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Solved problems · Propulsion · Crew answer

Why does my boat vibrate under power?

Vibration that grows with RPM usually comes from the running gear: a worn cutless bearing, a fouled or damaged prop, a bent shaft or poor alignment. Here's how to narrow it down.

Symptom

A rumble, buzz or shudder through the hull that increases with engine speed in gear, often worse than it used to be. It may ease in neutral.

Likely cause

The cutless bearing supports the shaft where it leaves the hull or strut. As its rubber lining wears, the shaft can move, which causes vibration that builds slowly over time. A prop with growth, a bent blade or a wrapped line is unbalanced. A bent shaft, loose coupling, worn engine mounts or poor engine-to-shaft alignment also cause vibration. Uneven wear at the two ends of the bearing points to misalignment.

Stop now if

The vibration starts suddenly after hitting something or picking up a line, a loud knock appears, or water is coming in around the shaft. Take the engine out of gear and stop it, check the bilge, and arrange a diver or haul-out.

The fix, in short

  1. Note when it happens. Compare neutral and in gear at the same RPM. Vibration only in gear points to the prop, shaft or bearing. Vibration in neutral too points to the engine or its mounts.

  2. Clear and check the prop. Have a diver look for line, plastic bags or growth on the prop and shaft, and for bent or chipped blades. Damaged blades unbalance the prop.

  3. Check the cutless bearing when hauled. With the boat out, lift and push the shaft next to the bearing. Noticeable movement, or wear at the bearing ends, means it's time to replace it.

  4. Check alignment and mounts. Look for cracked or collapsed engine mounts and loose coupling bolts. A mechanic can check engine-to-shaft alignment, ideally with the boat afloat.

  5. Check the shaft for straightness. If the bearing is new and the prop is good, have the shaft checked for runout. A bent shaft wears new bearings quickly.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Drivetrain: Propeller, shaft, cutless bearing, strut, coupling and engine mounts

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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