290

NICHOLLS'S SEAMANSHIP AND NAUTICAL, KNOWLEDGE

working ahead, the lower half was found to be inclined nearly 10°

to port, and the upper half at a slightly less angle to starboard. A

whole undivided rudder, which was free to move in either direction,

was found—when the screw was working ahead—to incline a little to port.

(b) Fwe-and-aft Component.—The after face of the blades is the

pressure surface of the screw when going ahead. The particles of water

are pushed astern past the rudder and give an underway steering effect

immediately the engines are started and before the ship gathers headway.

Screw Race Ejfect —

Going ahead—(a) Transverse component cants stern to port.

(b) Fore-and-aft component gives the steering

effect of headway.

Going astern—(a) Transverse component cants stern to starboard
(b) Fore-and-aft component gives steering effect

of sternway.
SUMMARY WITH RUDDER AMIDSHIPS -

Effect when starting from rest
Effect when going

Ahead
Astern
Ahead
Astern

I. Wake Current
No effect
No effect
Reduces
No effect

Current non-

steering
1

existent

power

Weakly at

first but

strongly as

ship gathers

headway

II. Transverse
Stern to star-
Stern to port
Stern to star-
Stern to port

Thrust
board

board

Strong
Strong
Fairly strong
Strong

III. Screw Race

(a) Transverse
Stern to port
Stern to star-
Stern to port
Stern to star-

Action

board

board

Weak 1
Weak
Weak
Weak

(b) Fore-and-aft
Gives steering efiect
Gives steering effect

Action
of headway '
of sternway
of headway
of sternway

Probable
Head swings
Head goes to
Head goes to
Head goes to

Resultant of
to port
starboard
p<?rt
starboard

L, II., III.,
Can be coun-
Cannot be
Can be coun-
Cannot, as a

acting
teracted by
counteracted
teracted . by
rule, be

together
rudder if
by rudder
rudder if
counteracted

desired

desired
by rudder

„

unless consid-

erable stern-

way is gath-

,
ered

The screw exerts its greatest turning effect when the engines are
going slow ahead or full astern, the radius of the ship's turning circle
increasing with her speed through the water.