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The “dry” exhaust

Keel Cooling

can be made a “wet” exhaust by a separateraw water pump. In principle this is similar to a heat exchanger cooled system butraw water circuit and heat exchanger have been replaced by pipes attached externally to the vessel’s keel. The expansion/header tank may be fitted separately in whichcase it must be connected directly to the circulating pump inletand the primary circuit adequately vented into the tank. Pipe bore and surface area must be adequate for effectivedissipation of heat from the primary circuit to sea or river water.In some installations where, due to system pressure losses through keel pipes, cylinder block and exhaust manifold, the flow capacity of the centrifugal circulating pump is insufficient, a Jabsco pump may be used (flow capacity approximately 30 GPM for every 100 HP at maximum engine load and rpm). Alternatively, the ship’s steel hull may be used as the cooling surface and the heat transmitted directly to the surrounding sea or river water from a tank welded inside to the bottom.
Engine CoolingIntroduction

Direct Cooling

Raw water is pumped directly through the engine block.However, as the temperature of the water entering the block isambient, the outlet temperature needs to be much lower than in an indirect cooled engine, to reduce the formation of scale and salt deposits and thermal stresses in the cylinder block. Lower operating temperature means that engine performancewill be considerably less efficient and direct cooling systemsshould therefore not be used on commercial craft engines. For weekend pleasure craft engines which operate no morethan 50-100 hours per year, reliability margins will probably bemaintained for a number of years until deposits in the engine cooling passages will begin to affect heat transfer to the cooling water. Engine operating temperature is best controlled by a marinethermostat, rather than a manually operated valve. For cold engine start, the thermostat will be closed and mostof the cooling water (about 10 GPM for every 100 HP at full loadand speed) will bypass the engine via a spring loaded back pressure valve and discharge into the exhaust manifold. A small bleed hole in the thermostat will ensure a slow circulation of cooling water through the cylinder block to prevent ‘hot spots’ while the engine is warming up.
Flexible Impeller Pumps -

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