Check the Cutlass Bearing

Posted: December 1, 2011

When it’s time to replace it, you should know how.

By: Deane Hislop

Sea water is the sole lubricant for your boat’s prop shaft as it spins thousands of rpm when you’re under way. The shaft is supported typically at its inboard end by a coupling that is attached to the transmission; its other end is held in place by a fluted rubber tube referred to as a cutlass bearing. This bearing can be found, depending on the boat’s design, either within a shaft log that passes through the hull or in a strut some distance from the keel.

The cutlass bearing is usually made of a hard, rubberlike substance known as nitrile. Longitudinal grooves in the rubber tube are designed to improve water flow and to flush away any grit or other hard material that might wear on the shaft. The outer shell of the bearing may be made of naval brass or a nonmetallic composite. Though durable, these bearings do wear out and must be replaced to avoid vibration or worse: shaft log or strut damage.

Indications of a bad bearing are rumbling or a vibration that slowly increases over time, a shaft that can be moved when lifted, or an out-of-tolerance cap between the shaft and the bearing surface (a 1¼-inch shaft should have clearance of .003 to .007 inches between the shaft and bearing surface, and a 1½-inch shaft’s clearance should be between .004 and .009 inches).

Removing a cutlass bearing can be challenging, particularly when it’s installed in a shaft log rather than in a freestanding strut, but it’s not impossible. Start by removing the prop shaft.

How To:

Replace a cutlass bearing

1. First, remove any setscrews or locking nuts that may be used to secure the shaft to the coupling at the transmission. Separate the coupling from the transmission, and then back off the locking nut located inside the coupling and remove the coupling. A shaft puller or a sliding hammer makes for easy shaft removal.

2. With the shaft out, it’s time to cut the cutlass bearing. Remove the setscrews that hold it in place. Then use a hacksaw to cut into the nitrile bearing material at one of the grooves opposite the setscrew. Continue cutting into the bearing shell until it’s paper thin, but don’t cut into the strut or shaft log. At this point, simply insert some hex-head bolts into the setscrew holes, tighten them evenly and collapse the cutlass inward, bending the bearing enough to grab it with ­pliers or vise-grips. Twisting in the direction of the bend and pulling will usually extract the bearing.

3. Next, clean the bearing mating surface with 320-grit wet sand­paper so you don’t damage or remove too much surface area. 

4. To install a new bearing in a strut, use a length of threaded rod that’s more than twice the length of the bearing and two thick washers that are larger than the diameter of the bearing and have holes that are just larger than the rod. Put the rod through the bearing, and use the washers as caps on either side. A little liquid soap on the outside of the bearing will help as you draw the washers together using a nut on either end.

5. For a shaft log, drive the bearing into place using a block of soft wood and a mallet. Finally, reinstall the bearing setscrews using blue Loctite thread-locking compound, and you’re done.