The screw propeller is still to a great extent an unsolved problem. We
have no definite rule by which we can fix the most important factor of
the whole, namely, the diameter. Mr. Froude has pointed out that by
reducing the diameter, and thus the peripheral friction, we can increase
the efficiency; and this is confirmed by cases--of Iris reduced 2 feet
3 inches, and the Arizona reduced 2 feet. This must, of course, be
qualified by other considerations. The ship has by her form a definite
resistance, and a certain speed is required; if the propeller be made
too small in diameter, the ship will not be driven at the required
speed, except at serious loss in other directions. This question was too
large and complicated to be dealt with here, and should, in the first
instance, be made the subject of careful and extended experiment, on
which a separate paper should be written.
To sum up the whole. Progress has been made during the past nine years,
and in the following particulars:
1. The power of the engines made and making show a great increase. 2.
Speeds hitherto unattainable are now seen to be possible in vessels of
all the various classes. 3. The consumption of fuel is reduced by 13.38
per cent. on the average; and numbers of vessels are now working on much
less coal than that average, while the quality of the coal is in nearly
all cases very inferior, so that it is not unfair to take credit for
20 per cent. reduction. 4. The working pressures of steam are much
increased on the average, and are still increasing; many steamers now
being built for 120 lb. per square inch, while 90 lb. is the standard
pressure now required.
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