ACTION OP RUDDE& AND PROPELLER 291
The churning of the crater is greatest when starting the engines
from rest, particularly when going astern* and especially so when
reversing from full ahead to full astern. The excessive vibration then
felt throughout the ship is dae to captation, the blades following each
other in quick succession into the same "hole" before the water has time
to effectively fill the cavity.
In view of the importance of reversing the engines when going ahead,
the following extracts are taken from the Report of a Committee of the
British Association appointed to discover the best rules for the guidance
of ships' captains in endeavouring to avoid collisions (see White's Naval
Architecture)
"The distance required by a screw steamer to bring herself to rest
from full speed by a reversal of her screw . . . generally lies
between four and six times the ship's length. It is to be borne
in mind that it is to the behaviour of the ship during this interval
that the following remarks apply:—
"It is found an invariable rule that during the interval in which a
ship is stopping herself by the reversal of her screw, the rudder
produces none of its usual effects to turn the ship; but that under
these circumstances the effect of the rudder, such as it is, is to
torn the ship in the opposite direction from that in which she
would turn if the screw were going ahead. The magnitude of
this reverse effect of the rudder is always feeble, and is different
for different ships, and even for the same ship under different
conditions of lading.
4'It also appears from the trials that, owing to the feeble influence of
the rudder over the ship during the interval in which she is
stopping, she is then at the mercy of any other influences that
may act upon her. Thus the wind, which always exerts an
influence to turn the stem or forward end of the ship into the
wind, but which influence is usually well under control of the
rudder, may, when the screw is reversed, become paramount, and
cause the ship to turn in a direction the very opposite of that
which is desired. Also the reversed screw will exercise an influ-
ence which increases as the ship's way is diminished, to turn the
ship to starboard or port, according as it is right or left-handed;
this being particularly the case when the ships are in light
draught.
"These several influences, the reversed effect of the rudder, the
effect of the wind, and the action of the screw, will determine