In a marine heat exchanger, which fluid cools the engine, and which cools the jacket water?

• A. Jacket water cools the engine; Raw water cools jacket water
• B. Raw water cools the engine; Jacket water cools raw water
• C. Oil cools the engine; Water cools oil
• D. Air cools the engine; Water cools air

Answer: (A)

The engine is cooled by a jacket water loop, which surrounds the engine block to absorb heat from the cylinders. That jacket water then passes through a heat exchanger where it is cooled by raw water (sea water). The raw water, drawn from the sea, runs through this separator/sea-water cooler to remove heat from the jacket water and is discharged back overboard. This two-loop setup keeps the engine protected without letting sea water contact the engine directly.

So, jacket water cools the engine, and raw water cools the jacket water. The other ideas don’t fit typical marine cooling arrangements: raw water directly cooling the engine would risk corrosion and fouling, and oil or air are not the primary cooling fluids in this system.