Engine

The auxiliary engine is normally a 30–60hp freshwater-cooled diesel. The closed loop circulates coolant through the engine and a heat exchanger; a separate raw-water pump draws seawater through a seacock and strainer, pushes it through the exchanger, then injects it into the exhaust. A rubber impeller inside that pump is a routine service part. The exhaust mixing elbow is where cooling water enters hot exhaust gas and is a corrosion/carbon trouble point. Engine mounts isolate vibration and hold alignment. Belts drive the alternator and sometimes the coolant pump. The starter, alternator, control panel, throttle/shift cables and stop system complete the installation.

Hours are useful only with records. A neglected 600-hour engine can be worse than a maintained 5,000-hour engine. “Rebuild” should mean documented machining, measurements and replaced internal parts, not paint and new external seals.

The engine is needed for harbor maneuvers, charging, making water on some boats and propulsion during calms or emergencies. Cooling loss can destroy it in minutes. Fuel, exhaust or electrical faults can cause fire, flooding or carbon monoxide. A failure on the ocean is inconvenient; a failure approaching a reef pass or lee shore is immediately dangerous.

A baseline service changes oil and filters, fuel filters, impeller and belts as due; checks coolant, valve clearance schedule, zincs, hoses, clamps, raw-water flow and leaks; and records RPM/temperature under load. Repair the actual fault: starter, alternator, heat exchanger, elbow, injector, mount or pump. Compression testing, oil analysis and injector testing are diagnostics, not ritual replacements.

A top-end job addresses head/valves; a true rebuild addresses wear inside the block. Repower is justified by a damaged block, uneconomic rebuild, obsolete parts, repeated unreliability or a badly mismatched engine—not faded paint. It usually triggers mounts, beds, exhaust, controls, wiring, shaft/coupling or saildrive interface work.

• Covered by a standard survey — external condition, fluids, leaks, mounts, hoses, belts, exhaust, controls, cold start if arranged and observed operation/sea-trial RPM. Many survey reports expressly disclaim internal machinery condition.
• Not covered — compression/leak-down, injector testing, oil/coolant laboratory analysis, heat-exchanger disassembly, ECU diagnostics and expert valuation of a rebuild. An engine survey at roughly $800–$2,000 is worthwhile before purchase when the engine is high-value, starts poorly, smokes, overheats, cannot reach rated RPM, lacks records or makes abnormal metal/noise. Insist the seller not warm it before arrival.

• The broker or owner — “Please leave the engine stone cold for inspection and send a clear photo of the model/serial plate and current hour meter.” “Send invoices for oil/filter, impeller, coolant, valve clearance, heat exchanger, exhaust elbow, injectors, mounts and any rebuild.” “Has it ever overheated, ingested seawater, lost oil pressure or failed to reach rated RPM?”
• The surveyor — “Record block temperature before start; video the cold start; note cranking time, smoke and discharge water; run at wide-open throttle long enough to stabilize; compare achieved RPM with the manufacturer rating; and recommend specialist tests only from observed evidence.”

Touch the block before anyone starts it. It should be ambient temperature. Watch the exhaust at start: a brief puff differs from continuous white steam, blue oil smoke or heavy black smoke. Check raw-water discharge. Run from idle to full load and record RPM, temperature and vibration. Pull the dipstick before and after: gray/milky oil, metal glitter, rising level or burnt smell is bad. Open the coolant cap only when cold and look for oil, rust or low level. Inspect beneath the engine with a white towel. Squeeze cold hoses, inspect belt dust, rusty mounts, leaking pump weep holes and soot/water around the elbow.