5G4 MOLESWORTHS POCKET-BOOK
Propulsion of Vessels. (Millar, ‘Inst. Nav. Arch.,’ 1877.)
“ The reaction of the stream of water acted upon by any
propelling instrument is the product of three factors.
“ fa") The mass of a cubic foot of water. ,
“ m The number of cubic feet acted upon m a second.
“ (c) The velocity in feet per second impressed on the
v*dt*T by the propeller. ’—Rankine.
“ = Speed of the ship in feet per second.
“ — ,, propeller in feet per second.
g L „ btream driven back = P — V = the slip.
A* — Effective area of propeller in square feet. .
N - Number of revolutions per second.
Q = Quantity of water acted upon in a second - A 1.
<; = Gravity = 32. .
T = Thrust in lbs. propelling the snip.
— 64 — 2 A. P S for sea wat er.
g
E = Energy expended = A I'S- for sea water.
Thus while the thrust varies as the velocity, the lost work
varies as the square of the velocity.
Slip of Paddle-wheels.
A = Length of arc of immersed portion of paddle-wheel
(effective circumference).
C — Length of the chord of the arc immersed.
S = Slip of the wheel.
2(A for radial floats.
A
3‘5 for feathering floats.
A
Approximately the slip of radial floats may be taken at 20
per ct nt., and of f athering floats at 15 per cent., on the effec-
tive circumference.
The effective circumference may be assumed to be that due
to th" extreme diameter, less |rdS of the depth of a float.
* From experiments on the performance of some river steamers, it
appears that only about oue-half the actual area of one float ou each
paddle-wheel ia effective.