52

MODERN STEAM PRACTICE.

that the main flue is large enough, and that the flame and heatedgases do not meet in opposite directions. As dredgers generallywork in harbours, wliere the water is very muddy, the mud beingstirred up from the bottom by the action of the buckets, smalltubulär boilers should be avoided; the tubes sliould be at least8 inches in diameter, with ample water space between them. Thetubes in such cases are joined to the tube-plates, with a Hange ofangle-iron rivetted to the tube. In this example there are two fur-naces, one at each side of the boiler meeting in a back flue, withreturn tubes at the same level as the furnaces. By this means amplewater above the tubes, and a large steam space, are obtained. Asit is an object to keep the weights low down, and as dredgingvessels are generally shallow, a low boiler should be adopted, placedwell below the deck, to give free passage fore and aft for the moor-ing chains, &c.

For ocean steam ships the multitubulär boiler is decidedly thebest, although some very good examples of flat flue overhead arrange-ments find favour. The tubes vary from 2^4 inches to 4 inches indiameter; and in the merchant Service they are placed over thefurnaces on the return principle. When for moderate power, and

Longitudinal Section. Front Elevation and Transverse Section.

Fig. 26.—Ordinary Tubulär Boiler.

aa, Furnace. b, Combustion chamber. c, Tubes. d, Smoke-box. e, Uptake.

arranged fore and aft, the boiler is generally made in one piece.Some of these boilers have no bottoms, but are simply fitted witha dry plate; while others, made in the usual manner, have dry plateslaid on the bottom of the furnaces, thus preserving the rivet heads