t once meet with the
difficulty that the wake in which the screw works has not a uniform motion.
Complex, however, as are the motions of the wake, the screw may be assumed
to work in a cylinder of water having such a uniform forward velocity as
will produce the same effect as the actual wake on the thrust of the screw.
It is then readily seen that the real slip is the sum of the apparent slip
and the speed of the hypothetical wake. To make this clear, let V be the
speed of the ship, Vs the speed of the screw, _i.e._, revolutions x pitch,
and V the speed of the wake; then--

Apparent slip = Vs - V.
Real slip = Vs - speed of ship with respect to the wake.
" = Vs - (V - V) = (Vs - V) + Vw.
" = Apparent slip + speed of the wake.

If the apparent slip be zero, the real slip is the speed of the wake, and
if the apparent slip be negative, the real slip is less than the speed of
the wake. The real slip is greater than the apparent slip, and can never be
a negative quantity. From Mr. Froude's model experiments, it appears that
this speed of wake for the A class of ship amounts to about 10 per cent. of
the speed of the A screw. If this value is correct, then the real slip is
(10 + 17.6) per cent., or 27.6 per cent. This is shown in Fig. 6, where O
is the point of no slip, being 17.64 from the point of real slip. Slips to
the right of O are positive apparent slips, slips to the left are negative
apparent slips. The vessel F would certainly have a wake with a speed
considerably less than that of A's wake. From the model experiments, the
wake for F is about one-half that for the A class, or, roughly, 5 per cent.
of the speed of the screw. For the ship F, O is the point of no apparent
slip, and the real slip is (5 + 11.4) or 16.4 per cent. For E, the point of
real slip is approximately the same as for F. For B and D, the positions on
the curve would be about the same. The ship B has a higher speed of wake
than D, but the screw D has the greater apparent slip. The influence of the
number of blades on the scale for the slip has been neglected. If this
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