Page 229 - Introduction to Marine Engineering

Shafting and propellers 209

simultaneously through an arc to change the pitch angle and therefore
the pitch. A typical arrangement is shown in Figure 11.7.
When a pitch demand signal is received a spool valve is operated
which controls the supply of low-pressure oil to the auxiliary servo
motor. The auxiliary servo motor moves the sliding thrust block
assembly to position the valve rod which extends into the propeller hub.
The valve rod admits high-pressure oil into one side or the other of the
main servo motor cylinder. The cylinder movement is transferred by a
crank pin and ring to the propeller blades. The propeller blades all
rotate together until the feedback signal balances the demand signal and
the low-pressure oil to the auxiliary servo motor is cut off. To enable
emergency control of propeller pitch in the event of loss of power the
spool valves can be operated by hand. The oil pumps are shaft driven.
The control mechanism, which is usually hydraulic, passes through
the tailshaft and operation is usually from the bridge. Varying the pitch
will vary the thrust provided, and since a zero pitch position exists the
engine shaft may turn continuously. The blades may rotate to provide
astern thrust and therefore the engine does not require to be reversed,

Cavitation
Cavitation, the forming and bursting of vapour-filled cavities or bubbles,
can occur as a result of pressure variations on the back of a propeller
blade. The results are a loss of thrust, erosion of the blade surface,
vibrations in the afterbody of the ship and noise. It is usually limited to
high-speed heavily loaded propellers and is not a problem under normal
operating conditions with a well designed propeller.

Propeller maintenance
When a ship is in dry dock the opportunity should be taken to
thoroughly examine the propeller, and any repairs necessary should be
carried out by skilled dockyard staff.
A careful examination should be made around the blade edges for
signs of cracks. Even the smallest of cracks should not be ignored as they
act to increase stresses locally and can result in the loss of a blade if the
propeller receives a sharp blow. Edge cracks should be welded up with
suitable electrodes.
Bent blades, particularly at the tips, should receive attention as soon as
possible. Except for slight deformation the application of heat will be
required. This must be followed by more general heating in order to
stress relieve the area around the repair.
Surface roughness caused by slight pitting can be lightly ground out
and the area polished. More serious damage should be made good by