1. CUTLESS OR CUTLASS?
Strictly, a modern propeller shaft bearing is a 'cutless' bearing, so named because, compared to earlier types of bearings, they 'cut less' into the propeller shaft.
Early propeller shaft bearings were made of various materials, including wood (often lignum vitae), bronze or brass and white metal.
However, all of these tended to cut into the shaft over time, leading to the development of rubber bearings that caused less wear to the propeller shaft.
This led to a manufacturer, Duramax Marine, using the name 'Cutless' to describe the bearings. The name stuck, just as 'biro' and 'hoover' stuck.
The use of the word 'cutlass' to describe bearings was done partially to avoid impinging on the 'cutless' bearing trademark, but it's likely there was also confusion over the actual name and the word 'cutlass' (actually a short sword often used by pirates) was accidentally substituted.
2. WHAT IS A CUTLESS BEARING?
The original cutless bearing design continues to this day: it is a rubber bearing with longitudinal grooves, which is often bonded into a metal (brass) or resin (phenolic) shell.
The grooves perform a couple of functions: they act as a conduit for the water which lubricates the bearing but also channel away grit and dirt that might otherwise damage the bearing.
A water-lubricated bearing must have water flowing through it continuously when operating.
Additionally, the grooves enable water to pass to/from the top of the stern tube to lubricate the stern seal(s).
Vetus cutless bearings are 'hydrodynamic lubricated' bearings, which means that the shaft runs on a film of water. This helps reduce noise and vibration by reducing friction and acting as a cushion, prolonging the life of the bearing and shaft.
With proper loading, rotational speed, clearances and alignment, the shaft will carry a film of water as it rotates. This film of water actually lifts the shaft, and the shaft then “rides” on the water film.
Operating in this way, both friction and wear are very low.
A tough resilient rubber compound is used which is relatively immune to the action of grit and sand.
If grit enters the bearing and gets trapped between the shaft and the rubber lining, it is depressed into the rubber and rolled into a water groove where it is flushed out of the bearing, minimizing damage to the bearing and shaft.
3. CLEARANCE
It is important to understand that the bearing rubber used is hygroscopic, which means it swells when soaked in water. For this reason, a new bearing requires a certain amount of clearance between itself and the shaft, to compensate for the rubber swelling.
Also, note that the rubber can take up to fifteen days to fully reach its working volume.
In addition, there is clearance factored in between the bearing and shaft to allow the lubricating water film to form during use.
To summarise: when new, clearance will be observed to allow for:
- hygroscopic expansion
- hydrodynamic lubrication
- manufacturing tolerances
It is not possible to give an exact indication of how large this gap will be, but, for example, a cutless bearing for a 25mm shaft may have up to 0.2mm tolerance all around it.
A further advantage of this is that when the dry cutless bearing is installed, there will be enough room around the bearing to enable the shaft to be slid through it without requiring lubricant or risking damage to the bearing.
4. MAINTENANCE AND STORAGE
During their working life, Vetus cutless bearings do not require any particular maintenance, but it is recommended to rotate the shaft every 2-3 weeks to prevent a build-up of marine growth which can potentially damage both the shaft and bearing.
The cutless bearing should be visually checked for wear once a season.
New Vetus cutless bearings should be stored in their original boxes in a cool environment, away from direct sunlight (ultraviolet), fluorescent lighting or ozone-generating equipment such as electric arc welders. The normal shelf life is up to ten years, provided the bearing is stored in the conditions recommended.
5. REPLACING A CUTLESS BEARING
The first step in removing a cutless bearing will usually be to remove the propeller shaft, the exact procedure for which will depend on the type of stern gear. In general, the propeller would be removed first and then the shaft disconnected from the engine coupling before it is withdrawn outwards.
The propeller shaft should be checked for wear, particularly around the location of the cutless bearing and the stern gland seals or packing. If wear is found, consideration should be given to replacing the shaft.
At this stage, it is worth confirming the diameter of the propeller shaft on an area that does not have any wear: for example, it is surprisingly common to confuse a worn 1" propeller shaft that should measure 25.4mm with a 25mm shaft.
We would advise using a good quality traditional vernier caliper for this.
The cutless bearing should then be removed from the stern tube or bearing housing. Firstly, any retaining screws should be removed: be aware that these may have been covered with filler, so a close inspection should be made.
As cutless bearings are usually a relatively tight press fit, simply knocking them with a hammer is unlikely to be the best approach: instead, a suitable bearing puller or threaded extraction tool is preferable, as this reduces the risk of damage to the stern tube.
In the instance of a completely stuck bearing, it can be possible to cut through the shell of the bearing and collapse it inwards, although care has to be taken so as not to damage the stern tube.
Before installing the new cutless bearing, the stern tube or housing should be thoroughly cleaned and checked for damage, corrosion or distortion. The internal dimension should also be checked with a vernier caliper to confirm that the correct cutless bearing has been selected.
The new bearing would usually be fitted using a suitable press or threaded installation tool, applying pressure to the hard shell rather than the rubber. The bearing should then be locked in place with new screws / grub screws using a thread sealant, but be careful to ensure that these are not over-long and that they do not distort the rubber.
When installed, check that the bearing grooves are not obstructed, so that water can flow freely in and out through the bearing.
Once the new cutless bearing has been installed, the propeller shaft can be refitted: this should pass through the bearing without the application of force. No grease or lubricant should be required. After this, the shaft should be reconnected to the engine coupling and the stern gland reinstalled.
It is advisable that engine alignment is checked at the time of reconnecting the propeller shaft to the engine coupling: misalignment may cause premature wear of the cutless bearing and stern gland seal and may lead to vibration when the engine is being used.
A properly installed cutless bearing should deliver long and reliable service, often for many years in typical usage. If the bearing experiences uneven wear or even failure in a short time (for example, under 500 hours running), the cause of the issue should be determined rather than just replacing the old bearing.
It is strongly advised to check everything very carefully when the boat is launched: the stern gland may require 'burping' to ensure water is all the way up the stern tube, or a water-feed pipe may need to be primed to purge it of air. If this is done, it will ensure water flow around the cutless bearing to keep it lubricated and ensure the long life of the bearing.
If you are unsure about any aspect of choosing the correct size of cutless bearing or about inspecting or replacing a bearing, our technical team will be happy to help. Please contact us for advice or assistance.