A vessel’s stern tube and stern tube system are critical parts of the propulsion arrangement. They allow the propeller shaft to safely pass from the inside of the ship to the outside sea.
The stern tube is a hollow, cylindrical structure built into the aft part of the ship’s hull. It houses the propeller shaft as it exits the vessel to connect with the propeller.
The stern tube supports the rotating propeller shaft by means of bearings, separates seawater from the ship’s interior and provides a controlled environment for lubrication and sealing
The stern tube includes several components:
- Stern tube bearings – support and align the shaft while it rotates
- Sealing system – prevents seawater from entering and lubricating oil from leaking out
- Lubrication system – reduces friction and wear (oil-lubricated or water-lubricated)
- Cooling (in some designs) – helps manage heat from friction
Failure of the stern tube system can lead to serious issues:
- Water ingress into the ship
- Oil leakage
- Shaft misalignment or seizure
- Loss of propulsion
In 2025, we had a total of nine cases related to stern tube damages compared to four in 2024 in Asia Pacific alone. The total claim of these nine cases amounted to approximately USD 8 million in cost.
Our investigation into these claims show that it is rarely due to a single cause. It is usually a combination of mechanical, operational and maintenance-related factors. Here below are the common causes that we have seen: –
Lubrication failures
- Loss of lubricating oil (leakage through seals)
- Contaminated oil (water ingress, debris)
- Incorrect oil grade or viscosity
- Inadequate oil circulation or cooling
Seal system failure
- Improper installation or alignment of seals
- Abrasion from rope, fishing nets, or debris
- Pressure imbalance in the seal system
Misalignment of shafting
- Improper installation during dry dock
- Hull deformation (hogging/sagging, grounding, heavy loading)
Excessive vibration and dynamic loads
- Propeller imbalance or damage
- Cavitation effects
- Engine torsional vibrations
- Operating outside design parameters
Foreign object damage
- Fishing nets, ropes, wires entangled in the propeller
- Floating debris or ice impact
- Contact with seabed during grounding
Poor maintenance and human factors
- Inadequate inspection intervals
- Failure to monitor oil condition
- Improper assembly during repairs
- Lack of crew training or procedural errors
Mitigating stern tube failure relies on consistent maintenance, careful monitoring and proper operation. Maintaining the correct lubrication – whether oil or water-based – with stable pressure and regular analysis helps prevent contamination and overheating. Seal integrity is equally important, requiring routine inspection and timely replacement to avoid leakage. Proper shaft alignment, especially after dry dock, reduces uneven loading and wear. Continuous monitoring of bearing temperature and vibration allows early detection of issues. Operating within design limits, avoiding sudden stresses and protecting against debris further reduces risk. Combined with corrosion protection, adherence to guidelines and trained crew, these measures significantly enhance reliability and can mitigate losses.
