A variety of cooling systems may be adopted for marine auxiliary engines but the most commonly used is the simple closed circuit system (Figure 7.5). Sea water is passed through the intercooler, the oil cooler and then the jacket water cooler in series flow. Engine driven fresh-water circulating pumps are normally fitted, but the sea-water pump may be either an independent unit or engine driven in tandem with the fresh-water pump. The cooling system may be arranged so that in an emergency sea water can be circulated through the engine jackets, after removal of certain blanks installed in the pipework.

In ships with diesel main propulsion engines cross-connections between the main and auxiliary engine jacket water systems are common. This enables the main engine to be kept warm in port from the heat in the auxiliary engine jacket water. To enable the auxiliary engine to be run in dry dock it is customary to arrange a connection from a double bottom or peak tank.