Definition
A cutless bearing is a water-lubricated bearing with a grooved rubber lining inside a metal shell that supports a propeller shaft where it exits the hull, relying on water flow through its grooves for lubrication and cooling.
The grooves along the rubber lining are not decorative: they carry the water that lubricates and cools the bearing. A shaft run dry, even briefly, damages the lining.
How it wears and what it causes
Wear shows as play between shaft and bearing, which allows the shaft to move where it should be supported. That produces vibration, accelerates wear in the stern gland and coupling, and eventually damages the shaft itself. Checking for play is a standard haul-out inspection.
What accelerates wear
Misalignment between engine and shaft, a bent shaft, grit and sand in the water, running dry, and a propeller that is out of balance. Alignment is the one owners most often overlook — see shaft seals, bearings and alignment.
Where you meet it
On every shaft-drive inboard, in haul-out quotes, and in the diagnosis of vibration and noise. Saildrive boats do not have one.