What is a cutlass bearing?
A cutlass bearing is a sleeve bearing that supports a boat's propeller shaft where it passes through the stern tube or an external strut. It has a rigid outer shell and a grooved rubber lining, and it runs on water rather than oil or grease.
How a cutlass bearing works
The shaft turns inside the rubber lining. Grooves run the length of the lining, and seawater flows along them to lubricate and cool the bearing. Once the shaft is turning fast enough, a thin film of water separates the shaft from the rubber.
That is why the bearing must be kept free of oil and grease, and why the water supply matters. Being afloat is not enough on its own: the grooves need to stay open so water can reach the whole length of the bearing.
Where it is fitted
Most inboard boats have a cutlass bearing at the aft end of the stern tube, where the shaft leaves the hull. Boats with a longer shaft often have a second bearing in a strut or P-bracket close to the propeller, and on some hulls the shaft support is built into the skeg. A skeg bearing can also belong to the rudder, so check which before ordering.
The bearing only supports the shaft. Keeping water out of the boat is the job of the shaft seal or stern gland. Our guide to propeller shaft and stern tube bearings maps each part along the shaft.
Shell and lining
Exalto's standard range has three shell materials, each lined with nitrile rubber: phenolic cutlass bearings, brass cutlass bearings and GRP cutlass bearings. Exalto also makes all-rubber cutlass bearings with no rigid shell. The shell has to suit the housing it sits in, so the material matters as much as the size.
Symptoms of a worn cutlass bearing
As the lining wears, the shaft has more room to move. Look and listen for:
- Vibration through the hull or engine mounts when under way, often at particular engine speeds.
- Noise from the stern, such as rumbling or knocking as the shaft moves in the bearing.
- More shaft movement than last time. With the boat out of the water, lift the shaft next to the bearing and compare the movement with your earlier records.
- Damage at the bearing ends: torn, swollen or missing rubber, or grooves that have worn away.
- Wear on one side of the lining. This usually points to shaft or engine misalignment, which needs correcting as well as the bearing.
- A shaft seal or stern gland that keeps leaking, because a shaft that moves in its bearing also moves in the seal.
No single sign proves the bearing is worn. A damaged propeller, a bent shaft or a misaligned engine can cause the same vibration, and a new bearing has some movement by design. Measure the clearance before deciding.
Cutlass bearing clearance
The bore of a cutlass bearing is made larger than its nominal shaft size. The difference is the running clearance, and it leaves room for the water film. Exalto publishes a bore tolerance for every size. For example, the phenolic bearing for a 25 mm shaft (MPSF25) has a bore of 25.27 to 25.37 mm, while the brass bearing for the same shaft (MBSF25) has a bore of 25.08 to 25.18 mm. The phenolic bore is larger because Exalto's tolerances allow for the phenolic shell absorbing some water.
To check clearance, slide feeler gauges between the shaft and the lining at both ends of the bearing, or measure how far the shaft lifts with a dial gauge. Compare the result with the new-bearing figures and with Exalto's tolerances and clearances. Every product page on this site lists the bore tolerance for that bearing.
How long does a cutlass bearing last?
There is no fixed service life. Wear depends on how many hours the boat runs, how well the shaft is aligned, whether water flows freely through the grooves, and how much sand or silt is in the water the boat runs in. Recording the clearance at every haul-out shows the trend, which tells you more than the bearing's age.
When the measurements or the symptoms say it is time, the cutlass bearing replacement guide walks through the job step by step, including the tools and costs.
Cutlass or cutless?
Both spellings refer to this type of bearing. Cutless is a Duramax Marine trademark; our page on cutless vs cutlass bearings explains the names and what to compare when changing brands.