The introduction of Siemens-Martin steel, about the year 1875, and its continued and extended use since, have however been really the means of rendering possible the construction of steamships of all sizes with high rates of speed now so common, and are undoubtedly the means whereby those ships can be so economically built and worked as to pay as commercial ventures. The construction of their hulls with a material fifty per cent. stronger than iron has rendered it possible to make such appreciable decrease in weight as to admit of fining their lines suitably for high speed without sacrificing carrying capacity. With this same steel, boilers can be constructed for a pressure of 150 pounds per square inch without weighing very much more than iron ones for 75 pounds. By using steel for castings, forgings, etc., the weight of the machinery has been reduced from 5 hundredweight to 2 hundredweight per I. H.-P., and when forced draught is employed it is as low as 1.6 hundredweight per I. H.-P. for large powers, and less still for such engines as are used in torpedo boats and catchers.
(Made by Earle’s Shipbuilding Company, Hull, England, for twin-screw fast cruiser for the British Navy, of 9,000 I. H.-P.)]
It has already been remarked that the consumption of coal, which enters as a most important factor into the question of high speed, both from the weight and cost, had been reduced, by the introduction of the compound engine, from 4 pounds to 2-1/2 pounds per I. H.-P., and latterly, as that engine was improved and higher pressures used, the consumption was further reduced to 2 pounds, and in some cases as low as 1-3/4 pound per I. H.-P. The triple expansion engine, developed within the past eight years, and later the quadruple expansion, have effected a still further saving, until with them and such other means as are now employed, the consumption is under 1-1/2 pound of coal per I. H.-P.
The success of the locomotive was very questionable until the exhaust steam was turned into the chimney so as to create a rapid draught, and the steam-blast to-day enables the locomotive to travel at its great speed by causing the comparatively small boiler to generate such a large amount of steam. When this form of boiler was tried on board ship its power would have been very much crippled had not some other means been adopted for forcing the draught, as the steam could not in this case be allowed to escape through the funnel, but must be condensed into water for the use of the boiler. By closing the stoke-hole and forcing into it by mechanical means a plentiful supply of air, this boiler was made to be as efficient for a torpedo boat as for a locomotive. This forced draught has now been adopted on large ships, and to-day the very high speed of naval vessels, and of many mercantile steamers, is due to it. Consequently, with the same weight of machinery, higher powers are developed with a corresponding increase in speed, and the cruiser Piemonte, constructed by Sir William Armstrong & Co., of which an illustration is shown on p. 91, had her speed increased by means of forced draught from 20 knots to 22.3 knots, at which speed she was going when the picture was taken.