Page 254 - Introduction to Naval Architecture

PROPULSION 239

number. Compared with non-cavitating conditions values of all three
parameters fall off at low advance coefficient, the loss being greater the
greater the cavitation number.
When cavitation is present the propeller can be viewed using a
stroboscopic light set at a frequency which makes the propeller seem
stationary to the human eye. Photographs can be taken to illustrate the
degree of cavitation present. A similar technique is used in propeller
viewing trials at sea when the operation of the propeller is observed
through special glass viewing ports fitted in the shell plating.
The propeller, particularly when cavitating, is a serious noise source.
It would be useful to be able to take noise measurements in a cavitation
tunnel. This is not possible in most tunnels because of the background
noise levels but in recent years a few tunnels have been built which are
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suited to acoustical measurements.

OTHER PROPULSOR TYPES

So far attention has been focused on the fixed pitch screw propeller as
this is the most common form of propulsor. Others are described
briefly below.

Controllable pitch propeller
The machinery must develop enough torque to turn the propeller at
the revolutions appropriate to the power being developed or the
machinery will lock up. This matching is not always possible with fixed
blades and some ships are fitted with propellers in which the blades can
be rotated about axes normal to the drive shaft. These are termed
controllable pitch propellers (CPPs). The pitch can be altered to satisfy a
range of operating conditions which is useful in tugs and trawlers. For
such ships there is a great difference in the propeller loading when
towing or trawling and when running free. The machinery can be run
at constant speed so that full power can be developed over the range of
operating conditions.
The pitch of the blades is changed by gear fitted in the hub and
controlled by linkages passing down the shaft Thus the GPP has a
larger boss than usual which limits the blade area ratio to about 0.8
which affects cavitation performance. It is also mechanically fairly
complex which limits the total power that can be transmitted. By
reversing the pitch an astern thrust can be produced thus eliminating
the need for a reversing gear box. Variation in thrust for manoeuvring
can be more rapid as it only involves changing blade angle rather than
shaft revolutions, but for maximum acceleration or deceleration there
will be an optimum rate of change of blade angle.