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.
Sterntube bearings are located in the Sterntube and serve to support the rotating tail shaft onto which the vessels propellors are fitted.
ATZ have the parts, service and experience to survey, supply and provide technical guidance on both white metal and composite derivatives.
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.
Two main types of Sterntube bearings exist, which are:
Although essentially tubes of material they have various features that allow them to perform. These include:
Both white metal and composite bearings are an interference fit in their respective stern tubes which means they must be press-fitted with significant hydraulic force when fitted. In the case of composite bearings the interference is much greater, and it is often best practise to shrink the composite bearings by cryogenic freezing before fitting.
The importance of reliable operation of the bearings is paramount. Through failure comes the possibility of:
It is important that the lubricant quality is maintained:
For an oil lubricated bearing arrangement that means clean filtered oil that is regularly sampled and changed at the correct service interval. It also means good maintenance and monitoring of the aft seal performance to avoid any seawater ingress into the bearings lubricating oil.
For a water lubricated bearing arrangement that means, clean filtered seawater pumped to the Sterntube at a flow rate at least that recommended by the maker.
If monitoring and maintenance protocols are carried out, then the bearings should last the life of the vessel. Of course, exceptional circumstances can occur leading to accelerated wear or failure of the bearings. In those situations, replacement is required.
In most cases bearing replacement require withdrawal of the tail shaft to gain access to the Sterntube and allow bearing removal and refitting. However, some composite bearing designs are not tubes but two half shells forming a tube – these can be exchanged with the tail shaft in position.
Whatever your bearing needs, ATZ have extensive experience with both types and can supply new bearings or provide advice on the operation, maintenance and troubleshooting of your equipment.
If the seawater has caused loss of hydrodynamic film, then you should see temperature spikes from your bearing sensors. Also, check your Sterntube oil sample in a laboratory, if there are traces of tin (from the white metal) it is likely they are damaged.
In a white metal bearing generally no. The white metal material is relatively thin and the oil lubrication should prevent anything other than minimal wear.
Composite bearings have much more wear allowance. The entire wall thickness of the material can support the shaft and wear is expected. Seawater is not nearly as good a lubricant as oil.
Ensure good condition of the Sterntube bore in which the bearing will be fitted. Measure well to calculate the outside diameter of the new bearing – to be machined on site. Note that the stern tube bore may need machining or repairing.
Check and double check the orientation of the bearing before fitting. There are external and internal features which must line up.
Check the condition of the shaft, any machining here will require machining a smaller than anticipated bearing inside diameter.
Cleanliness, lubrication and removal of sharp edges is key when hydraulically pushing up a white metal bearing. Any pickup on the bearing outside diameter could ruin your new bearing (and damage the stern tube too).
The experts in marine projects and mechanical engineering.
Our team of highly experienced technical engineers offer industry knowledge gained throughout their time at several multi-national marine propulsion design and service providers.
Free Download