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 the purpose of preventing
eddies behind the boss of the stern bracket, and to save the resistance of
the flat face of the screw boss. The edges of the blades are cast sharp,
instead of being rounded at the back, with a small radius, as in the usual
practice--the object of the sharp edge being the diminution of the edge
resistance. The driving key extends the whole length of the boss, and the
tapered shaft fits throughout its length.

[Illustration: FIG. 1.]

These points of detail have been features of all Admiralty screws for some
years.