How to Check the Cutless Bearing, Prop Shaft and Strut

Before you touch anything, write down what you feel on your next run. BoatUS's vibration guide sorts causes by when the vibration appears:

• Even in neutral: usually the engine itself or worn engine mounts, not the shaft.
• Only in gear: look at the prop first (fouled, bent or badly fitted).
• Gets worse at certain speeds: prop out of balance or a worn cutless bearing.
• Deep rumbling felt through the hull aft: a classic sign of a worn bearing (Practical Boat Owner).

A prop with rope, fishing line or weed on it, or a dinged blade, causes most stern vibration. BoatUS notes props are often the culprit. If a diver is available, have them check and clear it. Otherwise check at haul-out (see the BoatMend propeller guide).

Engine off, key out, transmission in neutral. Try turning the shaft by hand inside the boat, or have someone turn it. Cruising World's quick check: if the shaft takes a lot of effort to turn while the boat is afloat, something is binding, usually alignment, a bent shaft or a tight bearing. Note it for the pro.

Put a paint mark or a tape flag on the shaft. With the engine off, turn the shaft slowly by hand while a helper watches the shaft and coupling from the side. BoatUS suggests watching from several angles: a shaft that visibly wobbles or a coupling that "nods" can mean a bent shaft or a coupling that is out of true.

With the engine off, run a finger around the shaft where it enters the stuffing box or dripless seal. BoatUS points out that a groove or ridge you can feel means a worn shaft, which can cause shake and also stops the seal from sealing. Note any extra leaking at the seal too: a sloppy bearing lets the shaft move and works the seal.

With the boat on the hard, grab the prop with one hand and the shaft right next to the bearing with the other. Push and pull hard up and down, then side to side.

• Any clunk or visible movement between the shaft and the bearing: the bearing is worn and should be replaced (BoatUS).
• Fore-and-aft movement: there should be none. If the shaft slides in and out, BoatUS says suspect a loose prop, a loose shaft coupling or wear in the transmission. Have a mechanic check before launching.

Shine a light into the gap between the shaft and the bearing at each end. The rubber liner has lengthwise grooves. A cutless bearing distributor's rule is to replace it when those grooves are roughly half worn away. Also look for rubber that is torn, swollen, peeling from its shell, or pushed out of the housing (a line wrap can do that).

Check the strut for cracks, especially where the leg meets the hub and the base. Look for twisting or a bent leg, and for corrosion on bronze. Inside the boat, find the strut's through-bolts and check for weeping, cracked gelcoat or movement around them. BoatUS lists a cracked or twisted strut, or loose mounting bolts, as a cause of vibration that needs a repair or replacement by a yard.

Shaft zincs should be tight on the shaft and not in contact with the bearing housing. Replace them if they are half gone (see the BoatMend anodes guide). If you had the prop off, BoatUS also suggests checking the key, keyway and taper for damage, since a poorly seated prop vibrates.

The faces of the transmission coupling and the shaft coupling must be centered and parallel. Cruising World estimates more than half of the boats a yard manager sees have some misalignment, which causes vibration, early cutless bearing wear, transmission bearing failure and even broken shafts. A pro separates the couplings, measures the gap all the way around with feeler gauges against a published tolerance, and adjusts the engine mounts. BoatUS's quick test: with the shaft uncoupled from the transmission, if it suddenly turns easily, alignment is the likely problem. Final alignment is always done with the boat afloat, after it has settled for a while.

Look at each mount for torn or sagging rubber, rust or rubber dust under it. Worn mounts let the engine drop out of line with the shaft, so the alignment goes off again even after it is set. BoatUS lists worn mounts as a common cause of vibration both in neutral and in gear.

Replacing a cutless bearing is a haul-out job. The prop has to come off, and on a strut the shaft usually has to be pulled, which can mean dropping the rudder too. Old bearings are often stuck and need a puller or careful cutting. Then alignment has to be checked afterward. Practical Boat Owner calls it doable for an owner with mechanical experience; for most owners it is a yard job. Order the bearing by shaft diameter, bearing outside diameter and length, and confirm them by measuring.

Bearings also come with different outer shells, typically brass or non-metallic, depending on the maker's line. Some housings use a flanged bearing that bolts to the strut or stern tube; Pacific Marine's ordering notes ask for the flange diameter, thickness and hole pattern in that case. Match what came out of your boat unless your yard recommends a change, for example where the old brass shell corroded.

This bearing is cooled and lubricated by the water around it, so never run the shaft in gear with the boat on the hard.