Prop Shaft Alignment

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“Aligning the propeller shaft” is attempting to put the propeller shaft on "exactly" the same axis as the gear box output.

Here's a short list of the problems that can be caused by
engine/shaft alignment faults:
* Increased fuel consumption.
* Rapid cutlass bearing wear.
* Misaligned strut galls shaft, requiring shaft replacement.
* Stuffing boxes wear out and leak, not infrequently
sinking the boat.
* Bent or broken shafts.
* Drive system vibration can damage engine mounts and
the boat hull itself.
* Vibration causes damage to other systems.
* 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, flooding or sinking the boat.
* Shaft log failure, causing leaking and possible sinking.

When a mismatch between the engine's transmission flange and the propeller-shaft flange is as illustrated below, the engine must be raised or lowered by adjusting the jack nuts on all four engine mounts.

A mismatch, such as the illustration below, between the two flanges indicates that the engine's centerline is not parallel with the propeller shaft's centerline. In this case, one end of the engine must be raised or lowered.

Checking the Shaft Alignment

1. If the vessel is hauled out, check the position of the
shafts relative to the bearings. Cutlass bearings that are
worn more on one side than another are a positive
indicator that something is out of alignment.

2. Next, check the shaft-to-bearing alignment at both
the front and back ends of the bearing. If the shaft is
off-centered, either to one side or top to bottom, then
the shaft is not parallel with the bearing bore. In this
case, the entire system alignment must be redone,
including makeing sure that
the struts themselves are properly aligned.

3. Now check how the shaft is centered with the
opening in the stuffing box flange.
The shaft should be exactly
centered, but if there is a lot of clearance, it can be off a little
without causing harm.

4. When the vessel is in the water, the first thing to
do is check the engine mounts. If they are the
vertical stud type set in a rubber base, check the stud
to make sure that it is centered in the base with the
vessel at rest. If it is leaning in any direction, the mount
is stressed and the system is out of alignment.

5. Finally, check the shaft alignment by disconnecting the coupling
and checking the flange clearance with a feeler gauge (see the diagram below).

When your boat is hauled out for other work, like an AMS bottom paint job, it is a good time to check all the components of your running gear for wear or damage. With an AMS bottom paint job, we always provide a free bottom inspection which includes inspection of your running gear.

Sometimes we discover that there is excessive galvanic corrosion damage to the running gear (prop, shaft, or strut) or that your strut bearing is in need of repair.

Under these circumstances, it is recommended that the prop, prop shaft, and coupling be removed. Then, the prop shaft and coupling be sent to the machine shop so the shaft can be checked for corrosion damage and trueness and the shaft and coupling can be fitted and faced. It is also recommended that old coupling bolts be replaced with new ones and the prop be sent to the prop shop for checking.