What is a cutlass bearing?

How does a cutlass bearing work?

Inside every cutlass bearing is a grooved (fluted) liner. The bearing is pressed into the stern tube or strut, and the propeller shaft runs through the middle of it. As the boat sits in the water, seawater flows along those grooves. Once the shaft starts spinning, it lifts onto a thin film of water — engineers call this hydrodynamic lubrication — so metal never grinds on the liner in normal running.

The grooves do a second job: they flush sand and grit straight through the bearing instead of letting it sit there and grind away at the shaft. There are no grease points and no oil anywhere in the system. Water is the lubricant.

Where a stern tube bearing lives on your boat

The most common home for a cutlass bearing is the stern tube — the tube the propeller shaft runs through where it exits the hull. On boats with an exposed shaft, you'll also find them in the strut or P-bracket that supports the shaft just ahead of the propeller, and on some hulls in the skeg (see our skeg bearings guide). Wherever a shaft passes through a support underwater, one of these bearings is doing the work — they're the backbone of the wider family of marine shaft bearings.

Why boats use a water-lubricated shaft bearing

A greased or oiled bearing below the waterline creates problems a water-lubricated one simply doesn't have. With water as the lubricant, the supply never runs out while the boat is afloat, there's nothing to top up or replenish, and nothing to leak into the sea. And because the grooves flush abrasive particles through rather than trapping them, the bearing turns its worst enemy — grit — into something that mostly just passes by. That's why most modern shaft-drive boats run a cutlass bearing at the stern tube or strut.

Marine cutlass bearing materials at a glance

We stock 215 sizes of genuine UK-made Exalto cutlass bearings across four materials, covering shaft diameters from 3/4" (20 mm) up to 4" (100 mm):

How cutlass bearing sizing works

Every cutlass bearing is defined by three numbers: shaft (inside) diameter, housing (outside) diameter, and overall length. The standard marine proportion is a bearing length of four times the shaft diameter. Exalto publish the I/D and O/D as tolerance ranges: your shaft should sit below the bottom of the I/D range (that's the running clearance) and your housing above the top of the O/D range (that's the interference fit that holds it in place). Never add clearance yourself — the 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 shows how to take the three measurements, and the size chart lists every bearing we stock.

When does a cutlass bearing wear out?

A healthy bearing is silent. A worn one announces itself:

• A rumble or clunk from the stern under power
• Vibration you can feel through the hull
• Visible shaft movement when the shaft is levered at the bearing

The definitive check is done with the boat out of the water: grab the shaft at the propeller and try to move it up and down. Any perceptible knock or movement means the bearing is due. Most bearings last many years — commonly 5–10 or more depending on hours, water grit and shaft alignment; sandy water shortens the interval.

Replacement is a slipway or haul-out job — the old bearing is pressed or cut out of the tube and a new one pressed in — so it's usually done alongside shaft, seal or anode work to share the haul-out cost. (Our sister store, PSSSeals.com.au, covers PSS dripless shaft seals if yours is due at the same time.) Labour varies by yard and whether the shaft has to come out; the bearing itself is the cheap part of the job. Our replacement guide covers the whole process.

Cutlass, cutless or cutlas?

You'll see this part spelt several ways. Cutless® is a registered trademark of Duramax Marine (the Johnson Cutless range), while "cutlass bearing" is the generic Australian and UK term for the same water-lubricated shaft bearing — and cutlas or cuttless are common misspellings of both. They all refer to the same part, and you cross-reference between brands by dimensions (I/D × O/D × length), not by part code. The full story is in our cutlass vs cutless explainer.

Frequently asked questions

How long does a cutlass bearing last?

Many years — commonly 5 to 10 or more depending on engine hours, how gritty the water is, and shaft alignment. Sandy or silty water shortens bearing life; a rubber bearing with water grooves handles it best.

Can you grease a cutlass bearing?

No. A cutlass bearing is designed to run on water alone — seawater flowing along the grooves is the lubricant. It needs no grease or oil at any point in its life, which is exactly why boats use them.

How do I know what size cutlass bearing I need?

Measure three numbers: your propeller shaft diameter (I/D), the bore of the stern tube housing (O/D), and the bearing seat length. Match all three to a row in the size chart — our measuring guide walks through it step by step.

What's the difference between a cutlass and a cutless bearing?

Nothing — they're the same part. Cutless® is a registered trademark of Duramax Marine (the Johnson Cutless range), while "cutlass bearing" is the generic Australian and UK term. Cross-reference between brands by dimensions, not part codes.

Find your Exalto cutlass bearing

We stock the full Exalto range in Australia — 215 sizes across four materials, with most common sizes landing between roughly $200 and $500. Every price includes GST, Express Post is free Australia-wide, and in-stock orders placed before 2pm AEST are dispatched the same day (anything not in stock ships in 3–4 weeks).

Browse the size chart Email us your measurements

Not sure what you're looking at? Send us your shaft diameter and housing bore via the contact page and we'll reply with a specific Exalto part code.