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Function of solid fixed pitch propeller & Propeller mounting
The propeller consists of a boss with several blades of helicoidal form
attached to it. When rotated it 'screws' or thrusts its way through the
water by giving momentum to the column of water passing through it.
The thrust is transmitted along the shafting to the thrust block and
finally to the ship's structure.
A solid fixed-pitch propeller is shown in Figure above. Although
usually described as fixed, the pitch does vary with increasing radius
from the boss. The pitch at any point is fixed, however, and for
calculation purposes a mean or average value is used.
A propeller which turns clockwise when viewed from aft is considered
right-handed and most single-screw ships have right-handed propellers.
A twin-screw ship will usually have a right-handed starboard propeller
and a left-handed port propeller.
The propeller is fitted onto a taper on the tailshaft and a key may be
inserted between the two: alternatively a keyless arrangement may be
used. A large nut is fastened and locked in place on the end of the
tailshaft: a cone is then bolted over the end of the tailshaft to provide a
smooth flow of water from the propeller.
Fig: A solid fixed-pitch propeller
One method of keyless propeller fitting is the oil injection system. The
propeller bore has a series of axial and circumferential grooves
machined into it. High-pressure oil is injected between the tapered
section of the tailshaft and the propeller. This reduces the friction
between the two parts and the propeller is pushed up the shaft taper by a
hydraulic jacking ring. Once the propeller is positioned the oil pressure
is released and the oil runs back, leaving the shaft and propeller securely
Fig: Pilgrim Nut Operation
The Pilgrim Nut is a patented device which provides a predetermined
frictional grip between the propeller and its shaft. With this
arrangement the engine torque may be transmitted without loading the
key, where it is fitted. The Pilgrim Nut is, in effect, a threaded hydraulic
jack which is screwed onto the tailshaft (Figure above). A steel ring
receives thrust from a hydraulically pressurised nitrile rubber tyre. This
thrust is applied to the propeller to force it onto the tapered tailshaft.
Propeller removal is achieved by reversing the Pilgrim Nut and using a
withdrawal plate which is fastened to the propeller boss by studs. When
the tyre is pressurised the propeller is drawn off the taper. Assembly
and withdrawal are shown in Figure above.
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.
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
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
welding and subsequent heat treatment. A temporary repair for deep
pits or holes could be done with a suitable resin filler.
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