Describe the pattern before touching anything.
- Engine-speed vibration in neutral: engine, mounts, accessory, damper or cylinder balance.
- Only in gear: propeller, shaft, bearing, saildrive, gearbox or transmitted hull resonance.
- A narrow rpm band: resonance, mount stiffness, loose panels or exhaust contact.
- Rhythmic with shaft rotation: propeller damage, bent shaft, bearing or alignment.
- Squeal: slipping belt, bearing or seal, depending on location and timing.
- Rattle at idle that changes in gear: damper plate, gearbox gear rattle, mounts or loose hardware.
- Sharp combustion knock: injection or internal mechanical issue requiring prompt assessment.
Inspect for loose engine covers, exhaust contact, failed mounts, belt dust and movement at the coupling. Check whether the noise began after haul-out, propeller work, grounding, rope entanglement or mount adjustment.
Never approach a rotating shaft or belt with clothing, hair, tools or a listening device. Remote microphones and careful stationary tests are safer. If vibration increases suddenly, stop before it damages the seal, coupling, gearbox or hull support.