CHAPTEfi XIII,
THE EFFECTS OF THE SCREW RACE UPON THE STEERING OF
STEAMSHIPS.
THE turning effect of the rudder depends upon the force and direction
at which the passing flow of water impiDges on the rudder plate, already
referred to under "Parallelogram of Forces/' page 264.
The free flow aft of the water along the ship's side is interfered with
in the vicinity of the rudder by the local currents set up by the revolving
screw and the streamline form of the vessel. Experiments are still
being earned out with balanced, semi-balanced and unbalanced rudders
to discover the ideal streamline shape and the position and the method
of their suspension, so that they may react in the most effective way to
the actual flow of water in their immediate vicinity It is impossible to
determine in practice the ultimate direction of this current as it ia the
resultant of a complicated spiral commotion set up by the rotating
propeller, modified by the horizontal flow of water due to the steerage
way of the ship, so that deductions arrived at by theoretical analysis
are, more or less, of an empirical character.
The seaman is only concerned with the manoeuvring of Ms ship,
her responsiveness to rudder action in turning to port or starboard
when going ahead and going astern. The theoretical steering eflect
may not be exactly the same as that experienced in practice* The
successful pilot finds out what the ship will do under stated conditions
and then avoids, if possible, trying to make her do something else*
We shall discuss the steering forces under three heads:
L The Wake Current.
II. The Transverse Thrust.
III. The Screw Race (a) Transverse Component; (6) Fore-and-aft
Component.
L—The Wake Current.
The Wake Current is the simplest to comprehend. Hold a flat
piece of wood vertically in the water^ pull it forward first edge on and
then side on. Note the current effect, particularly when it is side on,
and observe the hollow, or cavitation, behind the wood and how the
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