Marine shaft bearings: a boat owner's guide
Between the gearbox and the propeller, your shaft runs through several very different bearings — and only one of them is the part most owners are actually searching for. Here's how the whole system fits together, which bearing does what, and how to size the one you need.
The propeller shaft bearing system, end to end
Follow the propeller shaft aft from the engine and you pass through the whole drivetrain in order:
- Gearbox coupling — the shaft bolts to the gearbox output flange. Gear and thrust bearings live inside the gearbox itself.
- Shaft seal (stern gland) — where the shaft exits the hull at the inboard end of the stern tube, a seal keeps seawater out of the boat.
- Stern tube — the tube carrying the shaft through the hull.
- Cutlass bearing(s) — at the aft end, a water-lubricated bearing supports the spinning shaft. It sits in the stern tube itself, or in a strut, P-bracket or skeg on boats where the shaft leaves the hull well forward of the prop.
- Propeller — hung on the shaft taper, just aft of that last bearing.
That aft bearing — the cutlass or stern tube bearing — is the wear item in the system, and it's the part we stock.
Which boat shaft bearing do you actually need?
"Marine bearings" covers three quite different parts, and it pays to be clear which one you're chasing before you buy anything:
| What it's called | Where it sits | What it does | Where to get it |
|---|---|---|---|
| Cutlass bearing (stern tube, strut or skeg bearing) | Aft end of the stern tube, or in a strut, P-bracket or skeg | Supports the spinning shaft on a film of seawater | Right here — 215 genuine Exalto sizes in Australian stock |
| Shaft seal / stern gland | Inboard end of the stern tube, inside the hull | Stops seawater entering the boat around the shaft | Our sister store, PSSSeals.com.au — PSS dripless shaft seals |
| Gearbox & thrust bearings | Inside the gearbox / engine | Carry gear loads and propeller thrust; oil-lubricated | Your engine or gearbox dealer — not a cutlass part |
We sell the first row: water-lubricated cutlass bearings. If it's a seal or a gearbox part you're after, the other two columns will point you in the right direction.
Why the stern tube bearing is water-lubricated
Unlike every bearing on the engine side, the aft shaft bearing runs on seawater — no grease, no oil. A grooved (fluted) liner sits in the stern tube, strut or skeg; water flows along the grooves, and once the shaft is turning it rides on a thin film of water rather than touching the liner. Engineers call it hydrodynamic lubrication. The same grooves flush sand and grit straight through instead of letting it grind the shaft. There's a full explanation on our what is a cutlass bearing page.
On the name: you'll see cutlass, cutless and stern bearing used interchangeably. Cutless® is a registered trademark of Duramax Marine for their Johnson Cutless range; cutlass bearing is the generic Australian and UK term for the same water-lubricated part. Exalto is the UK-made equivalent range we stock — cross-referenced by dimensions, not brand codes. More on that in cutlass vs cutless.
Sizing a stern bearing: three numbers
Every cutlass bearing is specified by shaft inside diameter (I/D), housing outside diameter (O/D), and overall length — with length conventionally four times the shaft diameter. I/D and O/D are each published as a tolerance range: your shaft must sit below the bottom of the I/D range (running clearance) and your housing above the top of the O/D range (interference fit). Never add clearance yourself — the published tolerances already build it in.
Part codes tell you the measurement system: MPSF50 is fully metric,
MIPSF40 is a metric shaft in an imperial housing, and IPSF20
is fully imperial. Our measuring guide walks through
taking the three measurements with callipers, and the
size chart lists every size we carry, from
3/4" (20 mm) shafts up to 4" (100 mm).
Four marine bearing materials — which one?
Size decides which bearings fit; material decides which fits your boating:
- Phenolic — the modern default and our biggest seller. Low friction, long life in clean saltwater, excellent vibration damping.
- Rubber — nitrile with longitudinal water grooves that flush particles through. The pick for sandy, silty or debris-laden water.
- Brass-shelled — a phenolic liner in a naval brass shell, for worn or oversize housings that need a metal shell's grip. Heavier and dearer, but very long-lived.
- GRP — glass-reinforced polyester shell that bonds directly into fibreglass stern tubes. Never use in a metal stern tube.
Worn boat propeller shaft bearings: the signs
A tired shaft bearing announces itself: a rumble or clunk under power, vibration through the hull, and — with the boat out of the water — perceptible movement when you grab the shaft at the propeller and try to lever it up and down. Any knock at the bearing means it's due. Most bearings last many years, commonly 5–10 or more depending on engine hours, grit in the water and shaft alignment.
Replacement is a slipway job: the old bearing is pressed or cut out and the new one pressed in. Labour varies by yard and whether the shaft must come out — the bearing itself is the cheap part — so most owners bundle it with shaft, seal or anode work to share the haul-out. Our replacement guide covers the whole process.
Genuine Exalto marine shaft bearings, in stock in Australia
Cutlass Bearings Australia is an independent Australian reseller of genuine Exalto UK-made bearings — 215 sizes across the four materials, priced from $115 to $2,395 inc GST, with most common sizes in the $200–$500 range. In-stock bearings ordered before 2pm AEST ship the same day with free Express Post Australia-wide; anything we don't hold ships in 3–4 weeks. Every price includes GST.
Browse the size chart Email us your measurements
Not sure of your size? Send us your shaft diameter, housing bore and seat length via the contact page and we'll match you to the right Exalto part code.