All You Need To Know About Bow Thruster Construction and Functionality


Bow thrusters are a type of propeller-shaped system fitted on the bow (forward part) and stern (known as stern thruster) of the ship. They are smaller in size than the ship’s propeller and help the vessel be more maneuverable at lower speeds.

About Bow Thrusters

Bow thrusters are generally used to maneuver the ship near coastal waters and channels or when entering or leaving a port during bad currents or adverse winds.

Bow thrusters help tugboats berth the ship to avoid unnecessary time and, eventually, money wastage because the vessel stayed less in the ports. The presence of bow thrusters on a vessel eradicates the need for two tugs while leaving and entering the port, thus saving more money.

Nowadays, ships have both bow and stern thrusters, which makes them independent of tugboats for maneuvering in the port limits (if the port regulation does not make it compulsory to use tugboats).

Installation Of Bow Thruster

Generally, side thrusters are transverse thrusters placed in a duct at the ship’s forward and aft end. The thruster set at the forward end is known as the bow thruster, and the one placed at the aft end is known as the stern thruster.

The requirement for the number of thrusters to be installed depends on the ship’s length and cargo capacity. The vessel’s route also plays an important factor as many countries have local regulations that stipulate the compulsory use of tugboats to enter or leave their port limits. For the installation of the side thrusters, the following things are essential:

  • The thruster compartment, also known as the bow thruster room, should be easily accessible from the open deck by the ship’s crew
  • Most seagoing vessels use an electric motor for the thruster, which is heat-generating machinery and must, therefore, be positioned in a dry and well-ventilated area.
  • The bow thruster room should have a high-level bilge alarm, and the indication should be provided in the engine control room and bridge.
  • The thruster room should be well-lit
  • The room should have at least one light from the emergency source.
  • In the case of installation of more than one panel, make sure to operate the thruster from only one panel at a time.
  • The thruster room should not be used to store flammable products in the area of the electric motor.
  • The tunnel or conduit containing the propeller must be installed perpendicular to the ship’s axis in all directions.
  • The propeller should not protrude out of the conduit
  • Grid bars may or may not be fitted at both ends of the tunnel (taking into account how much debris the ship bottom will experience in its voyage). The number of bars for to be kept at a minimum as they tend to reduce the thrust force and overall performance of the bow thruster (or stern thruster)
  • Sharp edges on the grid bars are to be avoided. A trapezoidal shape with no sharpness is a good choice of design for grid bars installed perpendicularly to the direction of the bow wave
  • The design and position of the thruster tunnel should not interfere with the water flow under the hull or should not add to hull resistance
  • Ensure that the material used for the installed thruster does not foul existing equipment inside the ship, such as steering links.

Construction and Working of Bow Thrusters

The bow and stern thrusters are placed in the through-and-through tunnels on both sides of the ship. There are two such tunnels at the forward and aft ends of the ship.

The thruster takes suction from one side and throws it out at the other side of the vessel, thus moving the ship in the opposite direction. This can be operated in both directions, i.e. port to starboard and starboard to port.

The bow thrusters are placed below the ship’s waterline. For this reason, the bow thruster room should be checked for water accumulation at regular intervals. The bow and stern thrusters can be electrically, hydraulically, or diesel-driven. However, the most commonly used are electric-driven thrusters, as in hydraulic-driven thrusters, there are many leakage problems.

Also, diesel-driven bow thrusters require more maintenance, and someone needs to go to the thruster room every time before starting to check the thrusters.

The thrusters used are usually of the CPP type, i.e., the blades on the propeller boss can be moved to change the direction of the thrust.

The boss, which carries the blades, is internally provided with a movable shaft (operated by hydraulic oil), also known as a Hydraulic Pod Motor-driven Thruster.

Once the signal to change the pitch is given, hydraulic oil will be supplied to operate the internal shaft (within the boss) to change the blade angle of the thruster. 

Operation Of Bow Thruster

A bow thruster consists of an electric motor mounted directly over the thruster using a worm gear arrangement. The motor runs at a constant speed, and whenever a change is required in the thrust or direction, the controllable pitch blades are adjusted.

These blades are moved, and the pitch is changed with the help of hydraulic oil, which moves the hub on which the blades are mounted. As the thruster is of controllable pitch type, it can be run continuously, and when no thrust is required, the pitch can be made to zero. Usually, the hydraulic valve block, which controls the pitch of the blades, is operated in the BT room to change the blade angle in an emergency.

When the Bow thrust is operated alone and the signal is given to operate the pitch at the port side, the thrust will turn the ship towards the starboard side from the forward part.

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Source: MarineInsight