The principle of a ship's anchor

From top to bottom, the structure consists of an anchor shackle, a bolt, an anchor rod, an anchor stock, and an anchor arm. The symmetrical parts on either side are called anchor claws. The anchor claws are the most critical part for the anchor to grip into the seabed. After the vessel drops anchor, the anchor sinks to the seabed under the pull of the anchor chain. Once at the seabed, thanks to the action of the anchor rod, the plane where the anchor arm is located becomes perpendicular to the seabed, at which point the anchor claws come into contact with the seabed.

The length of a ship’s anchor chain often exceeds the water depth; therefore, the portion of the anchor chain lying on the seabed remains flat. When the ship experiences disturbances—such as when encountering head seas—the anchor chain gets pulled, and the anchor resting on the seabed at the chain’s connection point is subjected to a horizontal force. At the same time, the anchor’s own weight exerts a downward force at the point where the anchor’s flukes make contact with the seabed. The combined effect of these two forces causes the anchor to move diagonally downward—a process known as the anchor embedding itself into the seabed. Once the anchor has been fully embedded in the seabed, it can provide the ship with the necessary mooring capability. It’s important to note that this mooring capability cannot be achieved by the anchor alone; rather, the long anchor chain also plays a crucial role.

The function of a ship's anchor

The primary function of a ship’s anchor is to secure and stabilize the vessel, which can be explained from the following aspects:

1. For an anchor to work effectively, the most basic requirement is that it must hook onto something on the seabed.

If the anchor chain isn't long enough, the anchor won't hold. If the seabed is flat, or if whatever the anchor is hooked onto isn't fixed, or if the hook isn't secure enough, it might be fine in calm weather—but once the waves become too high and the anchor can no longer grip the bottom, it will lose its effectiveness. This phenomenon is called "anchor dragging." During anchoring, if the ship starts to drag its anchor, it becomes extremely dangerous.

2. The weight of the anchor chain is negligible for the ship—it won’t make much of a difference.

Generally speaking, when a ship is at anchor, its anchor chain is usually straight. Do you think there would be any friction acting on the anchor chain at this point? If you’re by the seaside, you’ll notice that many small fishing boats have anchor chains made of thick ropes.

3. From above the ship, the anchor chain appears straight; however, beneath the water, a section of it runs nearly parallel to the seabed. In reality, this section of the anchor that lies close to the seafloor provides the gripping force, which is then transmitted via the anchor chain to the vessel, enabling it to resist the external loads imposed by ocean currents and waves and thus maintain its position. The reason for having a section that closely follows the seabed is to account for the effects of stress: by keeping a portion of the chain in contact with the seafloor, we create a margin of safety that remains effective within certain limits.

Although the weight of an anchor isn't significant, there are specific regulations governing which size of anchor should be used for ships of different sizes. This is due to factors such as the gripping power that an anchor can generate and its own inertial force. The frictional force exerted by the anchor chain also plays a role.

Types of ship anchors

There are many types of ship anchors. Depending on their structure and intended use, they can be categorized into shank-type anchors, stockless anchors, high-hold anchors, and special-purpose anchors. Merchant ships typically use stockless anchors as their foremost anchors, while stern anchors sometimes employ shank-type anchors or fluke anchors.

1. Rod anchor

Also known as the naval anchor, this type of anchor is an early design often referred to as the naval anchor. It features a simple structure, a high holding power-to-weight ratio, and good stability when set on the seabed. However, it is somewhat inconvenient to deploy and retrieve. The upward-curving flukes tend to become entangled with the anchor chain and can easily scratch the hull of the vessel. As such, this anchor is not well-suited for use as the primary anchor on commercial ships; it is typically employed on smaller boats or sailboats.

2. Rodless anchor

Also known as the "mountain-shaped anchor," commonly used types include the Hall anchor and the Spade anchor. These are stemless anchors with movable flukes (which rotate approximately 45° along the length of the anchor shank). In stemless anchors, the anchor shank and the anchor arms are cast separately. The flukes, anchor crown, and anchor arms are integrally cast as a single unit. The anchor shank is inserted into a rectangular opening in the anchor crown and secured by a pin and a cross pin that fit into two semicircular recesses at the lower part of the anchor crown, thereby enabling the anchor shank to rotate freely.

3. High-grip anchor

The high-hold capability anchor combines the advantages of both rod-type and rodless anchors; it is typically a rod-type swivel-fluke anchor. Commonly used models include the Marcy anchor and the Danforth anchor. These anchors are characterized by wide and long flukes that penetrate deeply into the seabed, providing excellent stability and thus achieving a high holding power. Their weight-to-holding-power ratio can reach 11 to 17 times. However, their fluke strength is relatively weak, making them prone to deformation. Moreover, due to their substantial resistance when being hauled out of the seabed, they are generally suitable only for engineering vessels; some are also used as standby anchors or stern anchors.

4. Special anchor

Special-purpose anchors are commonly used for permanent mooring of pontoons, buoys, light vessels, floating docks, and floating piers. These include single-fluke anchors, screw anchors, and mushroom anchors.

Daily inspection of ship anchors

1. Anchor inspection

1) Inspect the anchor shackle for wear and deformation, and pay attention to whether the cross pin is loose. Perform inspection and maintenance accordingly.

2) Pay attention to whether the anchor claws are bent, whether the rotation is smooth, and whether the angles are normal; perform inspection and maintenance accordingly.

2. Anchor chain inspection

1) Take turns using the left and right anchors in daily use.

2) Pay attention to whether the anchor chain markings are clear; if any have fallen off, they should be promptly reapplied.

3) Check whether the swivel ring is flexible and lubricate it with oil as needed.

3. Anchor Winch Inspection

1) Regularly check whether the brakes are in good condition.

2) Lubricate and perform a test run before each use. Before operating the steam anchor winch, drain any accumulated water from the cylinder until steam starts to emerge from the drain hole, then proceed with the test run.

3) Regularly lubricate the clutch to ensure smooth and flexible operation.

4) Pay attention to the wear condition of the sprocket teeth, worm gear threads, and other components.

4. Chain stopper

The friction surfaces of the chain stopper are regularly lubricated, while other parts should be coated with anti-rust paint. Regularly check the tightness of the fasteners connecting the base to the deck. Inspect the integrity of the handwheel guard on the chain stopper, and frequently lubricate the moving parts.

The above content provides a general introduction to the principles, functions, types, and daily inspections of ship anchors, as presented by the Xianji website editor. Ship anchors are indispensable equipment for the operation of vessels. Typically, a standard vessel is equipped with one anchor on each side—port and starboard—and an additional spare anchor to ensure that there’s an emergency backup in case either the port or starboard anchor is lost. Vessels designed for special purposes may be equipped with five or more anchors—for example, engineering vessels and salvage ships.