Marine shaft bearings: propeller shaft and stern tube bearings
On an inboard boat the propeller shaft runs from the gearbox, out through the hull and into the propeller. Along the way it is carried by one or more water-lubricated shaft bearings. This guide identifies each bearing position and the parts that are often mistaken for them.
The propeller shaft line, from gearbox to propeller
- Gearbox coupling. Bolts the shaft to the gearbox output flange. The gearbox has its own internal bearings, which the engine or gearbox supplier services.
- Shaft seal or stern gland. Stops seawater entering where the shaft passes into the hull. It seals the shaft but does not carry it.
- Stern tube. The tube through the hull that the shaft runs in, usually with a bearing at its aft end.
- Strut or P-bracket. An external bracket under the hull that holds the shaft close to the propeller, with a bearing of its own.
- Propeller. Mounted on the shaft taper and held by a key and nut.
Not every boat has every part. A short shaft may run only in the stern tube bearing, a long one may need a strut bearing as well, and some hulls carry the shaft through a skeg.
Stern tube bearings
The stern tube bearing sits at the aft end of the stern tube, where the shaft leaves the hull. On a boat without a strut it is the bearing closest to the propeller and carries much of the propeller's weight. Some installations add a second bearing at the forward end of the tube.
The tube material affects the choice of shell. Exalto does not recommend fitting a brass-shelled bearing straight into a steel or aluminium tube; its non-metallic phenolic and GRP shells are the alternatives for those housings.
Large commercial ships often use oil-lubricated stern tube bearings sealed at both ends. That is a different design from the water-lubricated bearings described here.
Strut and P-bracket bearings
A strut, called a P-bracket after its shape, holds the shaft below the hull just forward of the propeller. An A-bracket does the same job with two legs. The bearing is pressed into the bracket's barrel and is usually held with set screws. Because it is the support nearest the propeller, it carries much of the propeller's weight.
A strut bearing can often be changed without removing the shaft. The cutlass bearing replacement guide compares the removal methods and lists the tools.
Prop shaft bearings on boats and vehicles
"Prop shaft bearing" also describes the centre support bearing on a car or truck tailshaft. That part has nothing in common with a marine shaft bearing, and we do not sell it. For a vehicle, contact an automotive parts supplier.
Parts that are not propeller shaft bearings
- Shaft seals. A dripless seal or packed stern gland keeps water out where the shaft leaves the hull. It seals the shaft but does not support it, so it is sized and replaced separately from the bearing.
- Rudder bearings. A rudder stock or pintle moves with the steering and has its own load and clearance requirements, so a propeller-shaft bearing cannot be chosen for it by size alone. The skeg bearing guide explains the difference.
- Gearbox and engine bearings. These are internal parts, serviced by the engine or gearbox supplier.
Choosing a replacement shaft bearing
Each water-lubricated shaft bearing we sell is an Exalto cutlass bearing, identified by its nominal shaft diameter, housing bore and length. The measuring guide shows how to take those three dimensions, and the signs of a worn bearing help you decide whether it is due for replacement.