N38143 Chapter 4 Keel cooling or skin cooling Keel cooling or skin cooling is a closed circuit method of cooling that uses only coolant with 50/50 antifreeze mix. A properly designed and installed cooling system is essential for satisfactory engine life and performance. This system uses a group of tubes, pipes or channels attached to the outside of the hull below the waterline as a heat exchanger. Keel coolers are used in preference to the standard raw water cooled engine mounted heat exchanger when operating in areas that have heavy silt and debris in the water that would erode the heat exchanger tubes or block them.
Keel cooling is used in Arctic conditions to avoid the problems of freezing that is experienced with the raw water circuit on the heat exchanger cooling system. Keel coolers are available in standard designs from several manufacturers. These units are simple to install and are sized by the manufacturer for the engine model and boat application. Commercial coolers are made of erosion resistant materials and have a relatively high heat transfer efficiency. The disadvantage of external keel coolers is that they are vulnerable to damage and must be guarded.
An alternative to the commercially available coolers are fabricated keel coolers manufactured by the boat builder as part of the hull construction. These coolers are not as efficient and must be designed oversize to allow for a decrease in performance that follows the formation of rust, scale and marine growth on the keel cooler. If the genset is a replacement package and the original cooling system, keel cooler and expansion tank, is to be reused, then it is essential that the system thoroughly flushed to remove sludge that may be in the system. Failure to remove sludge could block air bleeds leading to the engine overheating.
Sizing the coolers Commercial keel coolers are manufactured in a variety of sizes and shapes. The keel cooler manufacturer will recommend a keel cooler when provided with the following data:- Engine model and rating Engine data sheet, see section 7 Heat Rejection (see table below) Engine coolant flow rates are at a system resistance of 28kPa (4 Ibf/in2) Max. coolant temperatures from grid cooler Maximum raw water temperature Pipe connections - see illustrations on following pages. Coolant - 50/50 antifreeze mix Warning!
Twin grid coolers are required for the 4.4TWGM, 4.4TW2GM and 6TWGM Page 17 Heat rejection data Engine model 415GM 422GM 422TGM 700GM 4GM 4TGM Heat rejection Jacket water - Engine Btu/min 1109 1302 2098 - 2250 3424 kW 19.5 22.9 36.9 TBA 39.5 60.2 Aftercooler - Intercooler Btu/min - - - - - - kW - - - - - - Water flow rate, thermostat fully open Engine US gals/min 12.9 13.7 13.7 25.6 25.6 Litres/min 49 52 52 TBA 97 97 Intercooler US gals/min - - - - - - Litres/min - - - - - - Max coolant temp (from grid cooler) 140 140 140 Engine oF 60 60 60 oC - - - Intercooler oF - - - oC