In the keel cooling system shown in figure 7-3, heat transfer takes place in the keel cooling coil that is mounted on the hull of the vessel below the waterline. In this system, which is used on small craft, the coolant is moved by a high-capacity freshwater pump from the bottom of the expansion tank through the engine oil cooler and marine gear (transmission) oil cooler to the cylinder block. (See directional arrows.) Openings in the water jacket around the cylinder bores connect with corresponding openings in the cylinder head through which the liquid passes to circulate around the valves and fuel injectors. A portion of the coolant is bypassed from the aft end of the water manifold into the aft end of the jacket surrounding the exhaust manifold and on to the expansion tank.

When the thermostat is open, the major portion of the coolant in the water manifold passes through the thermostat housing and flows directly to and through the keel cooling coils. When the thermostat is closed, that portion of the coolant entering the thermostat housing is bypassed directly to the engine water pump inlet where it remixes with the coolant from the exhaust manifold jacket. The coolant is then circulated through the cylinder block and cylinder head. With the thermostat closed, a quick warm-up is assured since the circulating water does not pass through the keel cooling coils.

Figure 7-3.-Typical keel cooling system.

Radiator and Fan Cooling System

The radiator and fan cooling system is shown in figure 7-4. In this system, the engine coolant is circulated through the radiator where it gives up its heat to the stream of air forced through the fins of the radiator by a fan. The fan is belt driven from the crankshaft. The water pump draws the cooling liquid through the oil cooler and discharges it into the lower part of the cylinder block. (See directional arrows.) Openings in the water jacket around the cylinder bores connect with corresponding openings in the cylinder head through which the coolant rises to circulate around the valves and fuel injectors. The coolant then circulates through a water manifold that is bolted to the cylinder head. From the water manifold, the coolant discharges past the thermostat and into the radiator.

With the thermostat open, the coolant circulates through the radiator before returning to the water pump. With the thermostat closed, the coolant is bypassed from the water manifold directly to the pump and recirculated through the engine. In this manner, a quick warm-up of the engine is assured since the circulating water does not pass through the radiator.