How Do Bow Thrusters Steer A Ship?
Navigating a ship through currents, winds, and tides is always a complex task. All three factors can cause the ship to drift sideways. The pilot must be aware of these effects and constantly take corrective action to ensure safe navigation.
In strong winds and tides, tugs often help the ship to berth sideways, and drift can be counteracted by towing, steering, or applying shear forces. These tugs are in addition to the existing engines, rudders, and thrusters (if any).
At any given time, berthing speeds are always low. In most cases, it is best to keep it below two knots. Bow or stern thrusters apply shear or lateral forces and are only effective at lower speeds. They can reduce the impact forces generated when the ship berths at a pier or mooring.
When the bow hits the pier hard, the bow thruster turns left or right, depending on the situation, to prevent a fall and pass the rope safely from the bow through the rigging, lifeboat, or motorboat.
When a ship lists (usually caused by a pilot approaching a dock), the stern turns to the left or right. As a result, the bow turns in the opposite direction, to the right or left. This phenomenon is known as “heeling.”
For a starboard propeller (i.e., a propeller that rotates clockwise as the ship moves forward), the stern tilts to the left and the bow tilts to the right. The opposite is true for a port propeller. This is when the bow thruster comes into play. In the first case, the bow turns to the left to limit its rotation to the right, while in the second case, the bow turns to the right.
Why does the bow and stern tilt when the ship lists? This is due to the effects of lateral thrust and propeller stroke, which will be discussed in a later chapter.
An electric motor drives the bow thruster. The power of the bow thruster usually depends on the engine power. A bow thruster (BT) rated at 800 to 1,000 hp is sufficient for installation on vessels with an overall length of 150 to 160 m, a beam of 22 to 25 m, and a gross tonnage (GRT) of about 10,000 to 12,000 tonnes. It can easily withstand a Beaufort 3 to 4 (11 knots) wind when at anchor.
In areas with higher wind speeds and larger vessel lengths and gross tonnage, a more powerful bow thruster (BT) is required.
Bow thrusters are usually installed to replace tugboats, which are hired by visiting ships. This can reduce the cost of ship operation. However, in some ports, hiring a tugboat is mandatory. In such cases, a BT tugboat can provide additional support to the pilot.
BT tugboats play an important role in both mooring and steering processes. During mooring, the bow tugboat (BT) keeps the bow away from the anchor chains.
Bow Thruster (BT)
The bow tug (BT) plays an important role when the ship is turning. In a narrow space, such as a crowded dock, the ship must turn around its axis. In this case, the ship stops and, with the help of the bow thruster, turns the bow to port or starboard, which in turn turns the stern.
Sometimes, the stern is also turned by the rudder and the engine. However, in this case, the natural rolling tendency of the ship when going astern is usually taken into account. If the ship is equipped with a starboard propeller, the bow will turn to the right, while the engine turns to the stern. Therefore, it is recommended always to use the right rudder; when going astern, the ship will continue to turn in the intended direction.
When going astern, it is recommended to use a tug at the stern and, if necessary, use the bow thruster. The bow thruster straightens the stern by pulling, thus counteracting the effect of the transverse thrust.
Therefore, the bow thruster plays an important role in ship maneuvering. However, how and when to use it depends on practical experience.
