Short answer
Insist on a genuine cold start, then run the engine under load rather than at the dock. Review the service records, inspect oil and coolant for cross-contamination, examine the exhaust and raw-water circuit for corrosion, and assess the drive separately. Age matters far less than documented maintenance and installation quality.
You will need: Torch, mirror, camera, oil sample kit, an independent surveyor
Hours tell you less than owners assume. A well-maintained high-hour engine routinely outlives a neglected low-hour one.
The non-negotiables
A genuine cold start. An engine warmed before you arrive hides starting faults, smoke and noise. If the seller has already run it, come back another day.
A load test. Dock idling proves almost nothing. Faults in cooling, fuel supply and propulsion appear under load and nowhere else — and you learn whether the engine reaches rated rpm, which is the most informative number available. See how to tell if a propeller is overloading the engine.
What to inspect
- Records: hours, dates, part numbers, who did the work. Gaps are the finding.
- Fluids: oil for water, metal and fuel smell; coolant for oil; oil for coolant.
- Cooling and exhaust: the mixing elbow, heat exchanger and raw-water circuit — corrosion here strands more boats than worn bearings do.
- The drive: saildrive diaphragm age and evidence, or shaft seal, bearing and alignment history. Assess it as a separate system from the engine.
- Installation: mounts, wiring, panel, fuel system, ventilation, and the quality of past repairs.
Where a professional pays for itself
An independent surveyor and, where warranted, compression or crankcase testing and oil analysis cost a fraction of a repower. Engage a surveyor who works for you and is independent of the sale — see used marine engine survey and ownership costs.