The marine boiler of to-day is in all its main features the same as it
was ten years ago. The single-ended boiler, made with two, three, and
sometimes four furnaces, is the simplest form, and for all powers
under 500 indicated horse power is the most generally adopted. The
double-ended form is largely used. It has been found more economically
efficient than the single-ended form, by as much as ten per cent, in the
writer's own experience. It is generally adopted for engines of large
power, but for small power is inconvenient, owing to its occupying more
room lengthwise in the vessel, and also involving two stokeholds and
therefore more supervision. At one time great difficulty was found
in keeping the bottoms of boilers of this kind tight. Owing to their
length, the unequal expansion due to different temperatures at the
top and bottom caused severe racking strains on the bottom seams and
riveting--so severe in some cases as to rend the plating for a large
part of the bottom circumference of the shell. This difficulty has now
been to a large extent got over, in consequence of the greater attention
given to the form and direction of the water spaces in the boiler
itself, so as to induce circulation of water; the introduction of the
feed-water at the top instead of near the bottom; the more careful
management now usual on the part of engineers; and lastly, the use of
larger plates, welded horizontal seams, drilled rivet holes, and more
perfect workmanship throughout. A modification of double-ended boiler is
that introduced by Mr. Alfred Holt. It has many decided advantages,
but is costly to make. The formation of the two ends into separate
fire-boxes leaves the bottom of the boiler free to adapt itself to the
variations of temperature to which it is exposed. The separation of the
furnaces from the combustion chamber, excepting through the opening
afforded by a connecting tube, is an advantage in the same direction,
and avoids almost entirely the racking strains due to irregular furnace
action. The weight of water carried is less, and that of the boiler
may also be made less; while the elliptical form of the two ends gives
greater steam space.
A type of boiler largely used in her Majesty's Navy, somewhat like a
locomotive boiler, is highly efficient in regard to weight and power
developed. Many examples have yielded one indicated horse-power in the
cylinders for every three square feet of heating surface, under natural
draught and with a very moderate height of funnel; and this with a
consumption of fuel not exceeding 2½ lb. per indicated horse-power per
hour under a working pressure of 60 lb. With the aid of a steam jet in
the funnel, the heating surface per indicated horse-power has fallen
below 2½ square feet. The large water surface afforded for escape
of steam secures almost entire freedom from priming, without the
incumbrance of steam domes; and the large combustion chamber allows of
the thorough combustion of the gases before their passage through the
tubes. The locomotive type of boiler has lately occupied the writer's
attention, with a view to its more definite introduction into marine
work. The difficulties, however, which lie in the way of applying it to
steamers going long voyages are very great. The principal difficulty
lies in the necessity of burning a large quantity of fuel in a very
limited space and time. This can only be done either by direct pressure
or exhaust action applied at the furnace. In other words, we must either
exhaust the funnel, which will absorb a large amount of power, but would
be comparatively easy of application; or our stokers, as is the case
with our miners, must work under a pressure of air.