The slips of these screws vary from 10 to 17½ per cent., which is certainly
not an extensive range, considering the widely different working
conditions. Slip, as an indication of the efficiency of the screw, is not
only an interesting subject, but it is often one of importance. In these
ships, however, there is nothing about the slips which would give rise to
any doubts as to the fitness of the screws for their work.

[Illustration: FIG. 5. & FIG. 6.]

The ancient fallacy that small slip meant a high screw efficiency was
supported by the great authority of the late Professor Rankine. Experience
proved that considerable slips and efficient screws were companions. The
late Mr. Froude offered an explanation of this general rule in a paper read
before this Institution in 1878, and gave a curve of efficiency with
varying true slip. In Mr. R E. Froude's paper last year there was a form of
this curve, with an arbitrary abscissa scale for the slip, devised to
illustrate in one diagram the wide conditions covered by his experiments.
In the screws now under consideration, the values of the pitch/diameter
vary only from 1.2 to 1.34, and for these the abscissa values for the same
slips do not differ much. Taking the mean value, and bringing the slips to
a common scale, Fig. 5 is obtained, which would approximately represent the
relation between the efficiency of any one of these screws and its true
slip, if this curve were applicable to full sized screws propelling actual
ships. The slips in Fig. 5 being real or true, are not the slips of
commerce, which are the apparent slips, such as those given in the table.
Let us endeavor to split up these real slips into the apparent slips and
another item, the speed of the wake. We then at 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