no means thorough
agreement as to whether single or twin screws have the greater
propulsive efficiency. What is required to form a sound judgment upon
the whole question is a series of examples of twin and single screw
vessels, each of which is known to be fitted with the most suitable
propeller for the type of vessel and speed; and until this information
is available, little can be said upon the subject with any certainty. So
far the following large passenger steamers, particulars of which are
given in table II., have been fitted with twin screws. It appears t be a
current opinion that the twin screw arrangement necessitates a greater
weight of machinery. This is not necessarily so, however; on the
contrary, the opportunity is offered for reducing the weight of all that
part of the machinery of which the weight relatively to power is
inversely proportional to the revolutions for a given power. This can be
reduced in the proportion of 1 to the square root of 2, that is 71 per
cent. of its weight in the single screw engine; for since approximately
the same total disk area is required in both cases with similar
proportioned propellers, the twins will work at a greater speed of
revolution than the single screw. From a commercial point of view there
ought to be little disagreement as to the advantage of twin screws, so
long as the loss of space incurred by the necessity for double tunnels
is not important; and for the larger passenger vessels now built for
ocean service the disadvantage should not be great. Besides their
superiority in the matter of immunity from total breakdown, and in
greatly diminished weight of machinery, they also offer the opportunity
of reducing to some extent the cost of machinery. A slightly greater
engine room staff is necessary; but this seems of little importance
compared with the foregoing advantages.

TABLE II.--PASSENGER STEAMERS FITTED WITH TWIN SCREWS.

-----------------+-----------+-----+-----------+-------+--------+-------+
| Length | | Cylinders, | Boiler |Indi- |
| between | | two sets in all |pressure|cated |
Vessels. | perpen- |Beam.| cases. | per |horse- |
| diculars. | |-----------+-------| square |power. |
| | |Diameters. |Stroke.| inch. | |
-----------------+-----------+-----+-----------+-------+--------+-------+
| Feet.