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.
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.
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!