efficiency curve were applicable to full sized screws propelling actual
ships, and if the determination of the wakes were beyond question, then we
should have a proof that our screws were at or near the maximum efficiency.
But, as we know, from the total propulsive efficiencies, that the screws
have high and not widely different efficiencies on these ships, we may
argue the other way, and say that there is good reason to consider that at
least the upper part of the curve agrees with experience obtained from
actual ships. Now take Fig. 6 and consider the general laws there
represented. Take the speed of the wake as 10 per cent. of the speed of the
screw, which is probably an average of widely different conditions,
including many single as well as twin screw ships. Then this curve shows
that considerable negative slips mean inefficient screws; that screws may
have very different positive slips without any appreciable difference in
their efficiencies; and that very large positive slips and inefficient
screws may be companions. For instance, a screw with a large positive slip
in smooth water is frequently inefficient at sea against a head wind, which
increases the resistance, and necessitates an increase of slip. I venture
to say that these statements, taken in a general manner, are not at
variance with experience obtained from the performances of screw ships.
Before it is possible to satisfactorily decide if this curve applies in a
general manner to full sized screws propelling ships, we require the
results of trials of various ships where the screws are working about the
region of no slip. Model experiments teach that the scale for the slip
varies with the design of the screw, and that with a given screw the speed
of the wake (which decides the point of no apparent slip) varies with the
type of ship and with the position of the screw with respect to the hull.
Remembering these disturbances, it is not improbable that it may be
possible to account for or explain what at first sight may appear
departures from the curve. The diameters of the screws in the table are not
compared with the diameters given by the method explained by Mr. Froude in