The dimensions and all details of a boiler to be built must be shown on suitable drawings for the guidance of the master boilermaker in laying out and directing the work of building the boiler. Supposing a cylindrical shell boiler is to be built, the drawings supplied must give at least a longitudinal and a transverse cross section of the boiler, besides views of the side and one or both end elevations, and sufficient enlarged views of single or combined parts to show the details of their construction.

From the drawings he proceeds to mark out to full or half size on a smooth laying-out board provided for the purpose, the front end of the boiler. The beginning of this work is shown in Fig.

220. Through the center, O, of the boiler, draw the horizontal and vertical lines as guides for laying down all parts of the boiler head and the openings therein. This board serves also to show on a large scale the relative positions of interior parts of the boiler, particularly the positions of the plates composing the combustion chambers.

The shell of the boiler is composed of two or more rings or courses of plates, each course consisting of one or more plates, as shown in Fig. 221. The plates in each course are butted end to end, and are fastened together by butt straps on both sides of the plates. An outer butt strap is marked in the figure. Two adjacent courses are joined together by overlapping the ends as shown, and the ends of the shell overlap the heads.

Fig. 221. - Double-End Cylindrical Boiler.

Referring to Fig. 220, the full-line circles represent the ends of the inner and outer course plates, and the dotted lines represent the neutral circles of these plates by which their curvatures are figured from the flat plates from which they are made. The lines through the center 0 divide the diagram into quadrants, determining four points of reference for laying out the shell.