Scientific American Supplement, No. 312, December 24, 1881 | Page 7

wait till the stern of the vessel is out of the water before it acts to check

the engine and reduce the speed. Nothing but the most sensitive, and,
indeed, anticipatory action of the governors can efficiently control
marine propulsion. Instances are on record of vessels having engines
without marine governors being detained by stress of weather at the
mouth of the Thames, while vessels having such governors, of good
design, have gone to Newcastle, have come back, and have found the
other vessels still waiting for more favorable weather.
With respect to condensation in marine engines, it is almost invariably
effected by surface condensers, and thus it is that the boilers, instead of
being fed with salt water as they used to be, involving continuous
blowing off, and frequently the salting up, of the boiler, are now fed
with distilled water. It should be noticed, however, that in some
instances, owing to the absence of a thin protecting scale upon the
tubes and plates, very considerable corrosion has taken place when
distilled water, derived from condensers having untinned brass tubes,
has been used, and where the water has carried into the boiler fatty
acids, arising from the decomposition of the grease used in the engine;
but means are now employed by which these effects are counteracted.
LIGHT ENGINES AND BOILERS.
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.
THE PERKINS SYSTEM.
The second marine engine to which I wish to call your attention is one
that has been made with a view to great economy. The principles
followed in its construction are among those suggested by the President
(Sir W.G. Armstrong) in his address. He (you will remember) pointed
out that the direction in which economy in the steam engine was to be
looked for was that of increasing the initial pressure; although at the
same time he said that there were drawbacks in the shape of greater loss,
by radiation, and by the higher temperature at which the products of
combustion will escape. We must admit the fact of the latter source of
loss, when using very high steam, it being inevitable that temperature
of the products of combustion escaping from a boiler under these
conditions must be higher than those which need be allowed to escape
when lower steam is employed; although I regret to say that in practice
in marine boilers working at comparatively low pressures the products
are ordinarily suffered to pass into the funnel at above the temperature
of melted lead. But with respect to the loss by radiation in the particular
engine I am about to mention--that of Perkins--there is not as much loss
as that which prevails in the ordinary marine boilers, because the
Perkins boiler is completely inclosed, with the result that while there is
within the case a boiler containing steam of 400 lb. on the square inch,
and the fire to generate that steam, the hand may be applied to the
casting itself, which contains the whole of the boiler, without receiving
any unpleasant sensation of warmth. By Mr. Perkins's arrangement,
using steam of 400 lb. in the boiler, it was found, as the result of very
severe trials, conducted by Mr. Rich, of Messrs. Easton and Anderson's
firm, and myself--trials which lasted for twelve hours--that the total
consumption of fuel, including that for getting up steam from cold
water, was just under 1.8, actually 1.79 lb. per gross indicated
horse-power per hour. That gross indicated horse-power was obtained
in a manner which it is desirable should always be employed in
steamboat trials. It was not got by using as a divisor the horse-power of
the most favorable diagram obtained during the