Test-Drive Trivia: Knock, Knock?

Mercruiser U-Joint
These are new MerCruiser universal joints. When U-joints get worn they can cause a thumping, knocking sound when stern drives are turned.

If you’re ever test-driving a used stern-drive boat and you hear a knocking or thumping sound coming from the transom, but only when the drive is turned sharply in either direction, it might pay — in purchase savings — to have some understanding about what might be causing that thumping sound.

You might think the motor mounts are loose or the drive is misaligned with the engine, and those are possible culprits — but the noise might also be also be coming from either the drive coupler or the universal joints in the drive. Worn U-joints can stem from not being lubricated properly, drive coupler failure, and poor engine alignment. The problem seems to be more common with Alpha drives compared to Bravo drives.

If the engine mounts become loose and are not corrected, it can eventually lead to a worn coupler and bad U-joints.

It would definitely be worth your while to pull the drive and have the alignment checked.  The factory alignment tool should slide in and out of the coupler easily.  Be sure to check the alignment with the engine rotated at different positions to check the coupler itself.  If the alignment tool does not slide in at one engine rotation position, but does at another, the coupler is either bad or not installed on the flywheel properly.  While the drive is off, check and service the universal joints.  If one of the U-joints is worn, it could also be a cause of the noise.

Whatever it is, any of these problems can be repaired without too much trouble or expense, so you might be able to use your knowledge to get a better deal.