An anchor dredge operation (Credit)

Vessels use anchors for a variety of reasons. They may be used during berthing or when manoeuvring through channels and other areas where restrictions in movement exist. Dredging anchors are often used to assist a ship in manoeuvring during the unavailability or inability of tugs to assist as required.

A dredging anchor allows a ship to move fore and aft while keeping the bow steady, shifting the ship's pivot point forward. Propulsion is used to overcome the drag of the anchor, giving good steering at low speed. Today, many ships are equipped with bow-thrusters to control the bow while going ahead or astern. These thrusters can be useful to manoeuvres large vessels in confined water or to assist smaller vessels in restricted waters.

To navigate a ship through a channel that is too narrow to allow passage while facing forward, it is sometimes necessary to move backwards. In this case, the ship's stern canting (a lateral shift in the position of the stern) is corrected by having a tugboat tow the vessel from behind, while its engines are used only enough to adjust its direction of travel. An additional tug, called a checking tug, tows behind the ship at the bow. It helps to control the vessel and prevent it from swinging waywardly. It also comes in handy when stopping the momentum of the ship so that its engines don’t need to respond immediately. When there is no bow thruster or tugboat to check the ship's movement, the anchor is used as a backup. The anchor will keep the ship steady and prevent it from moving too far.

In short-stay anchoring, the anchor is dropped at a depth of five to seven times the water’s depth. When the vessel puts in reverse, the anchor will drag over the seabed and dig in. By then, the pivot point has shifted to the bow of the ship near the hawse pipe. This enables the tug at the stern to exert more moment of force as its distance from the pivot point has increased. This also prevents yawing by shifting the pivot point forward to balance out any backward motion.

In this operation, the anchor is dragged over the soft mud rather than being dug into it. This prevents the cable from coming under huge strain and parting when the engine runs astern or when the towing tug tows. The machinery involved in this operation, like the windlass and bow-stopper that prevent the anchor from moving forward too far, can also get damaged if they are not handled correctly.

The length of the rope should not be greater than two times its depth in water, and it is generally advisable to have a thorough knowledge of the sea or river bed before performing this operation.

Learn more about dredging services and reach out to us via our website to learn more. You can also learn about dredging control systems here: https://www.vanderleun.cn/Automation.html