What is jacket water?

• A. The mixture of fresh water and coolant circulating in the engine block water jackets
• B. Pure water used for drinking
• C. Oil mixed with coolant
• D. Sea water pumped through the exhaust

Answer: (A)

Jacket water is the cooling liquid that flows through the engine’s water jackets around the cylinders. As it circulates, it absorbs heat from the engine and carries it away to be cooled, keeping the engine from overheating. It is not drinking water, not oil mixed with coolant, and not sea water pumped through the exhaust. In practice, jacket water is a mixture of water and antifreeze; in marine engines this coolant is often cooled by a separate sea-water loop, but jacket water itself is the liquid circulating in the engine block’s jackets. The description of a fresh-water and coolant mixture circulating in the engine block water jackets correctly captures what jacket water is.