How to Check Shaft Alignment in a Boat

Updated July 20, 2017

Marine engine alignment is extremely important to the performance and longevity of any boat configured with an inboard engine. Whether it's a sailboat, a ski boat with a small V8, or a cruiser with a large diesel engine, proper alignment will ensure the bearings in the strut and transmission will last, and the prop shaft packing will stay sealed. To keep your boat running well, the propeller shaft alignment should be checked every year as part of the boat's maintenance, and adjusted as needed.

Access the rear of the boat's engine compartment. Open enough access that the transmission and shaft are within easy reach. Look for the shaft coupler at the rear of the transmission. Clean off any rust or grease with a wire brush and rags.

Remove the bolts connecting the two halves of the coupler. Use the pipe wrench to hold the shaft if necessary. Separate the two halves of the coupler as much as space allows and use a wire brush to clean the face of each half. Inspect the face of the shaft coupler to identify the ridge on one half and the recess on the other. Ensure these surfaces are clean.

Bring the two halves of the coupler back together until the ridge is sitting in to the recess. Bring the two halves lightly against it so that a .015 inch feeler gauge can be slid in and out of the space with little force. Use the feeler gauge set to measure the gap between the two halves in four at 90 degrees apart. Ensure that these four gaps are equal. There should be no more than a .004-inch difference in the gap from one point to the point 180 degrees opposite.

Loosen and adjust the motor mounts on the front of the engine and on the transmission to bring the gap into specifications. Raise or lower the engine, or shift it from left to right as necessary to line up the shaft and transmission correctly. For example, if the gap is too large at the top of the shaft, lifting the front of the engine will bring it back in to specifications.

Tighten all engine mounting bolts, then measure the gap again to ensure nothing has changed while securing the mounting hardware. Reinstall the coupler bolts and tighten securely. Start the engine and check for vibrations and noise while in gear and at idle.


Replace coupler bolts with new hardware if they show excessive wear or are overly rusted. Ensure the replacements are the same grade of bolt and nut. Lightly coat the coupler faces with grease to prevent rust before reinstalling the bolts.


Engine alignment is extremely important. A poorly aligned engine can lead to excessive shaft packing wear, transmission failure and strut bearing failure. If you are not comfortable with the results of your alignment, get assistance from a professional. Alignment should be done while the boat is in the water to eliminate misreadings caused by hull twist and flex.

Things You'll Need

  • Socket and wrench set
  • Wire brush
  • Pipe wrench
  • Feeler gauges
  • Rags
Cite this Article A tool to create a citation to reference this article Cite this Article

About the Author