SCREW RACF 289
HI.—The Screw Race.
The screw worms its way through the water and, in doing so, creates
two spiral currents which corkscrew or curl sharply round each other
and criss-cross at different levels in the screw aperture whilst, at the
same time, the body of rotating water is driven astern by the thrust
impulse imparted to it by the after side of the blades when the engines
are going ahead. The twisting particles of water may be resolved into
two components, transverse and fore and aft.
Figure 1 is an effort to illustrate the spiral flow of the layers of
water acting against the rudder. Imagine the port blades of the propeller
to be coming up and the starboard blades to be going down, as they do
when going ahead with a right-handed screw
The downstroke to starboard induces and urges the water to pass
diagonally across the aperture from starboard to port under the boss
and to curl sharply upwards and back again, washing against the port
upper half of the rudder in its endeavour to get back to the starboard
side of the ship. That is to say, the water enters on the starboard
side, crosses to port below the boss, curls up again and is discharged
higher up against the top Half of the port side of the rudder.
This transverse component acting on the rudder pushes the stern
to starboard.
Simultaneously, the upstroke to port induces and throws the water
diagonally across the aperture from port to starboard above the boss,
the spiral impulse causing the water to curl sharply downwards and back
again and to wash against the starboard lower half of the rudder in
its endeavour to get back to its port side of the ship.
That is to say, the water enters on the port side, squelches across
to starboard above the boss, and is discharged against the starboard
side of the rudder low down when finding its way back to port.
This tranverse component acting on the rudder pushes the stern
to port.
So here we have a double transverse effect acting simultaneously
on opposite sides of the rudder, a flow of water acting on its starboard
side low down and another flow acting on its port side high up; but
the deeper flow acting on the starboard lower half predominates and
urges the ship's stern to port.
The rotary effect of the screw race has been demonstrated by an
interesting experiment, in which a Judder was horizontally divided into
two parts—an upper and lower half—each part being free to move
in response to any influences. When the screw (right-handed) was