cover. The check valve discharges PA into
at the ECSS operating station. The
drilled passages in the hub cone end cover.
resistance across the shaped potentiometer
The end cover passages connect with passages
from end to end is about 15 kilohms. The
in the hub cone and hub body to direct
word shaped is used to describe this poten-
the PA to the center post of each blade.
tiometer because the resistance is not evenly
The PA flows from the center post through
distributed along the potentiometer body.
a bushing connection to a drilled passage
Slightly more resistance (ohms per inch of
in the root of the blade. The drilled passage
potentiometer) is at the ends than in the
directs the air to a machined channel under
middle. This nonlinear resistance distribution
the leading edge. From the channel, the
is necessary to compensate for the increased
PA discharges into the water through small
sensitivity of the propeller pitch to the valve
orifices drilled in the blade edge.
rod positioning at the extreme ahead and
astern pitch settings. As maximum ahead
Two check valves in the PA tubing pre-
or maximum astern pitch is approached, a
vent entry of seawater into the system when
lesser change in valve rod position is needed
t h e PA is not being used. The primary
to obtain a certain pitch change. More change
check valve is installed in the hub cone
is needed if the blades are closer to zero
end cover as mentioned previously. It limits
pitch. This is caused by the decreased angle
seawater backflow to the propeller blades
between the valve rod and the lever arm
and hub cone. A backup check valve is
f o r m e d by the center post of the blade
located in the OD box at the connection
and the sliding block crankpin. So, an in-
between the PA tubing and the rotoseal.
crease in potentiometer resistance is necessary
Should the primary check valve fail, the
at both ends to reflect the same pitch change
backup check valve will prevent seawater
on the electronic readout devices with less
from passing through the rotoseal and filling
valve rod change (equal to follow-up rod
the shell of the PA cooler.
change).
PROPULSION SHAFTING SYSTEM
PRAIRIE AIR SYSTEM
The propulsion shafting system consists of the
Prairie air is emitted from small holes drilled
propulsion shafts, the line shaft bearings, the strut
in the leading edges of the CRP propeller blades.
bearings, the stern tube bearings, the bulkhead
The discussion presented here deals only with the
seals, and the stern tube seals. The following
PA flow in the OD box, along the propulsion
description of the propulsion shafting equipment
shaft, and from the hub assembly to the propeller
is representative of types found aboard the various
blade leading edges.
gas turbine ships. Consult the manufacturers'
technical manuals for specific information
PA is piped from the PA cooler to the
concerning your ship.
r o t o s e a l on the forward end of the OD
box. The rotoseal passes air from the sta-
tionary external piping to the rotating PA
SHAFTING
tube in the center of the OD box piston
assembly. The PA tube consists of sections
The shaft transmits propulsion torque to the
of seamless, stainless steel tubing connected
propeller and thrust to the Kingsbury thrust
by welded couplings containing O-ring seals.
bearing. In addition, the shaft is designed to
The tube runs aft through the centers of
accommodate the CRP propeller hydraulics and
the OD box piston assembly, the bearing
PA tubing. The PA tubing runs through the
support, the distance tube, and the valve
propulsion shaft at the center line. The center line
rod to the propeller hub assembly. The tube
is surrounded by the propeller valve control rod,
is held in the center of the valve rod by
as seen in figure 8-28. High-pressure hydraulic oil
welded-on guides that support the tube away
from the HOPM is supplied to the propeller hub
from the valve rod inner wall. At the aft
through the interior of the valve rod. Oil returns
end of the hub, the PA tube terminates
from the hub to the sump tank in the annular
with a check valve in the hub cone end
8-44
