A lot of parts make up the control of noise aboard your boat. We have tackled all these parts at one point or another. The type of noise you are experiencing is usually an indication of where the problem lies. The first we will look at are vibration-type noises.
The vibration type noises are usually related to the drive system and starts with the propeller. (see The Power of the Prop). A bent or damaged prop will definitely cause a vibration issue. If you have eliminated that as a problem you can work forward to the cutlass bearings. Cutlass bearings, being rubber, will wear over time. How much time depends on how much the boat has been used. You can also assume if you do a lot of boating in sandy bottomed waters that grit will get in and start chewing the rubber. Bear in mind that bearings are water lubricated and it may be hard to turn them while the boat is on the hard. That’s not a concern but if you feel the shaft wobble a bit while turning you will need to change the bearings.
After eliminating the bearings as the problem you will want to look at the engine mounts. These also have a limited life span and tend to last longer if you keep your engine in alignment (same goes with the cutlass bearings). Collapsed or partially collapsed mounts will transfer the engine vibration down to the stringers and throughout the boat. On small engines it is not a bad job but as you can imagine the heavier the engine the more work is involved as you need special equipment to get them swapped.
And speaking of engine vibration, you can expect a small amount of vibration from a running engine but there are some conditions that can increase it. Moving items like engine pulleys can be out of line causing an out of balance condition. More often though, bad injectors can be the culprit of an engine vibration problem. Also, although less often, transmissions can contribute to a vibrating engine.
The least likely vibration noise candidate is the shaft and although we have found a couple of bent or worn shafts over the years, they are usually bottom collision related rather than maintenance issues.
Any vibration will be more aggravated if there is equipment that is not mounted tight in the engine room — such as mufflers, water heaters, pumps or anything that is loosely mounted instead of properly guarded against vibration. Always use all the fasteners and rubber grommets that come with equipment when you install it.