288 NICHOLLS S .SEAMANSHIP AND NAUTICAL KNOWLEDGE
water swirls round the edges and follows up behind ;n an endeavour
to fill the cavity This follow up is called the Wake Current •
The same effect is produced by a vessel with or without a propeller
when being towed. She drags "dead" water after her to fill up the hole
caused by the volume she has displaced. This current is strongest
at the surface and gradually diminishes to zero strength at the keel
level; it is non-existent when the vessel is stopped and increases
in velocity as the ship's speed increases The wake current is more
pronounced in vessels of full form than in those of fine lines This is
the chief reason why flat-bottomed, square-sterned barges are dime ult
to steer as the wake current following up behind is strong and neutralises
more or less the effect of the flow aft due to the vessel's headway.
Effect.—
Going ahead—The wake current reduces the steering power of
the rudder.
Going astern—No effect; it does not exist at the stern.
II.—Transverse Thrust.
The obliquity of the blades sets up a current which may be resolved
into a transverse component and a fore-and-aft component; the transverse
force or thrust exerted by the screw is, however, small m comparison
to the fore-and-aft force which drives the ship ahead.
Effect of Right-handed Propeller.—
Going ahead—^Stern cants to starboard.
Going astern—Stern cants to port briskly.
The athwarthship component of the obliquity of the screw does not
account for the slewing of the stern as, in theory, the thrust to one
side should be equal to the thrust to the otheu side because the upper and
lower blades pass across equally ea'ch way in the course of a revolution,
the pressure low down being practically equal and opposite to the
pressure high up.
E it the propeller churns and breaks up the water near the surface
to a greater extent than deeper down, with the result that the lower
blades cut through more solid water and have to overcome greater
resistance than the upper ones. Think of a propeller half immersed
The upper half cuts through air, the lower half through water, and the
difference of these transverse pressures slews' the stern to starboard
when going ahead, but more briskly to port when going astern, as fcne
forward flow of water from the propeller washes up against the JavJl
and retards the stern way of the ship, thus reducing the lore-and-art
component relatively to the transveise component.