direction of its axis
beneath the surface, without giving motion to the superincumbent water, and
the inertia of this superincumbent water must, therefore, be taken into the
account. In the experiment upon the Minx, the depth of this superincumbent
column was but small. The total amount of the slip was 36.53 per cent.; and
there will not be much error in setting down about one half of this as due
to the recession of the water in the direction of the vessel's track, and
the other half as due to the lateral penetration of the screw blades.

601. _Q._--Is it not important to make the stern of screw vessels very
fine, with the view of diminishing the slip, and increasing the speed?

_A._--It is most important. The Rifleman, a vessel of 486 tons, had
originally engines of 200 horses power, which propelled her at a speed of 8
knots an hour. The Teazer, a vessel of 296 tons, had originally engines of
100 horses power, which propelled her at a speed of 6-1/2 knots an hour.
The engines of the Teazer were subsequently transferred to the Rifleman,
and new engines of 40 horse power were put into the Teazer. Both vessels
were simultaneously sharpened at the stern, and the result was, that the
100 horse engines drove the Rifleman, when sharpened, as fast as she had
previously been driven by the 200 horse engines; and the 40 horse engines
drove the Teazer, when sharpened, a knot an hour faster than she had
previously been driven by the 100 horse engines. The immersion of both
vessels was kept unchanged in each case; and the 100 horse engines of the
Teazer, when transferred to the Rifleman, drove that vessel, after she had
been sharpened, 2 knots an hour faster than they had previously driven a
vessel not much more than half the size. These are important facts for
every one to be acquainted with who is interested in the success of screw
vessels, and who seeks to obtain the maximum of efficiency with the minimum
of expense.[1]

[1] See Treatise on the Screw Propeller, by John Bourne, C. E.

PROPORTIONS OF SCREWS.

602. _Q._--In fixing upon the proportions of a screw proper to propel any
given vessel, how would you proceed?

_A._--I would first compute the probable resistance of the vessel, and I
would be able to find the relative resistances of the screw and hull, and
in every case it is advisable to make the screw as large in diameter as
possible. The larger the screw is, the greater will be the efficiency of
the engine in p