The boilers used on board steamships are of two principal types. The
older sort used for steam of comparatively low temperature, viz.: up
to 35 lbs. per square inch, is usually made of flat plates stayed
together, after the manner of the exterior and interior fire boxes of a
locomotive boiler.
Medium high pressure marine boilers, constructed for steam of 60 to
150 lbs. per square inch, are circular or oval in cross section, and
are fitted with round interior furnaces and flues like land boilers.
There are many variations of marine boilers, adapted to suit special
circumstances. Fig. 22 shows a front elevation and partial sections of
a pair of such boilers and Fig. 23 shows one of them in longitudinal
vertical section.