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Propeller Shaft Vibration
Vibration through the hull that rises with shaft speed is coming from the driveline, not the engine. It is worth chasing: vibration destroys cutless bearings, shaft seals and gearbox output bearings, and it usually gets worse.
Is it safe to keep going?
At reduced speed, usually. Severe vibration that started suddenly — after a grounding or a rope strike — should be treated as a damaged shaft or propeller and taken slowly.
Stop if it comes with water ingress at the stern gland, or with a noise that changes.
Likely causes, ranked
| Cause | How common | Confirms it | Needs haul-out |
|---|---|---|---|
| Fouled propeller | Very common | Growth or debris on the blades | Yes (or a dive) |
| Damaged or bent propeller blade | Very common | Inspection | Yes |
| Rope or line around the shaft | Common | Inspection | Sometimes |
| Worn cutless bearing | Common | Shaft moves vertically in the bearing | Yes |
| Engine and shaft misalignment | Common | Alignment check afloat | No |
| Sagging or perished engine mounts | Common | Excess engine movement | No |
| Bent shaft | Occasional | Runout measured on rollers | Yes |
| Unbalanced propeller after a repair | Occasional | Balance check | Yes |
| Worn gearbox output bearing | Occasional | Play at the coupling | No |
The first three are cheap and account for most of it. A rope strike or a season’s growth on the blades produces immediate, obvious vibration. Check before assuming anything structural.
Vibration or misfire?
If it changes with shaft speed — appears at a particular speed, worsens with throttle in gear, disappears in neutral — it is the driveline.
If it changes with engine speed regardless of gear, it is the engine.
Alignment must be checked afloat
Hull shape changes between a hard stand and the water. An alignment set on the hard is not the alignment the boat has floating, and a yard that offers to align during a haul-out should be asked how they handle that. Correct practice is afloat, after the boat has settled.
What you can check yourself
- When was the bottom last cleaned? Has anything been around the prop?
- Engine mounts for sag, cracking and movement under load
- Coupling bolts tight
- Whether the vibration appeared suddenly or built up gradually
When to call a mechanic
If it appeared suddenly, if there is play in the shaft at the cutless bearing, or if the stern gland has started leaking or running hot — those are consequences of vibration and they compound.
Propeller shaft and cutless bearing service · Shaft seal and stern gland service
What marine engine work costs in Australia
Common on these engines
Driveline rather than engine-specific, though gearbox output bearing wear shows up on high-hour ZF and Twin Disc installations.
→ ZF Marine: shaft vibration
Frequently asked questions
Is it the prop or the alignment? Both feel similar. A haul-out inspection settles it, and both are usually checked at the same time because the access is shared.
It started after I hit something — what now? Assume a bent shaft or damaged blade and run slowly. Get it inspected at the next haul-out, sooner if it is severe.
Can vibration damage anything? Yes — cutless bearing, shaft seal, gearbox output bearing and the shaft itself. It compounds.
Why does alignment need doing in the water? The hull changes shape between the hard stand and floating. Alignment set on the hard drifts once the boat is back in.
Practical summary
Use this fault guide safely
Stop now
Shut down for oil-pressure alarms, overheating, fuel spray, smoke in the engine space or loss of cooling-water flow.
Check safely
Start with visible leaks, strainers, fluid levels, alarms and loose connections—only when the machinery is safe and isolated.
Record evidence
Note rpm, temperature, smoke colour, alarm text, sea state and exactly when the symptom began.
Escalate
Give the evidence and engine model to a qualified marine mechanic; avoid repeated restart attempts.
Sources and verification
This page provides general Australian guidance. The current manual for the exact engine, gearbox or drive remains the authority for procedures, specifications and service intervals.
- 1Flywheel
- 2Marine gear
- 3Coupling
- 4Shaft & bearings
- 5Propeller