It should be noted that engine manufacturers pay special attention to the design of engines for direct cooling in order to minimise corrosion in the cooling passages. However many small craft owners marinise standard car engines successfully.
Raw water (river or seawater) is pumped directly through the cylinder block, exhaust manifold, exhaust silencer (if fitted) and exhaust pipe. To ensure efficient engine performance it is essential that an optimum operating temperature is being maintained by regulating the amount of cooling water through the cylinder block, either using 'Manual' or Automatic temperature control.
Manual Temperature Control
A hand valve is fitted in parallel to the engine block. When the engine is cold, the valve must be opened to reduce the flow of water through the block. As the engine temperature increases, the valve is closed gradually until optimum temperature is maintained.
IMPORTANT. Do not regulate the pump flow by fitting the by-pass valve between the pump discharge and suction or by restricting either the pump discharge or suction.
Automatic Temperature Control
A Marine thermostat is fitted after the cylinder block and is closed when the engine is cold allowing most of the cooling water to be pumped via a spring loaded back-pressure valve to the exhaust. As the engine temperature increases, the thermostat opens until the required operating temperature is maintained.
Heat Exchanger Cooling
A pump recirculates Fresh Water on a closed circuit through the cylinder block, thermostat and around the tubes of a heat exchanger (Primary Cooling Circuit) which is often an integral part of the vented expansion - or header tank. Cold Raw Water is pumped by a second pump through the heat exchanger tubes (Secondary Cooling Circuit) and maintains the fresh water in the primary circuit at an average temperature of 80-90oC.
Keel Cooling is basically the same as heat exchanger cooling, except that the fresh water is recirculated through the keel cooling pipes which are fitted outside to the bottom of the boat. The heat generated by the engine is dissipated directly to the raw water flowing around the pipes.
Raw Water Cooling
Raw water cooling systems demand a great deal from circulating pumps. They must have the unfailing ability to self prime at various engine speeds and must be able to pass such solids as sand and silt without impairing the pump's flow.
On average, petrol engines and fast running diesel engines require a raw water pump flow of approximately 8-8½gpm (36-39l) for each 100BHP for Direct Cooling systems and 14½-15½gpm (65-70l) for each 100BHP for Heat Exchanger Cooling systems.