Marine Bearings

The bearing at the after end of the stern tube carries the weight of the shaft and the propeller hanging off it, in seawater, with no grease and no attention for years at a time. DH Porter stock cutless and composite bearings in standard diameters from 3/4” through to 4”, and machine odd and metric sizes to order in our North Parramatta workshop.

Cutless is the original trade name and the one we use here. You will also see the same bearing written as a cutlass bearing, a stern tube bearing or a propeller shaft bearing.

How a water-lubricated bearing works

A cutless bearing does not run metal on metal. A nitrile rubber liner is bonded inside a rigid shell, and the bore of that liner is grooved along its length. As the shaft turns it draws water through those grooves, and the shaft lifts onto a thin film of water rather than riding directly on the rubber.

Two useful things follow. The bearing cools itself, because the water carrying the load also carries away the heat. And it clears itself: the sand and grit that would otherwise embed in the liner and score the shaft get flushed out along the grooves.

Both depend on water actually moving through the bearing. A bearing starved of water will run hot, glaze and wear out quickly no matter what it is made of.

Cutless or composite

We hold two families of bearing, and they solve the problem in different ways.

Cutless: a rubber liner in a rigid shell

• Naval brass shell (imperial and metric): the strongest shell we stock, and the one to reach for where a housing is worn, oversized or has been bored more than once. Naval brass carries tin in the alloy specifically to resist dezincification, and it machines and holds a press fit accurately. Suits bronze housings and traditional stern gear.
• Phenolic shell (imperial and metric): fabric-reinforced resin, non-metallic, and the sensible default for most replacements. It takes up a little water in service and swells slightly, which is why phenolic shells are bored with a touch more clearance than brass. That swelling works in your favour once fitted, since the press fit tightens rather than slackens.

Composite: solid material, no separate liner

• Fibre: a resin-bonded composite bearing with no metal in it at all. Tolerant of poor water quality and completely inert galvanically, which makes it a straightforward answer in steel and aluminium stern tubes.
• Railko: a woven fabric and thermoset resin composite with a solid lubricant through it. Hard-wearing under high bearing loads and in silty or abrasive water, which is why it turns up so often on commercial and workboat shafting rather than on yachts.

One rule worth remembering about brass

Do not fit a brass-shelled bearing into a steel or aluminium stern tube. Brass and steel, and brass and aluminium, sit a long way apart on the galvanic scale, and in seawater the less noble metal is the one that corrodes. That is your stern tube, not the bearing. In a steel or aluminium tube, use a phenolic shell or a composite bearing, all of which are electrically inert and sidestep the problem entirely.

Getting the size right

Three measurements identify a cutless bearing, and all three have to be right:

• Shaft diameter: the bore of the bearing, and the shaft it will carry
• Housing bore: the outside diameter of the shell, and the stern tube or bearing housing it presses into
• Overall length: usually about four times the shaft diameter, though the housing decides it

Imperial bearings step in 1/8” increments, metric bearings in 5mm. They are not interchangeable, and a metric bearing is not a rounded-off imperial one. If you are unsure which you have, measure the shaft rather than assume, or bring the old bearing in and we will match it.

When a bearing needs replacing

A worn bearing usually announces itself before it does any harm:

• Noticeable play when the propeller or shaft is levered by hand with the vessel out of the water
• Vibration that comes on with shaft speed and was not there last season
• A rumble or growl through the hull under way
• Squealing at low revs, which normally means the water film has broken down

Some play is normal in a new bearing, because the shaft is meant to sit on water rather than on rubber. It is a change from what the boat did before that matters.

What shortens bearing life

Most bearings that fail early were not worn out, they were prevented from working:

• Water starvation: blocked grooves, a fouled or painted-over intake, or an anode clamped hard against the forward face of the bearing
• Misalignment: an engine or shaft out of line loads one edge of the bearing and wears it into a taper
• The wrong shell: a brass shell in a steel or aluminium tube, which corrodes the housing rather than the bearing
• A worn shaft: a bearing pressed onto a shaft that is already scored or undersize will not last, and the shaft is the thing to fix first

If a bearing has worn unevenly or gone through more than one in quick succession, the bearing is the symptom. Shaft servicing and alignment is where that gets sorted.

Odd sizes, housings and stern tubes

Being a machine shop rather than only a supplier means we are not limited to what comes in a box. Metric and non-standard sizes are machined to order, worn or oversized housings can be bored and sleeved to take a standard bearing, and complete stern tubes and bearing housings can be made from scratch or matched to existing hardware. New shafting can be supplied cut, tapered and keyed to suit at the same time.

Ring the workshop on (02) 9890 1766 with your shaft diameter, housing bore and stern tube material, and we will tell you what is on the shelf and what needs to be made.