In comparing the performances of ships and propellers, much may
be done with simple means, and that without doing violence to
scientific truth, as by assuming that the tow-rope resistance of a
ship varies as the square of the speed and as the wetted skin, as indeed
it does in all well-formed ships for speed, rather below that of full
speed ; that for copper-sheathed ships, such as the old naval frigates
and corvettes, an allowance of 1\ lbs. per square foot at 10 knots may
be taken, in a general way, as fair, although with clean new copper
carefully nailed, 1\ would be sufficient; that the early iron ships
with such paints as were then used probably set up a resistance of l'l
to 1*125 lbs. per square foot ; and that with the modern spirit enamel
paints, freshly put on, an allowance of 1 lb. is enough.
Thab is the rule for —

Tow-rope resistance = WS x \jT\) x f'

S is the speed in knots.
f=l'Q for the best enamel paints.
_/=l'l for the older anti-fouling compositions.
/=1'25 for coppered ships.
THE SCREW PROPELLER. I 29

Tow rope horse-power - WS x \jz ) x/xfeet moved through by

the ship per minute

= WSxS 3 x/t 32,560.
This is the power required to move the ship through the water
without producing heavy waves, and is, of course, the ideal con-
dition. It may therefore be taken as the standard for comparison
when dealing with ships and their machinery and propellers. Hence —

General efficiency . . =Tr. H.P.-M.H.P,

Propeller efficiency . . = Pr. H.P. -i- Net H.P.
Engine efficiency . . ^Net H.P.-fl.H.P.
