Inboard Engine Alignment

engine alignment
Aligning the engine with the shaft is key to eliminating vibration.

If you have an inboard engine on your boat and have excessive vibration, it may be due to the engine not being properly aligned with the shaft. Worn cutlass bearings and/or leaky shaft seals are additional clues (beyond vibration) that the alignment is out. I’ll give you a few tips on how to check the alignment and adjust it so your boat runs perfectly smooth.

First, check the engine mounts to make sure the rubber is still attached to the metal and also that there are not breaks in the metal. Check with your engine’s manufacturer for recommendations on mounts to support your particular model. Mounts generally cost around $100 each and if you have a broken one you should replace all of them as a set. Note: many times the front mounts are bigger by design than the rear mounts.

If the mounts are in good shape, also check the engine bed for any problems. Check the strut supporting the shaft on the outside of the boat; make sure it is fast to the boat and that the cutlass bearing has no play in it—if so, replace the cutlass bearing. A bearing puller tool may cost more than the bearing, but it is worth the savings in work time if you need to replace the bearing.engine coupling

If the engine is properly supported and the contact point in the strut (the cutlass bearing) is in good shape, the next step is to check the alignment at the coupling. The coupling attaches the shaft to the end of the transmission. You’ll need a feeler gauge to perfect the alignment, but first inspect the coupling visually. There are typically four or more bolts attaching the two plates. Ideally, these should be tight and the coupling surfaces parallel. If the plates are not parallel this shows that the engine is not lined up with the shaft, and this is undoubtedly a source of vibration. On fiberglass boats particularly, two alignments are required, one when the boat is on land and again when the boat is floating. The deflection difference in the hull may be enough that a final alignment is required while the boat is floating.

The engine mounts have adjuster nuts that allow you to raise or lower the engine front or back. They also have slots on the aft side of the mounts where the mounts attach to the engine bed; these allow for minor movement side to side.  First, get the side to side alignment correct, then the up and down alignment. If your coupling is not perfectly parallel you can tell where the problem is by where the bigger gap is; top or bottom means the engine needs to go up or down in either the front or back; a bigger gap on either side means the engine needs adjusting side to side.

cutlass bearing
The brass tube with rubber sleeve is the cutlass bearing. If it is worn by a misaligned engine, it can be removed by backing off the set screw and either using a press or by cutting with a hacksaw blade.

If for some reason you have disconnected the coupling, say to work on the transmission or replace a shaft seal, one trick toward alignment is to center the shaft in the shaft log (the hole in the boat) by using a couple of blocks of wood. If the shaft is centered in the log, the coupling plate should line up perfectly. Otherwise go back to the step above to align the engine with the shaft. If the shaft is not centered in the shaft log you can expect to have leaks.

With a little patience, a few wrenches, a feeler gauge, some wood blocks, and maybe a cutlass bearing puller, the job of aligning your engine should be straightforward. Don’t forget to check the alignment again while the boat is in the water. Smooth boating!


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.