n
attained from half a knot to a knot higher than those here recorded. No
ship of the class C has yet been officially tried on the measured mile, but
as several are in a forward state, perhaps the actual data from one of them
may shortly be obtained. All these measured mile trials were made under the
usual Admiralty conditions, that is to say, the ships' bottoms and the
screws were clean, and the force of the wind and state of the sea were not
such as to make the trials useless for purposes of comparison. On such
trials the i.h.p. is obtained from diagrams taken while the ship is on the
mile, and the revolutions are recorded by ruechanical counters for the time
occupied in running the mile. Not less than four runs are made during a
trial extending over several hours. The i.h.p. in the table is not
necessarily the maximum during the trial, for the average while on the mile
is sometimes a little below the average for the whole of the trial. The
revolutions are the mean for the two sets of engines, and the i.h.p. is the
sum of the powers of the two sets. The pitch of the screw is measured. The
bolt holes in the blade flanges allow an adjustment of pitch, but in each
case the blades were set as nearly as possible at the pitch at which they
were cast. The particulars given in the table may be taken to be as
reliable and accurate as such things can be obtained, and for each ship
there are corresponding data; that is, the powers, speeds, displacements,
revolutions, pitches, and other items existed at the same time. There are a
few points of detail about these propellers which deserve a passing notice.
In Fig. 1 is shown a fore and aft section through the boss. It will be
observed that the flanges of the blades are sunk into the boss, and that
the bolts are sunk into the flanges. The recess for the bolt heads is
covered with a thin plate having the curve of the flange, so that the
flanges and the boss form a section of a sphere. This method of
construction is a little more expensive than exposed flanges and bolts,
which, however, render the boss a huge churn. With the high revolutions at
which these screws work, a spherical boss is extremely desirable, but, of
course, the details need not be exactly as shown in the illustration. The
conical tail is fitted to prevent loss with eddies behind the flat end of
the boss, and is particularly valuable with the screws of high speed ships.
The light hood shown on the stern bracket is for th