A bow thruster or stern thruster is a transverse propulsion device fitted to the bow or stern of a vessel to assist with manoeuvring. Bow thrusters allow the vessel to turn to port or starboard side, without using the main propulsion system, which requires some forward motion

ATZ with our partner, Aegir Marine, have the parts, service and experience to overhaul most thruster types.

Bow Thruster sand Stern Thruster

Tunnel Thrusters

Large vessels usually have one or more tunnel thrusters built into the bow, below the waterline. An impeller in the tunnel can create thrust in either direction which makes the ship turn. Most tunnel thrusters are driven by electric motors, but some are hydraulically powered. A bow thruster, also known as a tunnel thruster, may allow the ship to dock without the assistance of tugboats, saving the costs of such service. Ships equipped with tunnel thrusters typically have a sign marked above the waterline over each thruster on both sides, as a big cross in a red circle: (x).

Tunnel thrusters increase the vessel’s resistance to forward motion through the water, but this can be mitigated through proper fairing aft of the tunnel aperture. Ship operators should take care to prevent fouling of the tunnel and impeller, either through use of a protective grate or by cleaning. 

Maintenance of tunnel thrusters can typically be described as a minor overhaul or major overhaul.

Water Lubricated Stern Tube Bearings

Planned Dry Dock

During a planned dry dock, a minor overhaul can be carried out with the thrusters still attached to the thruster tunnel in the hull or removed and taken to a workshop.

The type of works carried out during a minor overhaul could be:

  • Propellor blade removal for repair in worst case or just cleaning and polishing
  • Replacement of blade foot seals 
  • Main shaft seal inspection.
  • Removal of the end cap for internal inspection of the thruster mechanicals
  • Backlash and bearing clearance checking
  • Overall external inspection
  • Normally a class surveyor would expect non-destructive testing to be carried out on the propellor blades
  • Refilling with new oil
  • Testing of thruster and calibration of pitch (on CPP type)

Major Overhaul

A major overhaul normally requires the removal and transportation of the thrusters to a workshop. To do this the thruster is disconnected from its main drive unit (normally an electric motor) and cut from its mountings to the thruster tunnel itself.

The type of works carried out during a major overhaul could be:

  • All the works normally carried out during a minor overhaul
  • Strip down of the main horizontal shaft to inspect all components
  • Strip down of the hub assembly and inspection of yoke, sliding blocks
  • Main shaft seal overhaul
  • New bearings fitted when required
  • Re-shimming bearing clearances.
  • Blasting, cleaning and painting of the thruster pod
Propulsion Overhaul

Thruster repairs

ATZ with our partner Aegir Marine have extensive experience with most bow thruster and stern thruster types and are in position meet your thruster parts and service needs. 

Thruster repairs are normally carried out at the shipyard, but we can offer all options:

  • Vessel afloat and trimmed sufficiently to allow access – where vessel design allows.
  • At the shipyard (basically anywhere), our Thruster expert will carry out or supervise any required works to your Thruster.
  • In-house, in our dedicated Thruster workshop an excellent repair team will carry out the repair.

All our repair procedures are set up in close partnership with the classification society and are followed by detailed reports.


What non-destructive testing results is a class surveyor likely to request during a major overhaul?

Vital equipment that require testing is normally gears, blades, blade carriers and hubs

Can a Thruster operate on a biodegradable oil?

This is largely dependent on the components that are most likely to be affected by the bio oil (particularly when mixed with seawater), and are normally elastomeric (gaskets, o rings, lip seals etc). It is essential that they are of a compatible material. 
Many thrusters are bio compatible or can easily be upgraded to be so.

Other than the planned maintenance described, is there anything else to look out for when overhauling thrusters?

Yes, pay attention to the input shaft seals as they are known to wear, particularly on the liner. Sometimes, shimming to re-position the liner will eliminate leakages.
Also, check any flexible couplings on the input shaft. They transmit a lot of torque and a little misalignment rapidly wears them out.

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