Jabsco Pump Catalog

Keel Cooling In principle, this is similar to a heat exchanger cooled system, but the raw water circuit and heat exchanger have been replaced by pipes attached externally to the vessel’s keel. Pipe bore and surface area must be adequate for effective dissipation of heat from the primary circuit to sea or river water. In some installations where the flow capacity of the centrifugal circulating pump is insufficient due to system pressure losses through keel pipes, cylinder block and exhaust manifold, a Jabsco pump may be used (flow capacity approximately 30 GPM [113 LPM] for every 100hp 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. The “dry exhaust” can be made a “wet exhaust” by a separate raw water pump. The expansion/header tank may be fitted separately. In which case, it must be connected directly to the circulating pump inlet and the primary circuit adequately vented into the tank. Direct Cooling Raw water is pumped directly through the engine block. However, as the temperature of the water entering the block is ambient, 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 performance will be considerably less efficient, and direct cooling systems should, therefore, not be used on commercial craft engines. For weekend pleasure craft engines which operate no more than 50-100 hours per year, reliability margins will probably be maintained for a number of years until deposits in the engine cooling passages begin to affect heat transfer to the cooling water. Engine operating temperature is best controlled by a marine thermostat rather than a manually operated valve. For cold engine start, the thermostat will be closed and most of the cooling water (about 10 GPM [38 LPM] for every 100hp at full load and 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. 96 11 Flexible Impeller Pumps INTRODUCTION