All About Engines

■Shi'* "i'l'l'll Jff""*. II -y n—»■—«■■»g» II 302 All About Engines utilising the manifest advantages of the turbine, and avoiding its disadvantages. So far, four of these have been devised. The first is the use of reciprocating engines and exhaust steam turbine. It will be recollected that the reasons for the high efficiency of the turbine with low pressure steam were given in Chapter VI., and seven or eight years ago experiment showed that the combination on a ship resulted in a reduction of coal consumption of from 12 to 15 per cent. The cruiser Bristol was equipped in this way, but the most famous example is the White Star liner Olympic. This 50,000 ton vessel has three screws, the outer ones driven by reciprocating engines, and the central one driven by the turbine. The former are triple expan- sion, having one h.p., one i.p., and two l.p. cylinders of 54-inch, 84-inch, 97-inch bore and 75-inch stroke. Each set develops about 15,000 horse-power at 75 revolutions per minute. Each engine bedplate weighs 195 tons, and each pair of columns supporting the cylinders 21 tons. Each l.p. cylinder, with its liner, weighs 50 tons. Between these is a low-pressure turbine of the Parsons type, receiving steam at 9 lb. and exhausting it at 1 lb. per square inch. The rotor is 13 feet 8 inches long, 12 feet in diameter, and weighs 130 tons. It is fitted with blades vary- ing from 18 inches to 25J inches in length. The whole turbine weighs 420 tons, and develops 16,000 horse-power at 165 revolutions per minute. When going astern or manoeuvring it is put out of action, and the steam passes directly from the reciprocal-