Defect of the Week: Cutless Bearings

On the evening of 30 October 2017, about six miles off South Padre Island in Texas, the shrimp trawler Ben & Casey began taking water through her shaft seal. The propeller shaft had sheared. The Coast Guard came out and helped the crew pump, the flooding outran them, and the crew stepped across to the Coast Guard boat. She sank at about a quarter past ten. Nobody was hurt. The investigators noted that a scallop dredger, Lady Gertrude, had sunk off New Jersey the year before in a similar way, with the shaft fractured forward of its stuffing box.¹

Both were working boats, much bigger than the yachts and narrowboats on a surveyor's autumn list. The lesson applies to any shaft on the hard. Scoring where a shaft runs in a bearing or under a seal can become the starting point of a fatigue crack, corrosion of the shaft can hurry it along, and a visual inspection has real limits when it comes to finding one. Haul-out is the one time most of the sterngear can be seen, measured and recorded.

One Load Path, Seven Things to Grade

"Sterngear in good order" covers a lot of hardware. Thrust travels from the propeller, along the shaft, through the coupling to the gearbox, and the shaft passes through the hull at a seal that keeps the sea out. The parts fail in different ways, so the record grades them one by one.

On a long-shaft installation the shaft leaves the hull through a stern tube and runs in a water-lubricated rubber bearing, usually in a P-bracket (or an A-bracket) aft. On long-keeled yachts and many motor cruisers the bearing sits instead in the outer end of the stern tube, where the shaft leaves the keel or deadwood, and many boats carry one in both places. The rubber bearing is commonly called a cutless bearing, often spelled "cutlass". Cutless is in fact a trade name, held today by Duramax Marine, which became the word for the type.² Saildrives and sterndrives have no shaft of this kind and are graded differently; the saildrive's own weak point comes up below.

Play Has a Direction

The standard test at haul-out is to take hold of the propeller and move it. Movement at the propeller has a direction, and the direction usually points at the worn part.

Radial movement (lift and push the propeller sideways) belongs to the bearing end. A worn cutless bearing lets the shaft nod in it. If the bracket itself moves, the problem is at its root, in the laminate or the bolts, and that is a structural finding. Turn the shaft by hand too, with a dial gauge or fixed pointer on the shaft as close to the propeller as it can be placed, and again midway between the hull and the bracket. Run-out there suggests a bent shaft, which no new bearing will cure; it is confirmed with the shaft out. Check separately that each blade tip passes the same mark: a single bent blade is a propeller finding, and it can look like a bent shaft to anyone watching only the tips.

Axial movement (push and pull the propeller fore and aft) belongs at the other end. A properly installed shaft has almost none beyond any end float the coupling or thrust bearing maker allows. Movement here means the coupling is slipping on the shaft, the gearbox output bearing is worn, or, on installations with a thrust block or a flexible drive coupling, that component is tired. The coupling deserves particular care. If a coupling lets go of its shaft, propeller thrust going astern draws the shaft aft and out of the boat, and whether the rudder stops it short of clearing the stern tube depends on the installation. Check that the coupling is keyed, that its set screws are present and locked or wired, and that its bolts are tight. Then check the gap between the propeller and the rudder blade and note it.

Wherever possible, put a number on it. Movement at the propeller mixes bearing clearance with bracket flex and overhang, so measure at the bearing itself: with the shaft resting on the bottom of the bearing, a strip of plastic feeler gauge slid in at the top between the shaft and the rubber ridges gives a clearance that can be written down. Check too that the bearing is still held by its grub screws and has not walked aft out of its housing. There is no single published replacement clearance for small-craft bearings, and the figures in circulation vary by maker and shaft size. A practical rule from one of the maker's distributors is to renew the bearing once its water grooves are about half worn away.³ A measured clearance and a note of the groove depth make a record that a second surveyor could repeat next year.

