchor to lift out of the seabed or drag along the bottom, which would significantly reduce its effectiveness. Maintaining a slack chain allows the anchor to adapt to some movement, ensuring it remains engaged effectively.

Scope of Chain

The scope of the chain (the amount of chain deployed) plays a crucial role in how well the anchor holds. A longer scope with more slack allows the anchor to work more effectively by reducing the angle of pull on the anchor. A general rule of thumb is to have a 3:1 scope ratio, meaning that for every foot of water depth, at least three feet of anchor chain should be deployed. This ratio ensures that the anchor is set properly and maintains its stability.

Shock Absorption

A slack anchor chain also serves as a form of shock absorption. When faced with waves or wind-induced movements, the slack in the chain helps to absorb some of the forces, preventing sudden jerks that could dislodge the anchor. This makes the overall anchoring process more reliable and safe.

Understanding Holding Power

The holding power of an anchor is determined by its design and the weight of the anchor itself. Common types of merchant ship anchors include stocked anchors, conventional stockless anchors, and high-power holding anchors. For conventional merchant ship anchors, the holding power is estimated at 7 to 14 times the weight of the anchor. To achieve this maximum holding power, the anchor chain adjacent to the anchor should always be in a horizontal position on the seabed. If the chain is lifted even slightly, it can significantly reduce the anchor's holding power, sometimes by up to 50%.

Adjusting Scope for Maximum Effectiveness

The amount of cable paid out (the scope of the chain) can greatly impact the anchor's holding capability. There is no one-size-fits-all formula, but it is generally recommended to use at least a 3:1 scope ratio. However, the specific scope depends on several factors, including the water depth, the type of seabed, expected weather conditions, and the anchor's design.

A commonly accepted rule suggests using 4 times the depth as the scope, but for a more precise measurement, the formula ( 30sqrt{d} ) is often used, where ( d ) is the water depth in meters. For example, if the water depth is 25 meters, the effective scope would be ( 30sqrt{25} 150 ) meters, which would be approximately 5.5 shackles (or 27.5 meters each).

Conclusion

Given the above factors, it is clear that maintaining the anchor chain in a slack state is crucial for ensuring the anchor sets properly, resists environmental forces, and provides stability for the ship while at anchor. Seafarers should always adhere to the correct anchoring techniques to ensure the safety and reliability of their vessels.