screw. The Rattler was fitted with a
dynamometer, which enabled the actual propelling thrust of the screw shaft
to be measured; and the amount of this thrust, multiplied by the distance
through which the vessel passed in a given time, would determine the amount
of power actually utilized in propelling the ship. Both vessels were fitted
with indicators applied to the cylinders, so as to determine the amount of
power exerted by the engines.

583. _Q._--How many trials of the vessels were made on this occasion?

_A._--Twelve trials in all; but I need not refer to those in which similar
or identical results were only repeated. The first trial was made under
steam only, the weather was calm and the water smooth. At 54 minutes past 4
in the morning both vessels left the Nore, and at 30-1/2 minutes past 2 the
Rattler stopped her engines in Yarmouth Roads, where in 20-1/2 minutes
afterward she was joined by the Alecto. The mean speed achieved by the
Rattler during this trial was 9.2 knots per hour; the mean speed of the
Alecto was 8.8 knots per hour. The slip of the screw was 10.2 per cent. The
actual power exerted by the engines, as shown by the indicator, was in the
case of the Rattler 334.6 horses, and in the case of the Alecto 281.2
horses; being a difference of 53.4 horses in favor of the Rattler. The
forward thrust upon the screw shaft was 3 tons, 17 cwt., 3 qrs., and 14
lbs. The horse power of the shaft--or power actually utilized--ascertained
by multiplying the thrust in pounds by the space passed through by the
vessel in feet per minute, and dividing by 33,000, was 247.8 horses power.
This makes the ratio of the shaft to the engine power as 1 to 1.3, or, in
other words, it shows that the amount of engine power utilized in
propulsion was 77 per cent. In a subsequent trial made with the vessels
running before the wind, but with no sails set and the masts struck, the
speed realized by the Rattler was 10 knots per hour. The slip of the screw
was 11.2 per cent. The actual power exerted by the engines of the Rattler
was 368.8 horses. The actual power exerted by the engines of the Alecto was
291.7 horses. The thrust of the shaft was equal to a weight of 4 tons, 4
cwt., 1 qr., 1 lb. The horse power of the shaft was 290.2 horses, and the
ratio of the shaft to the engine power was 1 to 1.2. Here, therefore, the
amount of the engine power utilized was 84 per cent.

584. _Q._--If in any screw vessel the power of the engine be dimi