The Shaft Under the Bearing

Once the bearing is worn, look harder at the shaft. The NTSB noted that scoring at bearings and seals can become a site for fatigue cracks.¹ On a stainless shaft there is a second mechanism: crevice corrosion, where seawater sits still and starved of oxygen under a bearing or a seal through a long lay-up. It is the same mechanism our keel integrity post describes at keel bolts. A shaft can only be seen where it is exposed, so record the exposed lengths, and note that the lengths inside the bearing and the stern tube were not seen unless the shaft was drawn. Where scoring or pitting shows at the bearing, seal or rotor journals, recommend dye-penetrant testing of those areas, which finds what the eye can miss. If the shaft is drawn when the bearing is renewed, as it often is, its whole length can be tested.

Grading the Seal

Seals come in three broad families, and each one is graded on its own terms.

• The packed gland (stuffing box) is meant to drip a few drops a minute while the shaft turns, and not at all when it stops. Around ten drops a minute, or any drip with the shaft still, is the usual cue for adjustment or repacking.⁴ Drip rates can only be seen afloat, before the lift or on a sea trial. Ashore, the packing can be examined only for age, weeping, the condition of the adjusting nut and locknut, and how much adjustment is left on the nut (one taken up almost to its end means repacking is due), so record what was observed and what was not.
• Face seals, with a carbon ring running against a stainless rotor and held by a rubber bellows, are drip-free. The bellows is the part that ages, and the maker publishes a replacement interval for it. Check the bellows is still under compression and that the rotor's set screws are present, doubled and tight, because a rotor that slips lets the seal open. Record the installation date, and if nobody knows it, the record says so.
• Lip seals are drip-free too, with a maker's replacement interval and, on some, a greasing schedule. Face and lip seals generally need burping of trapped air after each launch unless a vent or water feed is fitted. Record the type, and that it needs burping after launch.

In every case the short hose that joins the seal to the stern tube on many yachts is a below-waterline hose, and it is graded like one: the grade of hose the seal or gland maker specifies, double-clamped with all-stainless clamps, and supple, as in our through-hull seacocks entry.

Saildrives follow the same logic. The rubber diaphragm that seals the leg to the hull carries a manufacturer's replacement interval (Volvo Penta's is seven years) and the boat's records should show when it was last changed.⁵

Propeller, Bracket and Anodes

What is sold as "manganese bronze" is a high-tensile brass. It has the zinc content of a brass, and it needs cathodic protection to resist dezincification in seawater.⁶ The tell is a pink, coppery tinge on the propeller or the bracket, and a dull note in place of a ring when a blade is tapped. Dezincified metal keeps its shape and loses its strength. Dezincification in a P-bracket matters more than in a propeller, because a bracket that lets go allows the shaft to whip and takes the seal with it. Record the colour of the propeller as well as its edges, any bent or chipped blade, cavitation erosion, the hinge pins and gears of a folding or feathering propeller, and whether the propeller nut is locked by a split pin or tab washer.

The shaft anode should be tight on the shaft and sit clear of the bracket and the bearing housing. Record its condition as a percentage remaining, and whether its metal suits the water: zinc or aluminium in salt water, magnesium in fresh. If a rope cutter is fitted, note the blades and how it is fixed.

Shaft alignment cannot be confirmed with the boat ashore, because the hull changes shape when it floats. Final alignment is checked afloat, across the coupling faces with feeler gauges.⁷ Ashore, the record notes the engine mounts and any evidence of misalignment, such as a bearing worn on one side only (other than the normal wear on the bottom), and says plainly that alignment was not verified.

The Narrowboat Stern

A narrowboat's sterngear is simpler and just as easy to underrate. The shaft runs through a stern tube welded through the swim, usually sealed by a packed gland that is fed grease from a screw-down greaser, though some boats now carry a dripless seal. The usual advice to owners is a turn of the greaser at the end of each cruising day.⁸ An empty greaser, or a gland that has clearly not been fed, is worth noting, and so is standing water in the engine-bay bilge under it, though rain through the deck boards is a common source too. Inspect the weld where the stern tube enters the hull, where corrosion tends to start. Wasted plate around the stern is a job for the ultrasonic gauge, as set out in our steel-hull thickness post.

