Plate XVI.MACHINERY OF H.M.SS. "HECLA" AND "HECATE," 1839.Larger image
Plate XVI.
MACHINERY OF H.M.SS. "HECLA" AND "HECATE," 1839.
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TheGreenock, built in 1848, was the first war vessel by the Scotts fitted with the screw-propeller. We have already referred to her construction in iron, and to her launch. She had a displacement of 1835 tons, and her engines were of719 indicated horse-power. The speed realised on the trial was 9.6 knots. TheGreenock'smachinery, which is illustrated on the next page, is specially interesting, as it represents one of the earliest attempts to drive the screw-propeller by gearing. Two horizontal cylinders were fitted, each 71 in. in diameter, with a stroke of piston of 4 ft. The gearing consisted of four sets of massive spur-wheels and pinions, in the ratio of 2.35 to 1, so that 42 revolutions per minute of the engines give 98.7 revolutions to the propeller-shaft. The propeller was 14 ft. in diameter, and was so fitted that it could be detached and raised to the deck. There were four rectangular brass-tube boilers, each with four wet-bottomed furnaces, and all the internal uptakes united in one funnel, which was telescopic, so that when it was lowered and the propeller raised out of the water, the vessel had the appearance, as well as the facility, of a sailing frigate.
As will be seen from the drawings, both the engines and boilers were arranged very low in the hull, to be safe from the enemy's fire. The engine and boiler compartment occupied 72 ft. of the length of the ship—about one-third of the total length—and the seating for the machinery was specially constructed, with a very close pitch of frames which were only 1 ft. apart. For comparison with the drawings of the machinery in theGreenock, we give on page 49 a similar drawing of the machinery of theCanopus, of 12,956 tons displacement, seven times that of theGreenock. To double the speed, the power of machinery had to be multiplied twenty times, and yet the space occupied is only about trebled.
MACHINERY OF H.M.S. "GREENOCK," 1848.Larger image
MACHINERY OF H.M.S. "GREENOCK," 1848.
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MACHINERY OF H.M.S. "CANOPUS," 1900.Larger image
MACHINERY OF H.M.S. "CANOPUS," 1900.
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In 1850 the largest of the steam vessels in the Navy[58]had a displacement of 3090 tons, but the most noted was theDauntless, of 2350 tons displacement, with engines of1347 indicated horse-power to give a speed of 10 knots. It is true that there were three smaller vessels of greater speed, one of 196 tons steaming 11.9 knots; but this was the highest rate reached in the Navy service. By this time some of the fast mail steamers made 13-1/2 knots. These latter were suited for war service, but we have already dealt with them.
Following the adoption of the screw-propeller in warships came the abandonment of gearing for the engines. For many years various forms of horizontal engine were used; first with return-connecting rods, and subsequently with direct-acting rods. Steam pressures steadily increased, largely owing to stronger materials being available. It was, however, not until the 'seventies that the cylindrical boiler, the compound engine, and the surface condenser admitted of an increase to 60 lb. per square inch[59]—several years after these improvements had been introduced in the Merchant Marine.
The Scotts had worked steadily at the solution of the problem from their trials with theThetisin 1858(see page 34ante). In 1860 the late John Scott, C.B., laid before the Admiralty a system of water-tube boilers and compound engines, but objection was raised to the system. The French Naval authorities, with whom the Scotts then had close business connection, took up the scheme, largely because of the favour with which it was viewed by M. Dupuy de Lôme, the head of the Department. The first ship fitted was a corvette of 650 tons displacement; the boilers worked at a pressure of 140 lb., while the initial pressure at the compound three-cylinder engines was 120 lb. These were the first engines of the compound type in the French Navy.