When it comes to yacht maintenance, one of the least understood and most overlooked systems that require maintenance, engine and shaft alignment probably heads the list. Here's a short list of the problems associated with incorrect alignment and poor or worn out engine mounts:
Rapid bearing wear.
Causes stuffing boxes to wear out and leak, not infrequently sinking the boat.
Bent or broken shafts
Drive system vibration that can damage transmissions, engine mounts and the boat hull itself.
Transmission failure caused by increased stress on the rear output shaft bearings and gears.
Loosening of struts, causing leaking and possible sinking.
Oscillating propeller shaft causing stuffing box clamps to loosen and work free, usually sinking the boat.
Wear or worn out engine mounts cause drive shaft misalignment to stern drive, causing universal joints to oscillate and wear out.
These are but a few of the damages that can be caused by basic engine/shaft misalignment. Now comes a discussion of why, unlike in an automotive applications, boat engine drive systems do not indefinetly remain in alignment and need to be periodically checked and realigned. People tend to think of boats and yachts as being rigid structures, but they're not. The fact is that boats bend and twist in all directions, albeit not to any degree that's visually detectable. Expecially fiberglass boats, because they are a thermosetting plastic, can change their shape over time. This can happen either as a result of improper hauling and shoring, or it can happen simply as a factor of age and the effects of gravity.
However, the primary reason why drive systems do not stay in line indefinitely is due to wear and tear on all the components that make up the system. Stress, vibration, friction, slamming and pounding are all factors that contribute the the changing size and shape of all physical objects. Foremost among these are the engine mounts. Remember that it is the engine mounts that carry nearly all of the thrust load of the propeller and transfers it to the hull.
A basic rule in engine alignment system is that that larger the vessel, the longer and larger diameter the shaft, the more critical alignment is. That's because any misalignment becomes magnified over the length of the shaft; the longer the shaft, the greater the potential error. A shaft that's centered at the strut bearing can easily be off by several inches with only a change in a degree or two of angle at the engine. This factor is what accounts for why shaft misalignment can cause so much damage on any shafting system of considerable length.
Weak Struts This is a very common problem on small to midsize boats. If you want to understand just how important strong struts are, just take a look at the huge struts used on larger Bertrams, Hatteras or Viking yachts. They don't spend all that money on massive struts for no reason. When there are hundreds of thousands of dollars at stake in machinery, the drive support system must be strong.
Another general rule is that the larger the shaft diameter and the more bearings it has, the more critical are the engine mounts. Unless the mounts are holding in the engine in place and not permitting any movement, there's no point in trying to achieve proper alignment of anything. If the engine moves, no other part of the system will stay in alignment either.