Page:Cassier's Magazine Volume XV.djvu/413



the period which we have been taking as the beginning of our survey, the marine engine was very heavy for the power developed. This was due to four causes,—the low steam pressure carried; the low piston speed and rate of rotation; the low rate of combustion in the boilers; and the inferior quality of the materials used, as compared with what obtains to-day. The engine was also a very complex machine. For the purpose of securing varying grades of expansion, ingenious but complicated cut-off mechanisms were employed; all the pumps were worked from the main engine; and there was a tendency to run to very intricate castings about the condenser and pumps. This state of affairs did not diminish with the introduction of the compound engine at first, but toward the close of the compound period there was a decided improvement all along the line, due to increase of pressures, increased rotational speeds, and to better material. As has already been stated, the possibility of the successful use of the triple- expansion engine was due to the improved material for boilers.

With the increase in steam pressure, and the number of stages of the expansion, there was an increase of simplicity, due both to the fact that the complicated mechanisms would not work so well under the higher pressures, and that they were no longer necessary, inasmuch as the cylinder ratios themselves provided a sufficient expansion, and any variation needed was secured by the link.

In naval practice one of the greatest improvements was the increase in rotational speeds, which caused a material reduction in weight, and this increase has continued steadily up to the present time. With these fast-running engines it was not so easy to work the pumps from the main engine, and they were gradually made independent until the best naval practice of to-day is to leave for the main engine nothing to do but drive the propeller, the condenser being entirely separate with its pumps, and all other pumps also being worked independently. This has, of course, had its drawback in the fact that the economy of these small engines, which are either simple or compound, is very much less than that of the main engines, and, indeed, the wastefulness of the auxiliaries has become a serious problem. This will be discussed more fully under another head.

Although forced draught for increasing the rate of combustion was used in the United States Navy during the Civil War, it did not come into general use for naval vessels until about 1882, and in the merchant service still later, but since that time its use has become universal. Indeed, were it not for forced draught, boiler weights would be so great as to have long ago set a limit to speeds of the faster classes of vessels. When natural draught alone was used, the maximum rate of combustion with the best free-burning coal and good chimney draught did not reach 20 pounds per square foot of grate. With forced