Most modern yacht diesels use two linked cooling circuits.
The closed circuit contains treated coolant circulated through the block and cylinder head. A thermostat controls temperature. The coolant transfers heat through a heat exchanger and may also heat a calorifier.
The raw-water circuit draws seawater through a seacock and strainer. A flexible-impeller pump pushes it through coolers and the heat exchanger. The water commonly enters the exhaust mixing elbow and exits overboard.
Follow the raw-water path
Seacock → strainer → raw-water pump → gearbox or charge-air cooler if fitted → heat exchanger → exhaust elbow → waterlock → outlet
The actual order differs by engine. Trace yours.
Common restrictions
- Closed or partly obstructed seacock
- Plastic bag or marine growth at the inlet
- Air leak at the strainer lid
- Worn, damaged or spinning impeller
- Missing impeller vanes lodged downstream
- Scaled heat-exchanger tubes
- Crushed or delaminated hose
- Blocked exhaust injection elbow
An engine may overheat even when some water exits the exhaust. Conversely, steam can result from insufficient flow while the temperature sender has not yet reacted. When an alarm sounds, reduce load and stop as soon as navigationally safe. Never remove a pressurised coolant cap while hot.
Coolant is not simply coloured water. Use the chemistry and concentration approved for the engine, and do not mix incompatible types. Raw water and coolant should remain separate. Investigate unexplained coolant loss, an overfull expansion tank, bubbles, oil contamination or salt deposits.
Service pages: Impeller replacement · Heat exchanger and coolant · Overheating diagnosis