"Mid Size Power Boats": A Guide for Discreminating Buyers - by David Pascoe

Troubleshooting Problems Involving
Engine/Shaft Alignment

by David H. Pascoe, Marine Surveyor

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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.


    Page One
  • List of Problems
  • Reason why drive systems do not stay in line

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 indefinitely 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.

Photo #2

This type of mount is fine for gas engines but not for heavy diesels. This mount, on a brand new 45 footer, is badly distorted. Note how it is cocked toward the engine and forward. In this case, the mounts allowed the engine to move so much that the shaft coupling came apart.

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 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. Obviously, then, the engine mounts should be well-designed and strong. Unfortunately, more often than not the builder has selected the most inexpensive mounts that he can buy, mounts that are not going to do a very good job of keeping the system in alignment.

In fact, the most commonly used engine mounts for gas and small diesels is a mount that does a very poor job of maintaining alignment. Shown in the photo above, this mount not only permits the engine to rock forward and back, but up and down and laterally also. It literally allows the engine to move like a hobby horse.

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.

This is also why small boats can get away with a great deal of misalignment, and why they can get away with using cheap, inferior mounts. A shaft that is only ten feet long can tolerate ten times the amount of misalignment of a shaft that is twenty feet long. But by the time you get up to a forty-footer with a 15' shaft, the amount of movement and misalignment in the system that it can tolerate without causing damage declines dramatically.

At this point, the thickness and mass of both the shaft, propeller and the drive load all begins to play an increasingly important role. The reason is simply because small diameter shafts are much more elastic than large diameter shafts. All boat shafting systems are self-centering to some degree, but the larger the diameter shaft, the less self-centering it becomes. The shaft does not bend but transfers all that misdirected energy (vibration) to other components such as struts, bearings, transmission and the engine mounts. Continued to Page Two

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First posted on April 19, 1997 at David Pascoe's site www.yachtsurvey.com.
Page design changed for this site.

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About Author:
David H. Pascoe is a marine surveyor (retired) with 40 years' experience.

He is author and publisher of power boat books:

"Mid Size Power Boats"
"Surveying Fiberglass Power Boats" 2E
"Buyers' Guide to Outboard Boats"
"Marine Investigations"

Visit  yachtsurvey.com  for more than 160 online articles.

David Pascoe's biography

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