Skeg bearings: a cutlass bearing in a different housing

What is a skeg?

A skeg is the fin or keel extension at the aft end of the hull that carries the propeller shaft's aft support on many yachts, motorsailers and workboats. As well as holding the shaft steady right where it matters — just ahead of the propeller — the skeg protects the prop and rudder. You'll also see the part inside it called a skeg bush or a rudder skeg bearing; different names, same component.

Not every boat has one. On many hulls the same job is done by a strut bearing or a P-bracket bearing — an appendage that hangs the aft bearing below the hull instead of building it into a fin. Whichever your boat has, the bearing inside is identical; only the housing around it changes.

A skeg bearing is a standard cutlass bearing

Inside the skeg sits a grooved (fluted) liner. Seawater flows along the grooves, the spinning propeller shaft rides on a thin film of water — hydrodynamic lubrication — and the grooves flush grit through instead of letting it grind the shaft. No grease, no oil: water is the lubricant. That's a cutlass bearing, whether it lives in a stern tube, a strut, a P-bracket or a skeg. Our what is a cutlass bearing guide covers the mechanism in full, and if you've seen the part spelt "cutless", that's the same bearing too.

The practical upshot: you don't need a special "skeg bearing" product. Any bearing from our marine shaft bearing range that matches your dimensions will fit — cross-reference by size, not by name.

How to size a skeg bearing

Identical to any cutlass bearing — three numbers:

• Shaft diameter (I/D) — measure the propeller shaft with callipers. Your shaft must sit below the bottom of the bearing's published I/D range so it has running clearance.
• Housing bore (O/D) — measure the inside of the skeg's bearing seat. The housing must sit above the top of the O/D range so the bearing holds with an interference fit.
• Overall length — measure the seat end to end. Standard marine proportions put bearing length at four times the shaft diameter.

Never add clearance yourself — Exalto's published tolerances already build in both the running clearance and the press fit. The range covers shaft diameters from 3/4" (20 mm) up to 4" (100 mm) across three part-code systems: fully metric M_SF (e.g. MPSF50), metric shaft in an imperial housing MI_SF (e.g. MIPSF40), and fully imperial I_SF (e.g. IPSF20). Our measuring guide walks through the measurements step by step, and the size chart lists every size we carry.

Which material for a skeg bearing?

Exalto make all four materials in the UK. For skeg, strut and P-bracket applications the choice comes down to your water and your housing:

• Phenolic — the modern default and our biggest seller. Low friction, long life in clean saltwater, excellent vibration damping. It swells slightly when submerged; Exalto's tolerances already account for that.
• Rubber — nitrile with longitudinal water grooves. The pick for sandy or silty estuaries and debris-laden water, because the grooves flush particles through rather than letting them grind the shaft.
• GRP — glass-reinforced polyester shell for direct bond into a fibreglass skeg. Never fit a GRP-shelled bearing into a metal housing.
• Brass-shelled — a phenolic liner in a naval brass shell, for a skeg bore that's worn or slightly oversize and needs a metal shell's grip. Heavier and dearer, but very long-lived.

Replacing a skeg bearing

The check is the same wiggle test as any shaft bearing: with the boat out of the water, grab the shaft at the propeller and try to move it up and down. Perceptible knock or movement at the bearing means it's due. Afloat, the warning signs are a rumble or clunk under power, vibration through the hull, and visible shaft movement when levered at the bearing. Bearings commonly last 5–10+ years depending on engine 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 extracted from the housing (often cut and collapsed first) and the new one pressed in. The good news is that a skeg or strut bearing is often more accessible than one buried deep in a stern tube. Labour varies by yard and by whether the shaft has to come out — the bearing itself is the cheap part of the job. Most owners bundle it with shaft, seal or anode work to share the haul-out cost; if a PSS dripless shaft seal is on that list, our sister store PSS Seals Australia stocks those. For the full process, see our cutlass bearing replacement guide.

Skeg bearing FAQs

Is a skeg bearing the same as a cutlass bearing?

Yes. A skeg bearing is a standard water-lubricated cutlass bearing — the name just describes where it's housed. The same part is called a strut bearing or P-bracket bearing in those housings. Match it by dimensions, not by name.

How do I measure a skeg bearing?

The same three numbers as any cutlass bearing: shaft diameter (I/D), housing bore (O/D) and overall length. Don't add clearance yourself — the published Exalto tolerances already build in running clearance on the shaft and an interference fit in the housing.

What material should I choose for a skeg bearing?

Phenolic is the modern default for clean saltwater. Choose rubber for sandy or silty water, GRP for direct bond into a fibreglass skeg, and brass-shelled where the skeg bore is worn and needs a metal shell's grip.

How do I know my skeg bearing is worn?

Out of the water, grab the shaft at the propeller and try to move it up and down — perceptible knock means the bearing is due. Afloat, listen for a rumble or clunk under power and feel for vibration through the hull.

Skeg bearings in stock in Australia

We hold genuine Exalto bearings in 215 sizes at our Spotswood, Melbourne warehouse. Free Express Post Australia-wide, same-day dispatch for in-stock items ordered before 2pm AEST (anything not in stock ships in 3–4 weeks), and GST is included in every price. Measured up? Find your size on the chart — or email us your three numbers and we'll confirm the exact part code.

Find your size on the chart Email us your measurements