How to Troubleshoot a Propeller Shaft
You might think of your boat as a rigid, indestructible structure. But like any vehicle, it is subject to mechanical failure and the results of normal wear and tear. Hulls flex and twist, motor mounts wear and crack, water causes corrosion, seals leak and split, and propellers often strike underwater objects. All of these maladies can affect the operation and integrity of the propeller shaft, which takes a repeated pounding during even mild operation. Propeller shafts that perform at high speeds and become even minutely unbalanced are subject to severe vibrations and poor performance.
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Things You'll Need
- Pliers
- Screwdriver
- Pointed wood dowel
- Wood blocks
- Masking tape
- Socket set
- Breaker bar
- Ratchet wrench
- Hammer
Instructions
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Observe any noticeable vibration in the hull structure that translates up through the the deck and railing while the boat cruises or rides on plane. This is a good indication that the propeller and propeller shaft drive system has become misaligned. Trailer the boat home and park it. Place the engine in the full tilt down, vertical position. Disconnect the battery with a socket and wrench and remove the key.
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Examine the propeller blades for bends, nicks and gouges in the surfaces. The blade tips should be uniform and not flattened or worn down. Remove the cotter pin from the castellated prop shaft nut with pliers. If equipped with a tang-type locking ring, bend the tabs back with a screwdriver.
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Place a socket and breaker bar on the shaft nut and hold it still. Twist the propeller hard left and right with your free hand. There should be no movement. Any movement will indicate a worn prop shaft, or a worn key or keyway.
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4
Wedge a block of wood between a propeller blade and the ground to immobilize the propeller. Use a ratchet wrench and socket to remove the prop shaft nut. Pull the propeller off the shaft. If it's stubborn, use a hammer to tap the back of the propeller heel until it slides off. Examine the thrust washer on the front of the prop shaft. It should have a clean, uniformed taper, with no flat spots or wear. Replace if it deformed.
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Examine the keyway along the shaft -- it should be uniform and sharp with a perfect rectangular shape. Check the key, if it still sits in the keyway. It should have a perfect half-moon shape, and not deformed on its ends or top. Replace it if deformed. The propeller shaft should have no shiny grooves or burrs in its surface.
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Shift the boat transmission lever in neutral. Place some heavy wood blocks underneath your lower unit, high enough to be even with the propeller shaft. Use masking tape to tape a wooden dowel that has a pointed end to the top of the wooden blocks.
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Rest the pointed end of the dowel against the edge of the propeller shaft. Rotate the shaft with your hand and watch the clearance between the dowel and the shaft as the shaft rotates. A bent shaft will shows a perceptible gap during 180 degrees of rotation.
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8
Pull firmly out on the propeller shaft and then firmly push it in. Repeat this quickly several times. If you see any any movement or hear a "clack" or "knock," it means the cutlass bearing has developed too much slack, due to lack of lubrication, misalignment or wear. It must be replaced or serviced, because it will set up harmful vibration and affect the transmission and drive gears.
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