Several things had combined in the latter part of this decade to bring about this advance. The great change between 1860 and 1872, from the causes already noted, which had reduced coal consumption by one-half, was followed by the introduction of corrugated flues and steel as a material for both boilers and hull. With this came still higher pressures, which were carried from 60 to 80 and 90 pounds. In August, 1881, a very interesting paper was read by Mr. F. C. Marshall, of Newcastle, before the Institution of Mechanical Engineers, in which he showed that a saving of 13.37 per cent. in fuel had been arrived at since 1872. The general type of engine and boiler had remained the same in these nine years, but the increased saving had been due chiefly to increased pressures. It is curious that at the reading of both the paper by Sir Frederick Bramwell, in 1872, and that of Mr. Marshall, in 1881, there should have been pretty generally expressed a feeling that something like a finality had been reached. So little was this opinion true that, though over thirteen per cent. saving had been effected between these two dates, a percentage of gain more than double this was to be recorded between the latter date and 1886. In these matters it is dangerous to prophesy; it is safer to believe all things possible. Certainly the wildest dreamer of 1872 did not look forward to crossing the Atlantic at 20 knots as a not unusual speed.
In 1874 triple-expansion engines had been designed for the Propontis by Mr. A. C. Kirk, of Napier & Sons, of Glasgow, which, on account of failure in the boilers which were used, did not give at first the results hoped for. In 1881 the Messrs. Napier fitted the Aberdeen with engines of the same kind, steam at 125 pounds pressure per square inch being used. In the next two years the change proceeded slowly, but by 1885 the engineering mind had so largely accepted it that a very large proportion of the engines built in that year were on this principle, and at the present it may be regarded as being fully accepted as was the compound engine ten years since. The saving in fuel is generally reckoned at from twenty to twenty-five per cent., or, to put it more graphically, in the words of Mr. Parker, Chief Engineer Surveyor of Lloyds, in his interesting paper, read in July, 1886, before the Institution of Naval Architects: “Two large passenger steamers, of over 4,500 gross tonnage, having engines of about 6,000 indicated horse-power, built of the same dimensions, from the same lines, with similar propellers, are exactly alike in every respect, except so far as their machinery is concerned. One vessel is fitted with triple-expansion engines, working at a pressure of 145 pounds per square inch; while the other vessel is fitted with ordinary compound engines, working at a pressure of 90 pounds per square inch. Both vessels are engaged in the same trade and steam at the same rate of speed, viz., 12 knots an hour. The latter vessel in a round voyage of 84 days burns 1,200 tons more coal than the former.”