The propeller sits in its aperture above the skeg, and the rudder hangs from a stock through the counter with its heel located on the skeg. Check the stock and top bearing for play, the heel pin and its cup for wear, and the skeg for distortion after years of grounding on the cut. Then the weed hatch. It sits only a little above the waterline at rest, and under way the water in it can rise over the lid, so the lid, its clamp and its seal are graded with the same seriousness as a seacock. The wider inland picture is in our narrowboat survey primer.

Classification: A, B or C

The test is the one that runs through the Category A, B and C system: does the defect endanger the vessel or the people aboard if she is used?

Category A. A dripless seal that leaks at rest, any seal leaking beyond what adjustment can stop, or a bellows that is cracked or perished; rotor set screws loose or missing; a coupling that is loose on the shaft or missing its locking; a bracket that moves at its root, shows cracking there, or is extensively dezincified; a shaft that is cracked, or bent enough to see against a fixed pointer; a rudder stock cracked or bent, or a heel fitting insecure; a weed hatch whose lid or seal will not make a watertight closure. Each one can let the sea in, lose the shaft, or leave the boat without steering. Rectify before the vessel is used. Leaks at rest are graded where they were seen afloat.

Category B. A cutless bearing with its grooves about half worn, or play enough to knock when the propeller is shaken; any dripless seal or saildrive diaphragm past the maker's replacement interval, or of unknown age; scoring or pitting on the exposed shaft; a slight bend measurable only on a dial gauge; pink dezincification on a propeller; a packed gland dripping at rest or beyond the servicing rate (where seen afloat), or with little adjustment left; a propeller nut without its locking; rudder bearing or heel-pin play that can be felt as a knock; an anode more than half consumed. Defined action within a set period; for sterngear this is often most economical at this lift-out, while the boat is ashore.

Category C. Small nicks at blade edges, an anode partly used, an empty greaser, dull rope cutter blades. Monitor and maintain.

As with the battery and shore power entry, the temptation is to grade down because the boat motored to the hoist without complaint. Wear at the stern builds quietly between lift-outs, so grade what was found on the day.

Documenting It

Inadequate: "Sterngear inspected, satisfactory."

Professional: "Category B. Cutless bearing, P-bracket (aft). 25mm shaft. Feeler-gauge clearance 0.6mm at the top of the aft end; movement at the propeller approximately 1.5mm; water grooves about half worn, wear on the lower side as expected; bearing retained. No axial movement; no shaft run-out seen against a pointer. Bracket secure at the root, no cracking seen. Exposed shaft clean with light scoring at the bearing entry; shaft within the bearing and stern tube not seen. Recommend the bearing be renewed at this lift-out, and the scored area dye-penetrant tested, along the whole shaft if it is drawn. Face seal fitted; installation date unknown, so bellows age unknown; recommend the bellows be renewed unless the date can be established. Alignment not verified: vessel ashore. Photographs attached."

The second record names the part, the direction and size of the movement, the evidence, what was not seen and why, and the action. Our post on what makes a survey report defensible explains why the "not seen" clauses carry as much weight as the findings.

Marine Inspect's survey templates carry checklist items for the stern: on the sailing yacht template, the shaft seal and cutless bearing, the propeller, the anodes and the rudder; on the narrowboat template, the stern gear, the weed hatch, and the rudder and skeg. The surveyor's measurement, grade and photographs go against the item at the stern, by voice or by hand, and carry into the report. The sample reports show how that reads: the Calypso Dawn yacht sample and the Meadow Lark narrowboat sample.

Read next in the series: Defect of the Week: Through-Hull Seacocks, Defect of the Week: Battery and Shore Power, and Defect of the Week: Chainplate Failure.