Introduction to water lubricated stern tube bearings

Global effects of CoVid-19 may result in significant changes to the operation and maintenance of commercial vessels. With reduced opportunity for planned dockings and restricted travel for visiting service technicians, this guide will help ensure the ongoing reliability of your water lubricated stern tube bearings.

Sterntube bearings are of the journal type. That is bearings in the shape of tubes fitted to the inside of the Sterntube itself. They are lubricated by the bath of lubricant that they are submerged in and operate hydrodynamically when the shaft is rotating.

Water lubricated bearings are generally 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 the polymeric or composite bearings in the tube, are provided by seawater.

Achieving good performance and life from the bearings

This is dependent on several factors which include:
  1. Correct design and modelling of the shaft line – bearings may be built with slope and offset as required to maximise shaft support and load on the bearing itself.
  2. Accurate installation and alignment of the bearings – installations differing from the required configuration as a result of bending or hull distortion when afloat can lead to overload.
  3. Adequate system cooling and filtration – cleanliness of the pumped seawater flush is essential. Debris and heat will contribute to accelerated wear.

Indications you have an issue with your Stern tube bearings


Your Sterntube bearings may have temperature sensors fitted.

Temperatures consistently above normal trend may indicate an issue, after considering local sea temperatures. More heat comes from more friction, so it is likely the shaft loading has changed.

Temperature spikes may be considered more alarming. A sudden increase in bearing temperature may indicate a total breakdown in the fluid film resulting in high bearing stresses.


You may notice this as significant vibration through the ship or just that recorded on shaft sensors.

Bear in mind that the reason for the vibration could be something other than the Sterntube bearings so consider other evidence such as bearing temperatures to help. Heavy vibration at the Sterntube bearings may indicate a significant issue such as shaft misalignment or even a breakdown of the bearing material.


In more extreme cases of fluid film loss and resulting shaft/bearing friction, the torque consumed by the shaft line may increase. Use your shaft line monitoring data to identify any unusual patterns.

Failure modes of a water lubricated bearing

Generally speaking, there are two failure modes:

Delamination (overheating).      

Occurs due to a loss of the hydrodynamic film and an inability to dissipate heat. Normally initiated at the bearing aft end this leads to melting of the bearing material.


Water lubricated bearings are very forgiving with regards wear. The entire wall thickness is theoretically available to support the shaft. However, with increasing wear comes increasing bearing clearance and shaft misalignment so accelerated wear may lead to a worn-out bearing.

This can occur due to several reasons including shaft misalignment, abrasives in the seawater and high dynamic loads resulting from extreme manoeuvring.

Minimising the risk of damage to water lubricated stern tube bearings

  • If the shaft is stationary for a significant period (such as a lay-up), rotate often to avoid marine growth build up. Typically, using the turning gear to rotate for up to 30 mins every 7 days is enough.
  • Ensure your pumped seawater filtration is to specification and in good working order. Typically, this is about 100 microns.
  • Ensure your pumped seawater flow rate is to the specification of the bearing supplier. A typical minimum flow rate is 0.15 l/min/mm of shaft diameter. So, 500mm shaft bearings require 4,500 l/hr. Your pump capacity should exceed the hydrostatic pressure.
  • Ensure your pumped seawater temperature is within the limits set by the bearing supplier. Typically, this would be 40°C maximum.
  • If sensors are fitted, closely monitor bearing temperatures. If a high temperature is seen, consider reducing rpm.
  • Avoid extreme high-speed turning conditions
  • Avoid high rpm partially submerged propeller operation
  • Avoid sustained low rpm that leads to boundary/mixed lubrication at the stern tube bearings, and correspondingly increased wear rate.
  • Try to minimise stirring up abrasive sediment in shallow waters. If the abrasives embed in the bearing material the wear rate may increase.

Frequently asked questions

A dive inspection has revealed that the aft bearing has wear, should I be concerned?

In most cases, water lubricated bearings have significant wear down allowance. In theory, you can support the tail shaft even if most of the wall thickness has been worn away. Normally the limit will be set by the classification society and is typically 10mm for a 600mm diameter shaft.

My existing water lubricated stern tube bearings are failing, can AtZ supplied bearing material be fitted in their place?

Yes, we can retrofit virtually any length, diameter or design of water lubricated bearing. All we need is the inside diameter of the stern tube, the shaft diameter and the length of the existing bearings. With a copy of the drawing of the existing arrangement its even easier to provide you with a retrofit option.

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