The Essential Guide to 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 manoeuvrable at lower speeds.

Bow thrusters are generally used to manoeuvre 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 manoeuvring in the port limits (if the port regulation does not make it compulsory to use tugboats).

Related Read: A Detailed Explanation of How a Ship is Manoeuvered to a Port

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.

Bow Thrusters

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.

Related Read: Understanding Design Of Ship Propeller

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.

bow thruster

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 (as shown in the video).

Related Read: 10 Precautions to Take Before Operating Controllable Pitch Propeller (CPP) on Ships

Bow thruster Parts

The motor shaft drives the thruster shaft via a pinion gear arrangement. The sealing gasket is provided in the motor casing which holds the water which is in the tunnel.

The Thruster assembly consists of the following components:

  • The electric motor with safety relays
  • The flexible coupling between the motor and thruster
  • Mounting and casing for the electric motor
  • The connecting flange and shaft
  • Motor casing seal
  • The tailpiece with shaft seal
  • Bearings
  • The propeller shaft
  • The zinc anodes
  • Grid with bars at both ends of the tunnel

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.

The bridge controls the thruster, and the directions are given remotely. In remote failure, a manual method for changing the pitch is provided in the thruster room and can be operated from there.

Related Read: How Bow Thruster is Used for Maneuvering a Ship?

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 Thruster 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.

Bow Thruster Operation

Similarly, when the Bow Thruster is operated alone, and the signal is given to run the pitch at starboard side, the thrust will result in turning the ship towards the port side from the forward part.

When the stern and bow thruster are operated together on the same side, the ship will move laterally towards the opposite side.

As seen in the above diagrams, the bow thruster and the stern thruster provide excellent manoeuvrability to the ship.

Things To Note While Operating Side Thrusters

  • Ensure that the motor is started well ahead of the thruster operation and that the hydraulic lines are opened.
  • Never operate the thruster beyond its rated load, else it may lead to tripping of the motor.
  • Gradually increase the capacity and shift the pitch. Avoid sudden changes in the BT movement.
  • The side thrusters are considered as an “on load” starting device, i.e. they should only be operated when they are submerged in water.
  • Before operating the thruster, check for small craft, swimmers, boats and tugs adjacent to the thruster tunnel.
  • Never touch any moving parts or the electric motor in operation
  • In the case of installation of more than one panel, ensure the thruster is operated from only one panel at a time

Maintenance Of Bow Thrusters

1) The insulation must be checked regularly and kept dry. This is done because bow thrusters are not used frequently and thus there are chances of damages by moisture. Moreover, because of the frequent idle state of the bow thrusters, there can be a reduction in insulation resistance, especially in colder regions.

2) The space heater is checked for working condition so that the insulation can be kept dry.

Related Read: Importance of Insulation Resistance in Marine Electrical Systems

3) The bearings of the motor and the links are to be greased every month.

4) The condition of hydraulic oil is to be checked every month for water in oil and samples should be sent for lab analysis for further checking.

5) The thickness of the contactors is to be checked from time to time.

6) Checks are to be made for any water leakages in the bow thruster room which is an indication of seal leaking.

7) The flexible coupling between the motor and thruster should also be checked.

8) Check and inspect all the cable connections for cleanliness and tightness

9) Vacuum or blow clean the motor grid to remove the carbon grid, which may increase the operating temperature

Major Maintenance Of Bow Thrusters 

The major overhauling and maintenance of the bow and stern thrusters are done during the dry dock when the ship’s hull is out of the water, and the thruster blades and tunnel can be easily accessed.

Following maintenance is usually done in the dry docking:

  • Replacement of the O’ rings and the sealing rings
  • Removal of the pinion shaft
  • Inspection and maintenance/ replacement of gear set
  • Replacement of the bearings
  • Repairs, cleaning and replacement of the blades
  • Inspection of hub and repair if needed
  • Inspection and overhauling of the oil distribution box (for operating propeller blades)

Advantages Of Using Bow Thrusters

1) Better manoeuvrability at low speeds of the ship.

2) Safety of the ship increases when berthing in bad weather.

3) Saves money due to the reduction of stay in port and less usage of tugboats.

Disadvantages Of Using Bow Thrusters

1) a huge induction motor is required, which takes a lot of current and load, and thus large generator capacity is required.

2) Initial investment is high

3) Maintenance and repairs are costly when there is damage.

The thrust force produced by the motor to move the ship will depend on various parameters, such as the hull design, power source, tunnel design, use of grids, draft and load of the vessel, etc.

The weather’s condition and the water’s state also play a vital role in BT performance.

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Disclaimer: The authors’ views expressed in this article do not necessarily reflect the views of Marine Insight. Data and charts, if used, in the article have been sourced from available information and have not been authenticated by any statutory authority. The author and Marine Insight do not claim it to be accurate nor accept any responsibility for the same. The views constitute only the opinions and do not constitute any guidelines or recommendation on any course of action to be followed by the reader.

The article or images cannot be reproduced, copied, shared or used in any form without the permission of the author and Marine Insight. 

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  1. Hi Mohit,
    I am a Marine and saftey advisor for a inland tanker company
    I have read your article about bowthrusters. Interesting. But I am looking for a graph or table or whatsoever on which you may indicate what the effect of the bowthruster is by increasing speed of the ship/barge.

    In case the ship has no speed the turning point is about amidships. By increassing the speed the turning point shifts into the direction of the bow. Also the transverse force of the bowthruster (water force) will be in my opinion reduced by the by the water flowing longitudinally along the ship’s side. I thinkls at 3 knost speed the effect will be dramatically reduced. What is your opinion about that? .

  2. Hello, this information is very amazing !
    Thanks, for providing this useful information.

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