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Q&A Engine Alignment

Q: When should you have your engine aligned?

Q: When should you have your engine aligned?

A: Having your alignment checked on a regular basis is a good idea, but sometimes a check is needed because of symptoms you may notice. Various things can cause your engine to become misaligned with the shaft.

Boats, as a rule, “work,” which means the hull moves or flexes. This occurs more in some boats than in others. For example, a well-built, tough small skiff could have much less hull movement than a large, fast motoryacht with many flat panels in its hull and inadequate stringers. A boat that’s run slowly in calm water will probably flex less than the same boat running at a greater speed in rough water. Large ships are usually designed to accommodate hull movement. This isn’t necessarily so with all pleasure boats.

Obviously a poorly designed and/or poorly built boat will have a greater tendency to flex, but most hulls flex to some extent even if well-built. This can occur when the boat is being pushed along by its engine, when it encounters waves, when it’s hauled (particularly with only two slings on a long, heavy hull), when it’s being blocked up, when it’s being put back in the water, and from other causes.

Some boats have inadequate backing for the propeller strut. As the boat runs and the strut absorbs thrust and torque, the hull will flex a bit in that area, making the strut deflect the shaft. Any shifting of the hull, particularly around the engine and strut, can cause misalignment of the engine with the shaft.

The engine mounts also can cause misalignment. Usually they include rubber or some other flexible material to absorb vibration. If the vibration-absorbing material degrades, the engine can become cocked. This also can happen if the engine wasn’t mounted properly in the first place. Each time you run the engine its force is pushing the entire boat through the water against the engine mounts and their bolts.

Sometimes the shaft itself is out of alignment because the strut is cocked. If there are more than two bearings for the shaft, these may be out of alignment with each other, thus deflecting the shaft. In some instances the shaft could have become slightly bent, causing a problem no matter how well your engine is aligned.

As you can see, many things can cause your engine to become out of alignment with the shaft. When this occurs you may notice unusual vibration, excessive movement of the engine mounts, whipping of the shaft, movement or excessive leaking of the shaft seal, movement of the engine and other symptoms.

It’s what you don’t notice that may be the worst problem. A misalignment can cause damage to your transmission, engine, engine mounts, shaft bearings and other components. When you stop to think that the power of the engine is being transferred through the rotation of the shaft and propeller in order to move the boat, and then consider that rather than a “straight run” it’s being deflected to the side, you can begin to imagine the possible consequences of misalignment.

In theory an alignment check is easy to do, though not in practice. Many think that all you have to do is disconnect the coupling between the transmission and the propeller shaft, back the shaft off just slightly, then check the distances between the flanges on the couplings with a feeler gauge. In theory the distances will be the same, within a certain tolerance, if the engine is in proper alignment. The tolerance accepted will vary from mechanic to mechanic, and with different engines and installations and shaft diameters.

This sounds easy enough, but there is far more to a good alignment job. There are issues that you might not expect. For example, the weight of the shaft and the coupling will deflect or bend the shaft. The weight of the prop also may affect what you are seeing inside the boat, particularly if you have only one cutless bearing at the point of exit through the hull. All of this will depend upon the weight and diameter of the shaft, the weight of the prop and the coupling, and the distance of the shaft run from its support(s).

Also, as noted above, if components such as the bearings or strut are out of alignment, your feeler gauge measurements may not be giving the true story. In addition, most couplings have a lip that can create a false impression of alignment. An experienced person will be able to use various methods to detect misalignment and correct it. To the inexperienced this can be a formidable problem.

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