I wish, before quitting this section of my subject, to call your
attention to two very interesting but very different kinds of marine
engines. One is the high-speed torpedo vessel, or steam launch, of
which Messrs. Thornycroft's firm have furnished so many examples. In
these, owing to the rate at which the piston runs to the initial
pressure of 120 lb. and to very great skill in the design, Messrs.
Thornycroft have succeeded in obtaining a gross indicated horse-power
for as small a weight as half a cwt., including the boiler, the water
in the boiler, the engine, the propeller shaft, and the propeller
itself.
To obtain the needed steam from the small and light boiler, recourse
has to be made to the aid of a fan blast driven into the stoke-hole.
From the use of a blast in this way advantages accrue. One is, as
already stated, that from a small boiler a large amount of steam is
produced. Another is that the stoke-hole is kept cool; and the third
is that artificial blasts thus applied are unaccompanied by the
dangers which arise, when under ordinary circumstances the blast is
supplied only to the ash-pit itself.