The Compound Steamship Engine (1896)
Randolph, of Randolph, Elder & Co., Glasgow, first introduced the compound engine in paddle boats for the Pacific Company in 1856, but it did not come into general use until 1870.
Alfred Holt, of Liverpool, tried it successfully in his boats in the China trade in 1865, running direct from Liverpool to the Mauritius, 8500 miles, without re-coaling. In 1868, the National Company tried it in the Italy.
The opening of the Suez Canal on the 17th November, 1869, gave an immense impetus to screw steamships for the Indian, Chinese, and Australian trades. This, combined with the compound engine, created a revolution in the carrying trade of the world, which has proved almost fatal to sailing ships, especially as they are unfit for the Canal or the Red Sea.
To understand this it is only necessary to remember that the first Cunard boats could only carry 225 tons cargo and 90 passengers, and could only steam 8.7 knots per hour, on an average, consuming 4.7 pounds of coal per I.H.P. per hour. While the first of their screw compound boats, the Bothnia, built in 1874, carries 3000 tons of cargo, and 340 saloon passengers, besides steerage, and steams on an average 13 knots, consuming only 2.2 pounds of coal per I.H.P. per hour, her engines being only 507 H.P. nominal, against 425 in the Acadia, of 1840.
In the simple engine, the steam passed at low pressure from the boilers to the cylinder, where it did its work, and then passed direct to the condenser. However, in the compound it passes at very high pressure into a small cylinder and thence by expansion into a large one, and thence, in the triples, to a still larger one, before it passes into the condenser.
The Mongolian's cylinders, for example, are 30, 50, and 80 inches in diameter respectively, with 5 feet stroke. The Friesland’s are 35.5, 56, and 89 inches, with 4.25 feet stroke. Of course, they do not do three times the work of the old engines, but, as the cranks are set at different angles, much greater power is obtained.
Steam is now used in marine steel boilers up to 200 lbs. pressure to the square inch, instead of at 13 lbs. in the early boats; but as it requires very little more coal to raise 200 lbs. than 13 lbs. the consumption of coal has been gradually reduced from about 5.5 lbs. to 1.5 lbs. per I.H.P. per hour.
Experts tell us that to convert a quantity of water at 32 degrees into 10 lbs. of steam requires one cwt. Of coal; into 40 lbs. it requires only 1.012; and into 90 lbs. only 1.024 lbs. One of Napier's engines, in the Russian ship of war isinope, recently consumed only 1.45 lbs. per hour at full power, and the Empress of Japan, of the C. P. R. line, consumed only 1.56 lbs. on her trial trip.
“Forced draught " has also been introduced, which causes a more perfect combustion of the fuel; but much depends on the quality of the coal, the work of the firemen, and the character of the boilers. Warm water is also returned from the condenser to the boilers, which is another economy, and the steam is “super-heated " to increase its power.
The Daventry, by using an “evaporator," has raised the water to 170° Fahrenheit, and the Enchantress, by means of a " feed-heater," has raised it to 2100, which must economize the consumption of coal.
The Compound Engine Used in Steamships (1896)
Someone professes to have discovered a means of returning steam to the boilers, which, if successful, would of course produce another revolution in steam engines. Quadruple cylinders have also been adopted in a few ships.
There is still plenty of room for further reduction in the consumption of coal, as, according to Mr. Merrifield, F.R.S. ('Text-books of Science'), no steam engine, as yet, does one-fifth of the work which, theoretically, it ought to do if all the heat produced by the combustion of the coal were utilized. It will doubtless soon be reduced to 1 lb. per I.H.P. per hour, or even less.
The early screws could only run about 3000 knots at full speed without re-coaling, but compound engines now enable them to run about 10,000 knots, and still more at reduced speed. Large steel boats of moderate power and speed now carry immense cargoes of dead weight, so that sailing ships have no chance against them.
Thus, the Rossmore, 4360 tons gross, carries 6800 tons of cargo, besides coal, and steams 12 knots with engines of only 2500 I.H.P., or about 500 nominal. The Georgian, of 5800 tons gross, is said to carry 7000 tons of cargo besides coal, or nearly 60 per cent, over her tonnage; and the White Star freight-boat Cevic recently cleared from New York for Liverpool with the following enormous cargo :—
- 144,000 bushels of grain
- 9,000 bales of cotton
- 896 head of cattle
- 1,130 tons flour, copper, meats and hay
- 3,000 boxes cheese
- 2,600 barrels oil and wax
- 2,000 bales hides
Iron and wooden sailing ships only carry 40 to 45 per cent, over their tonnage. Steamships, however, have been greatly overdone, and freights have been reduced to ruinous rates. Thus, wheat has been carried from Montreal to Liverpool at 4.5 cents per bushel (formerly 25 to 30 cents), and iron from Glasgow to New York at is 6d per ton (formerly 25s), and grain from New York to Liverpool at 4 cents per bushel, or less.
Fry, Henry, “Fourth Epoch: The Compound Engine, Epochs in Atlantic Steam Navigation,” in The History of North Atlantic Steam Navigation with Some Account of Early Ships and Ship Owners, London: Sampson Low, Marston and Company, Ltd. (1896): P. 46 -49