It having been found desirable in steam vessels to economize to the utmost the space occupied by the apparatus, a material difference in the construction of the boilers becomes necessary from those used in land-engines, in order to the economical combustion of the fuel; and in consequence of the different arrangements, marine boilers are more complex, heavy, and costly, than land-boilers of similar power. In general, marine boilers are composed of an external shell or casing, with rectangular sides, stayed together by tie-bars; and within the shell are contained the fireplaces or furnaces, and the flues for the passage of the flame and smoke, the flues being deep rectangular channels, which, after making various turnings, unite in one common chimney or funnel; the furnace, ash-pit, and flues, being entirely surrounded by water. In order to maintain a considerable depth of water over the flues and furnaces, and at the same time afford sufficient space for the accumulation of steam, an elevated compartment, called the "steam-chest," is formed for the purpos upon the top of the boiler, and the bulk of the boiler is occupied with water.
In boilers of large power, and especially in sea-going vessels, in order to prevent the water from accumulating on the lee side of the boiler, and leaving the weathermost flues uncovered by water, the boiler is formed in two or three longitudinal compartments, with a free steam communication between each; or one or two divisions of plate-iron are formed within it, running from front to back, and extending from the bottom upwards, higher than the water line in the boiler. At the bottom of the boiler are pipes, passing through the vessel's sides, and furnished with stop-cocks, or valves, for "blowing off"' the water from time to time, as it becomes saturated with salt; and there are apertures called mud-holes, closed by doors, for the purpose of raking out the mud or other deposits.
We proceed to give a few of the forms of boilers which are employed in steam vessels.
The engravings on the opposite page represent a boiler of 180 horse-power, constructed by Messrs. Fawcett of Liverpool.
From the very great extent of flue through which the smoke and heated air has to pass before it reaches the chimney, these boilers are found to raise steam very rapidly, and with a small consumption of fuel.
The boiler is composed of three separate parts, the sides of the middle boiler serving as a side to each of the side-boilers. At some distance above the water-line, large apertures are cut in the sides of the middle boiler, to form a steam communication between the three compartments, and a water communication is established below by a pipe from each boiler opening into the blow-off main. Each boiler has two fire-places, and the flues a a of the side-boilers branch into the flues b b of the contiguous fire-place of the middle boiler, immediately at the back of the bridge; and, after taking the circuitous course shown in the plan by the arrows, they unite at the back of the boiler, and form one large flue c, from which the chimney d rises at about the middle of the length of the boiler. The flues are not divided horizontally, but extend the whole depth from b to b, as shown in the vertical section. Towards the fore part of the middle boiler is a steam-chest e, from which proceeds the main steam-pipef, which conveys the steam to the cylinders. g is the feed-pipe, and h one of the feed-valves.