140	j<IGHOLLS'S  SEAMANSHIP AND NAt/TICAL KNOWLEDGE
The Pitch of tie screw is the distance it would move the ship ahead
in one revolution, supposing there is no slip, or supposing it to revolve
in a solid instead of in water,
The Slip is the difference between the actual speed of the ship and
the speed of the propeller or engine speed. It is due to the yielding of
the water to the pressure exerted on it by the screw as it forces the ship
ahead. The slip is increased when the wind and sea are ahead, also
when the ship's bottom is foul and her progress through the water
retarded.
The Engine Speed is the rate at which the propeller would drive the
ship if there was no slip.
Example.—Required the speed of a ship, the pitch of propeller
being 18 feet, and making 70 revolutions per minute, no slip.
Speed =Fpitch Xrevs. X 60 minutes-f-ft. in nautical mile
Speed=18 ft. X70 x60-f-6080=1243 knots
Example.—A propeller has a pitch of 20 feet and makes 65 revs, per
minute.    The log registers 10*5 miles in one hour, what is the slip?
Engine speed =pitch Xrevs. X 60—6080
=20 ft. X 65 X 60-4-6080=12-8 knots
Actual slip—engine speed—speed of ship=12'8—10*5=2-3 miles
per hour
Slip is usually expressed as a percentage.
Slip per cent. =actual slip X100--engine speed
„	=2-3 X100—12-8=18 per cent
FUEL CONSUMPTION AND SPEED,
Equations connecting coal consumption and speed are merely
approximate and have been deduced from experiment and experience
as so much depends upon the quality of the coal, the design and efficiency
of engines and boilers, the trim of the ship and the state of the weather.
Bad steering reduces the speed.
The consumption of coal or oil varies approximately as the "cube
of the speed*5 and also as the "speed squared multiplied by the distance"
for moderate speeds up to about 14 knots, after which the ratio of
to speed increases very steeply.
I.	new consumption     new speed3	new displacement*
old consumption """ old speed3 ~~ old displacement*
II,	new consumption     new speed2 X new distance
old consumption ~~ old speed2 X old distance