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.
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 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
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