How to Troubleshoot a Marine Propeller Shaft

Inspecting the propeller shaft is part of a regular maintenance schedule.

Marine propellers and shafts take a constant beating when subjected to rough seas, constant acceleration and deceleration, load and water chemicals. Besides the engine, the propeller performs most of the work, pushing a vessel through the water. A propeller shaft that goes out of balance, cracks or deforms in any way should be inspected and replaced. Inspecting propeller shafts for operational condition should be a regular service ritual, whether the boat sits at the dock or makes frequent long-distance cruises. Inspecting your propeller shaft can alert you to problems that might lie ahead.

Things You'll Need

• Owner's manual
• Dial indicator (if applicable)
• Pointed dowel
• Straight edge ruler
• Socket set
• Ratchet wrench
• Degreaser
• Shop light
• Magnifying glass

Instructions

1. Place a pointed dowel along the vertical edge of the rudder line. Extend the pointed end of the dowel down to the center of the machined end of the propeller shaft. Rotate the shaft several revolutions and watch for any perceptible wobble in the shaft. Any deviation will indicate a bent shaft or a propeller bore that has become cocked or out of alignment. If you have use of a dial indicator, use it to read any off-angle wobble on the propeller shaft.

2. Make certain the cutlass shaft bearing does not allow excessive movement. Pull the blade back and forth and notice any free play that seems inordinate. Consult your owner's manual for the maximum allowable clearance. Some waterproof marine bearings require a certain amount of free play, but they should not surpass the manufacturer's specifications. Excessive slop in the bearing will indicate the bearing needs replacement and could have possibly damaged the propeller shaft.

3. Remove the propeller from the shaft, using the correct socket. Pull the propeller outward from the shaft, while noticing how easily the propeller separates from the shaft. Use a hammer to tap the back of the propeller hub if it proves difficult to remove. Inspect the key and key way. The key should appear as a small rectangular block of metal with fairly sharp edges. If the key has deformities like crushed or rounded sides or ends, inspect the shaft for damage. The shaft key way must not have any deformities -- it should be deep with clear cut lines. If not, the shaft should be replaced.

4. Slide the propeller back onto the shaft without installing the key. Make sure the propeller does not wobble on the shaft taper. Rotate it back and forth and notice if it binds in any direction. If it does, the shaft might have to be lapped with valve grinding compound to remove small burrs. This will ensure a better fit.

5. Clean the propeller shaft with a good degreaser. Hold a straight edge ruler over the top of the shaft, in line with its length. Look for any high or low spots between the propeller shaft and ruler edge. Such deviation will point to a bent propeller shaft.

6. Inspect the clean propeller shaft with a bright light and magnifying glass. Look for hairline pressure cracks from the output portion of the shaft to the end. Inspect the end of the shaft for spider cracks. Notice if the shaft has any grooves, marred surfaces or any unusual scratches. Look for small pits in the metal surface -- this will indicate rust and contamination. Make sure the taper of the shaft appears uniform with no flat spots. Replace the propeller shaft if you see any of these damages.

References

• Prop Scan USA: Propeller Checklist
• Yacht Survey: Troubleshooting Problems Involving Engine/Shaft Alignment