The Democratic advocate., 1891-03-21
Triple-Screw Propulsion.

Triple-ScreW Propulsion.
In the course of an illustrated article de-
scribing the new triple-screw steel-proteeed
cruiser No. II, the Iron Age publishes the
following from the pen of Horace See, the
well-known engineering authority;
There is no subject upon which such
diversity of opinion exists as that of screw
propulsion. He who designs propelleror
system of propulsion is likely to meet with
more adverse criticism on his scheme of
work than the poor Indian did on his Ca
noe In the latter case this was confined
to the shape of the boat but in that of the
screw propeller the shape is only one of a
great number of subjects to be criticised.
These multiply and become more compli-
Gated by the addition Of each screw
1t is fortunate that the United States
Government is coming forward to make an
experiment with triple screws, as it is far
better able to suffer loss if the effort is not
a perfectly successful one than the mer-
chant or the builder.
In building this experimental vessel
they may not attain what is expected, but
are sure to secure Il vessel which will be a
valuable addition to the navy, as there are
many advantages to be gained by the use
of the three screws, even il the height of
extreme speed calculated upon is not
reached.
The speed of war vessel varies with
the condition of the service in which she is
engaged. Today at full speed, to overtake
or escape an en9my, or tomorrow at an easy
jog, while making tour Of duty where
time is no object In the first case the end
must be attained at any cost, but in the
| last at the least.
The engine is built to secure the primary
obiect-extreme speed. This means, with
the use of the single screw, a reduction of
revolutions at low speed, and with it pro-
portiOnate increased coal consumption per
horse power employed per hour. Ii is pos
sible with twin screws to improve on this
either by duplicating the engines on each
shaft, discontipuiDg the forward set when
low speeds are desired, and running the
after ones at full speed, or by running the
port or starboad engines separately at full
speed.
With triple screws We can 90 still fur
ther than this. By their use the speed
can be cut down in two ways, either by
running the middle screw alone or either
one Of the outside screws separately at full
speed. The idea of running the middle
screw separately has governed the adoption
of the triple system in the 73OO-ton cruiser
to be built for our navy. Some points,
however, in their application to this vessel
are opca to criticism. We refer to the
position of the outer screws and diameter of
the middle screw. If in dropping or di-
verging the screw the shaft is inclined or
the centres at the wheel made greater than
at the engine, its pitch will vary through
every part of its revolution in proportion
to the amount of inclination, Consequent
ly its action will be irregular and the speed
of the ship retarded,
By making the middle screw less in di-
ameter than the outer ones we shall have
this vessel of 1900 tons displacement pro-
PEELED, if the engine is worked up to its full
natural draught power, by twelve-foot
screw to which about 5000 indicated horse
power will be applied. This will no doubt
act with some loss of efficiency and conse
quent waste IF fuel, so that the end aimed
at will be but partly secured. If, however,
it is correct to employ such small diam
eter here, it is all wrong in the merchant
practice to employ sixteen or seventeen
foot screw to do work of similar charac-
ter.
The rivers of our country, on account of
shadows in some places, present field for
the use of the multiple screw as great as
that on the ocean, so that we shall await
with some interest the building and trial Of
this vessel