Wear, inspection & replacement
A worn cutlass bearing announces itself — vibration, rumble, a squeal at the dock — but the proper check takes thirty seconds at every haul-out. Here's how to do it, what the limits are, and what actually kills bearings before their time.
The wiggle test, done properly
With the boat hauled, grab the propeller (or the shaft right beside the bearing) and lever it firmly up-down, then side-to-side. A new bearing runs on a designed clearance of a few thousandths of an inch — ABYC's shafting standard allows roughly 0.010″ on a 2″ shaft — so barely perceptible movement is normal. Visible wiggle, or a clunk you can hear, means the bearing is due.
Two refinements separate the proper check from the casual one. First, look at the wear pattern: rubber worn at the top of one end and the bottom of the other means the shaft is misaligned — fit a new bearing without fixing alignment and it will wear out just as fast. Second, know the test's blind spot: a badly misaligned shaft can bind solid in the bearing and show nomovement at all while wearing rapidly. If the boat vibrates under way but the bearing "passes", dig deeper.
Afloat, the symptoms are vibration that builds with revs, a rumble through the hull near the stern, and squealing or chirping at low speed — the rubber grabbing a dry shaft before the water film forms.
What kills cutlass bearings early
A bearing that dies young almost always had help. Fit the replacement after finding the killer, or it goes the same way.
Water starvation
Anodes clamped hard against the bearing end, barnacles or antifouling blocking the grooves, paint over stern-tube water slots — the film never forms and the rubber runs hot and dry.
Misalignment
The most common cause of premature wear. The shaft loads one edge of the rubber instead of floating centred — diagnosed by the diagonal wear pattern, fixed by aligning before the new bearing goes in.
Fishing line & rope
Line worked in past the prop wraps the shaft, melts into a collar and chews the rubber. Check behind the prop every haul-out.
Grit and silt
The grooves flush normal grit, but boats living in silty rivers or working off beaches wear bearings faster — harder-wearing options exist; ask us.
Galvanic corrosion
A brass shell in a steel or aluminium housing sets up a galvanic cell — shells go pink and crumbly (dezincification). Composite shells (phenolic/GRP) are immune; that's the rule, not a preference.
Long lay-ups
Rubber pressed against a stationary shaft for months can take a set or stick. Turn the shaft a couple of rotations weekly during long lay-ups, or block the shaft clear of the bearing.
How replacement works
Out with the old: set screws come out first (heat helps if they're thread-locked). Strut bearings can often be pulled in place with a proper puller; otherwise — with the shaft withdrawn — the classic method is a careful longitudinal hacksaw cut almost through the shell, collapse with a cold chisel, and twist out.
In with the new: press it — never hammer it. Impact shock can separate the rubber from the shell invisibly. A threaded rod with thick washers makes a fine field press. Don't lubricate the housing or shell for the press fit; on the shaft and rubber use only water-soluble soap or glycerine — never petroleum grease or oil, which attacks nitrile and clogs the grooves.
Retention: cone-point set screws seat into dimples drilled part-way into the shell — never through into the rubber — fitted with thread-locker and recessed flush. Then check the water path: bearing ends unobstructed, anodes clear, and stern-tube installations with positive water feed where fitted.
- Haul-out job — commonly bundled with antifoul and anode changes
- Press, don't pound: shock breaks the rubber-to-shell bond
- Water-soluble soap or glycerine only — no petroleum products
- Set screws bear on the shell, never the rubber; thread-locker, recessed flush
- Chilling the bearing to shrink it is fine if gradual — never dry ice
- Check shaft alignment before the new bearing goes in
- Keep bearing ends clear: anodes and rings off the bearing faces
Wear & replacement FAQs
How much shaft movement is acceptable?
Design clearance is only a few thousandths of an inch — ABYC's shafting standard (P-6) allows around 0.010" of bearing clearance on a 2" shaft, scaling with size — but the hand test feels that clearance amplified by leverage. The practical field rule: barely perceptible movement is fine; around 1.5 mm (1/16") at the prop is borderline; clearly visible wiggle or an audible clunk means replace. When in doubt, a shipwright with a dial gauge settles it in minutes.
Why does my shaft squeal at low revs?
That chirp or squeal is the rubber grabbing the shaft before the water film forms — stick-slip. Occasionally it happens to healthy bearings at very low RPM, but persistent squealing usually means the bearing is running dry: blocked grooves, an anode hard against the bearing end, paint over the water slots, or a stern-tube bearing without adequate water feed. Find the water problem before it becomes a bearing problem.
How long should a cutlass bearing last?
There's no fixed life — alignment and water supply decide it. A correctly aligned, properly fed bearing commonly delivers many years and over a thousand engine hours; surveyors treat failures inside a few hundred hours as premature and go hunting for the cause (misalignment, starvation, grit) rather than just fitting another bearing to wear out the same way.
Can the bearing be replaced without pulling the shaft?
Often, yes — strut (P-bracket) bearings are routinely changed in place with a proper bearing puller once the prop is off and the set screws are out. Stern-tube bearings more often need the shaft withdrawn, which may mean dropping the rudder on some boats. Either way it's haul-out work, commonly bundled with antifouling; we can recommend the right approach if you tell us your setup.
Bearing due? Get the right one ready for haul-out.
Send your measurements or the old bearing's code and we'll confirm the exact Exalto replacement, so the new bearing is waiting when the boat comes out.
Three measurements identify any cutlass bearing: shaft diameter, outside diameter and length. Send what you have — even a photo of the old bearing — and we'll confirm the right Exalto part.
(03) 5973 6444