Definition
A heat exchanger transfers heat from an engine's closed coolant circuit to raw water drawn from outside the vessel, passing both through a shared assembly of tubes so that heat moves between them without the two ever mixing.
Indirect cooling — the arrangement almost all modern marine diesels use — keeps seawater out of the engine itself. The engine runs on treated coolant at a controlled temperature; raw water carries the heat away and goes overboard through the exhaust.
What goes wrong
Both sides foul. The raw-water side collects scale, marine growth and debris — including rubber from a past impeller failure, which lodges at the tube stack inlet. The coolant side accumulates sediment as inhibitor degrades. Either reduces heat transfer, and the result is overheating that appears under load first.
A perforated tube lets the circuits mix, contaminating both. That is a re-core or replacement, not a clean.
Where you meet it
In every overheating diagnosis, on service schedules as a longer-interval item, and in the cleaning procedure. In the tropics it needs attention on a shorter cycle than temperate schedules assume.