So far all the trials had been made with two crank engines; so it was now decided to construct another set of engines for 150 lb. pressure, having a crank to each cylinder. These engines had cylinders 20½ inches, 33 inches, and 58 inches diameter by 36 inches stroke, and were fitted into the screw steamer Rosario, whose dimensions are 275 feet 3 inches between perpendiculars, 34 feet 3 inches beam, and 19 feet 2 inches depth of hold, 1,862 tons gross, and the deadweight capacity 2,550 tons. In March last year she was loaded with 2,530 tons deadweight, and did the voyage to Bombay at an average speed of 8.6 knots on a consumption of 10.5 tons per day of South Yorkshire coal, and burnt on the voyage 347 tons. This result is superior to that of the Draco when the size of the ship is taken into account, but is not so much so as might have been anticipated from the increase of pressure and the rate of expansion, which was 14.4 in the Rosario and 12 in the Draco. Another set of engines was made from the patterns of those of the Draco, but with the high pressure cylinder 20 inches diameter, steam at 150 lb. pressure being supplied from two single ended boilers, having a total heating surface of 2,200 square feet.
They are fitted in the S.S. Finland, a cargo boat 270 feet long, 35 feet beam, by 18 feet depth of hold, and 1,954 tons gross register. In January she was loaded with 2,500 tons deadweight, and sailed for Rangoon. The average speed attained was 8.42 knots per hour, or 202 miles per day, on a consumption of 10.3 tons of Welsh coal per day, the rate of expansion being 12. It should be mentioned that all these ships named are fitted and steered with steam stearing gear, so that in comparing these results and those published of the engines made by an eminent engineer in the north of England, an allowance should be made, as in that ship there was no steam stearing gear.
I have chosen to make all these comparisons by reference to the ships' logs, and to give results such as a shipowner looks for rather than those which engineers prefer to use in forming a judgment on the merits of different engines. I do this for two reasons: first, because the commercial success of the triple compound engine depends on the saving it can effect in a long voyage; and secondly, because I had no reliable indicator diagrams from which the consumption per indicated horse power could be calculated with any degree of accuracy. On trial trips with the steamers already named, the consumption of ordinary South Yorkshire coal was 1.6 lb. per indicated horse power, and the consumption of water per indicated horse power calculated from the high pressure indicator diagrams was 1.41 in the Draco, 13.2 in the Rosario, and 13.16 with the Finland, or taking the medium pressure diagrams, it was 12.2, 1.30, and 11.95 respectively. Twelve months ago we constructed for Messrs. Thomas Wilson, Sons & Co., two sets of triple expansion engines of 600 indicated horse power, one having two cranks and the other three cranks, the engines, boilers, and propellers being otherwise exactly alike and fitted into sister ships.
The water consumed in the three crank engine is 12.93 lb., against 13.0 in the two crank, but the former drives its ship nearly ½ knot per hour faster than the latter does its, and when both ships are driven at the same speed the consumption of coal in the three crank ship is considerably less than in the other.
We have now entirely given up the construction of two-crank triple expansion engines, because of the impossibility of equally dividing the work between the cranks; for, although the engine when running appeared to be perfectly balanced, the wear of the brasses of the crank having the two cylinders was always considerably more than that of the other. Placing the high pressure cylinder over the low pressure cylinder seemed to give the most satisfactory results, but even these were far inferior to those once obtained with the three cranks. We have lately constructed some very small three-crank engines from which exceedingly good results were obtained; the cylinders are only 11½ inches, 17 inches, and 30 inches by 18 inches stroke, which developed 218 indicated horse power with a consumption of 12.8 lb. of water per indicated horse power, and this, together with some other observations, leads me to believe that the best economical results will be obtained by running triple expansion engines at a much higher number of revolutions than is usual, and with a rate of expansion not less than 12 for a steam pressure not less than 140 lb. (155 absolute). The largest engines we have made of this type so far are those of S.S. Martello, which have cylinders 31 inches, 50 inches, and 82 inches diameter by 57 inches strokes and indicate at sea 2,400 horse power when running at 60 revolutions with steam of 150 lb. pressure; the consumption of Yorkshire coal is 37 tons per day average throughout a New York voyage.
Had Welsh coal been used in every case, the results would have been very much better, for, in addition to the superior evaporative power of Welsh coal, it is slow burning and much more easily controlled, especially on the comparatively short grates of these modern boilers, the quick-burning Yorkshire coal causing the safety valves to frequently blow off when working near the load pressure unless great care is taken by the firemen.
I trust these few particulars may be of interest to the Institution, and especially to those members of it who are particularly interested in the commercial success of our mercantile navy. I have purposely avoided engineering details and technicalities of any kind, giving only such information as will tend to give British shipowners faith in that form of engine which will undoubtedly help them to successfully tide over bad times, and keep the bulk of the carrying trade of the world in their hands.Paper read before the Institution of Naval Architects, March 27, 1885.