Engineering Wonders of the World
Volume I

THE PROPELLING MACHINERY OF A SHIP. 31 Fig. 3.—DIAGRAMS OF SINGLE-ENDED SCOTCH BOILER, SHOWING GRATE G, COMBUSTION CHAMBER D, FIRE-TUBES E, FUNNEL F, AND VALVE H. In the right-hand diagram (end view) the circles a, b, and c indicate the positions of the three furnaces. square inch, while the triple-expansion engine receives it at about 160 lbs., and the quad- ruple-expansion engine at about 200 lbs., or at an even higher pressure. In addition to great economy in steam consumption, the quadruple engine, with its four cranks, offers a more per- fectly “ balanced ” shaft than that given by either a two-crank or three-crank engine, and therefore the quadruple engine reduces vibra- tion to a minimum. Indeed, on the score of vibration, there is little or nothing to choose between the well-balanced quadruple engine and the marine steam-turbine. The arrange- ment of three or four cylinder machinery now universally adopted in screw steamers is to place the inverted cylinders, which are sup- ported by columns from the bedplate, in a line directly over the crank-shaft. And now let us offer a few comments on the types of marine boilers at present in use. The essential difference between the Scotch and the water-tube boiler is that while in the former the heated products of combustion pass through tubes surrounded by water on the outside, in the latter the operation is reversed, the water being inside and thø fire outside the tubes. The chief advantages of the water- tube boiler are its lightness and the quickness with which Scotch v. Water-tube Boilers. it raises steam —qualities which have secured its almost uni- versal adoption in warships at the present day. On the other hand, it requires more attention and upkeep, and does not stand the wear and tear of continuous service so well as the ordinary cylindrical fire-tube or Scotch boiler, which still remains supreme in the mer- cantile marine. The principle of the Scotch boiler will be better understood by reference to Fig. 3. The boiler illustrated is single-ended—that is, fired from one end only, and it lias .i r i • The Scotch three furnaces, a, b, and c, in „ Boiler. which the coal is burned. The ashes fall through the grate, g, whence they are readily removed as occasion requires. At