Inboard Marine Engine Cooling Systems

A marine internal combustion engine must be operated at a specific working temperature. Overheating causes excessive expansion, which can cause an engine to seize. Most marine engines are water-cooled. Their cylinders are surrounded by cooling water spaces known as a water jacket. With few exceptions, the transmission oil, too, is cooled by either the engine's water jacket or a water jacket of its own.

On rare occasions, you may come across an air-cooled marine engine or an open circuit seawater cooling system. The latter is a direct seawater (raw water) cooling system.

Most marine engines are cooled by a closed-circuit fresh water system. The fresh water carrying the engine heat is circulated through an outboard mounted keel cooler or an inboard mounted heat exchanger, where it is cooled by seawater. Seawater does not enter the keel or skin cooler. It makes contact on the outside. In the case of the heat exchanger, the seawater is pumped in and circulated through it. It is then discharged overboard either through the exhaust or directly over the side. If discharged through the exhaust, it is known as a "wet exhaust" system. The keel cooling system has on a "dry exhaust".

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The heat exchanger system is the most common form of cooling system employed in marine engines. The heat exchanger can be engine-mounted or remote from the engine. It has two water circuits: the closed circuit, which circulates through the engine block, and the open (or exchange) circuit. The latter uses raw water which, having been used once is rejected. Two pumps are required; one to circulate the fresh water through the engine and the heat exchanger and the other to feed raw water to the heat exchanger.

The advantages of the heat exchanger system are:

• The raw water only comes into contact with parts intended to withstand it. These parts are resistant to corrosion.

• A sufficient flow of water is available for cooling the exhaust pipe and exhaust gas.

• The amount of water in the sealed circuit is relatively small. This means that the engine will warm up fairly rapidly. The flow of water is easy to control with a thermostat.

• Addition of anti-freeze to protect the engine block in winter.

Aftercooling

The term aftercooling refers to turbocharged marine engines where compressed air blown by the turbocharger is also cooled by a heat exchanger before entering the combustion chamber. Cooler air is denser and allows more fuel to burn.

Closed Circuit Keel Cooling System

In this system, closed circuit fresh water is cooled by passing it through pipes or, in steel hulls, a tank fitted beneath the hull. The former is known as keel cooling and the latter is known as skin tank cooling. A pump circulates the fresh water through the engine as well as through the pipes or the skin tank. The cooler components in contact with the outside water should be of highly corrosion resistant material, such as copper-nickel or equivalent.

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Source: Dick Gandy's Australian Boating Manual