The stern tube is at the heart a traditional shaft driven propulsion system. It provides the conduit from inside the vessel and the engine to outside and the driven propeller. It is a tube which provides bearing support and cooling for the rotating shaft, a watertight seal and structural support to transfer loads from the propulsion system into the vessel’s hull.
Containing a number of safety and propulsion critical items – it is a key part of vessel equipment requiring maintenance and periodic survey.

Stern Tube

The traditional layout of vessels sees inclusion of a single stern tube, but in some other applications two or even three stern tubes can be seen. They are usual placed symmetrically either side of the vessel centreline such that thrust propels the vessel ahead in a straight line.

As the stern tube provides the transition from inside the ship to outside underwater, there are at various points along its length sealing interfaces. Two main types of configuration exist, which are:

  • Oil lubricated stern tube – fitted to the majority of ships and is filled with oil providing white metal bearings with lubrication and cooling, and has a seal fitted at either end.
  • Water lubricated – fitted to newer ships and those operating within sensitive marine areas as well as military vessels requiring resistance to underwater explosions. These usually have only a single seal on the inboard side. Lubrication and cooling of composite bearings in the tube are provided by seawater.

From the forward (inboard) to the aft (outboard) end of the stern tube sit a number of system components which together support the function of the stern tube. These include:

  • A water or oil filtration, circulation and cooling system – feeding fluid into the tube to provide bearing lubrication
  • A rotating forward seal – preventing water or oil in the stern tube entering the vessel machinery space
  • Bearings which support the rotating tail shaft
  • Pressure and temperature sensors – allowing monitoring of the bearing and lubrication conditions
  • In the case of an oil lubricated stern tube only rotating aft seal – preventing oil leaking out, or seawater leaking into the stern tube.
  • A rope guard and cutters – providing additional protection for the aft seal in case of net, rope or cable being wound onto the shaft.

Sterntube Survey

Survey of a stern tube would cover all of the main system elements to ensure that they were operating within normal parameters. Review of the latest performance data and discussion with the onboard team to see whether any particular issues had been identified would provide basis of any focus areas.

While in active service, the sterntube survey would focus on equipment accessible from within the vessel.

Condition of the aft seal could be evaluated in the water with assistance from a dive team performing a basic visual check as well as taking any relevant measurements and looking for evidence of any damage or leaks. The rope guard and any cutters could also be checked as part of this review.

Within a dry dock, a more thorough evaluation of the system would be possible, to include inspection of bearing surfaces and shaft liners if the shaft is to be withdrawn.

Classification societies offer various schemes which support reduce need for withdrawal of a tail shaft provided adequate operational data and condition monitoring is undertaken during service.

AtZ can support stern tube survey activities as well as recommend potential system upgrades and any required parts and service. 


My stern tube is oil lubricated and seems to be leaking oil inboard. It is not causing any sea pollution and my oil analysis is fine – so can I continue operations?

You can continue operations, but we would strongly recommend rectification of the fault soonest. Oil leaking inboard suggests that the aft seal is damaged and sea water is filling the stern tube. Oil lubricated bearings do not perform well in seawater contaminated oil, and there is a risk of overheating and seizing them.
Your oil analysis sample is often drawn from the upper portion of the stern tube where there is most oil. At the tail end of the tube where the highest loaded bearing is operating – it could be completely immersed in seawater.
In this situation AtZ can review your system and suggest containment actions that will help to reduce differential system pressures and slow the leakage. Once docked, we can also provide the required parts and service to make the repair.

We want to change to an EAL stern tube oil – is this a straight swap?

No – each part of the stern tube system needs to be considered for compatibility first. A system flush may be required, and some seals and other equipment might need to be changed for a compatible material.
AtZ can review your system and advise any required steps for upgrade.

Our water lubricated seal is leaking inboard – can we do anything to reduce the leakage before dock?

Yes – depending on the seal type, it may be possible to make some small adjustments to reduce the leakage.
First – it is not recommended to do any work on the seal without at least an inflatable seal and preferably a second barrier (cofferdam) for security – also to prevent the clamp being pushed back on release.
The operating compression of a face-type seal can be checked. If wear has reduced the fitted value, it may be possible to move the seal shaft coupling to temporarily increase this. In the case of an axial lip type seal, it may be possible to exchange the running seal for the spare on the shaft.
Please get in touch with us to review your fitted products and from there we can recommend your best containment, monitoring and then repair options.

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