How to Determine if Your Prop Shaft is Bent

Written by eric cedric
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How to Determine if Your Prop Shaft is Bent
Use the wake to help determine bends in the prop shaft. (boats wake image by Chad McDermott from

When a boat engine's prop shaft is bent, the performance of the engine and boat are affected. Diagnosing a bent prop shaft is done in three different ways. Perform two of these with the boat in motion, paying attention to vibrations and cavitation -- air bubbles and turbulence -- in the wake, and if the boat planes out at speed. Use a visual inspection for the third method of identifying bent prop shafts.

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  1. 1

    Start the boat engine and slowly bring the engine to speed. Pay attention to any vibrations in the steering wheel at low speeds. Vibrations at low speeds are clear indicators of a bent prop shaft.

  2. 2

    Watch the wake as you bring the boat into the slow to medium speeds. If there is excess air bubbling around the prop shaft along with the vibrations, the shaft in most likely bent.

  3. 3

    Bring the boat to speed. A boat at maximum hull speed should "plane out", that is to say it will travel parallel to the water surface, or be on the same "plane". A bent prop shaft makes it harder for the boat to achieve this plane. The prop shaft is probably bent if the boat does not plane out.

  4. 4

    Pull the engine up and out of the water -- with the engine off -- and inspect the prop blades and the shaft. Look for clear bends in the shaft and chips, breaks and bends in the prop blades.

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