PROPELLERS
A propeller consists of a boss which has several helicoidal form blades. 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. The thrust block must therefore have a rigid seating or framework which is
integrated into the ship's structure to absorb the thrust. The propeller will usually be either of
the fixed pitch or controllable pitch type. ln addition some special designs and arrangements are
in use which offer particular advantages.
Fixed pitch propeller
Although described as fixed pitch, a solid single-piece cast propeller has a pitch which varies
with increasing radius from the boss. The pitch at any particular point on a blade is however
fixed and an average value for the complete propeller is used in as calculations. A fixed pitch
propeller is shown in Figure 5.40, where most of the terms used in describing the geometrical
features are also given. It should be noted that the face is the surface farthest from the stern and
is the 'working' surface. A cone is fitted to the boss to provide a smooth flow of water away
from the propeller. A
propeller which rotates clockwise, when viewed from aft, is considered to be right-handed.
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.
Fixed pitch propeller
Cavitation is the forming and bursting of vapour filled cavities or bubbles and occurs as a result
of certain pressure variations on the back of a propeller blade. The results of this phenomenon
, 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. The propeller, when turning in the
ship's wake, is a potential source of vibration excitation. To some extent this can be minimised
by having the leading edges skewed back. Skew back is an advantage when the propeller is
working in a varying wake as not all the blade is affected at the same time. Variations in the
thrust and torque are therefore smoothed out. Since the vibrations are blade excited. then the
number of blades is significant and determines the vibration frequency. Where severe vibration
problems exist, it may ultimately be necessary to change the
propeller for one with a different number of blades.
Propeller mounting
The propeller is fitted onto a taper on the tail shaft and a key may be inserted between the two;
alliteratively a keyless arrangement may be used. A large nut is fastened and locked in place on
the end of the tail shaft. A cone is then bolted over the end of the Tail shaft to provide a smooth
flow of water from the propeller.
One method of keyless propeller fitting is the oil injection system. The propeller bore is
machined with a series of axial and circumferential grooves. 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 fastened together.
The pilgrim nut is a patented device which pro vides a predetermined frictional grip between
the propeller and its shaft. With this arrangement the engine torque may be transmitted without
loading the key (where fitted). The pilgrim nut is, in effect, a threaded hydraulic jack which is
screwed onto the tailshaft (see Figure 5 .41). 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 pressured, the Propeller
is drawn off the taper. Assembly and withdrawal are shown in Figure 5.41.
Controllable-pitch propellers
A controllable-pitch propeller is made up of a boss with separate blades mounted into it. An
internal mechanism enables the blades to be moved simultaneously through an arc to change
the pitch angle and therefore the pitch. A typical arrangement is shown
in Figure 5.42.
When a pitch demand signal is received, a spool valve is operated which controls the supply of
low pressure ail ta the auxiliary servo-motor. This moves the sliding thrust block assembly to
position the valve rad which extends into the propeller hub. The valve rod admits high pressure
oil into one side or the other of the main servomotor cylinder. The cylinder movement is
transferred by a crankpin and ring to the propeller blades. The propeller blades rotate together
until the feed-back 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 from the
bridge. Varying the pitch will vary the thrust provided and since a zero-pitch position exists the
A propeller consists of a boss which has several helicoidal form blades. 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. The thrust block must therefore have a rigid seating or framework which is
integrated into the ship's structure to absorb the thrust. The propeller will usually be either of
the fixed pitch or controllable pitch type. ln addition some special designs and arrangements are
in use which offer particular advantages.
Fixed pitch propeller
Although described as fixed pitch, a solid single-piece cast propeller has a pitch which varies
with increasing radius from the boss. The pitch at any particular point on a blade is however
fixed and an average value for the complete propeller is used in as calculations. A fixed pitch
propeller is shown in Figure 5.40, where most of the terms used in describing the geometrical
features are also given. It should be noted that the face is the surface farthest from the stern and
is the 'working' surface. A cone is fitted to the boss to provide a smooth flow of water away
from the propeller. A
propeller which rotates clockwise, when viewed from aft, is considered to be right-handed.
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.
Fixed pitch propeller
Cavitation is the forming and bursting of vapour filled cavities or bubbles and occurs as a result
of certain pressure variations on the back of a propeller blade. The results of this phenomenon
, 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. The propeller, when turning in the
ship's wake, is a potential source of vibration excitation. To some extent this can be minimised
by having the leading edges skewed back. Skew back is an advantage when the propeller is
working in a varying wake as not all the blade is affected at the same time. Variations in the
thrust and torque are therefore smoothed out. Since the vibrations are blade excited. then the
number of blades is significant and determines the vibration frequency. Where severe vibration
problems exist, it may ultimately be necessary to change the
propeller for one with a different number of blades.
Propeller mounting
The propeller is fitted onto a taper on the tail shaft and a key may be inserted between the two;
alliteratively a keyless arrangement may be used. A large nut is fastened and locked in place on
the end of the tail shaft. A cone is then bolted over the end of the Tail shaft to provide a smooth
flow of water from the propeller.
One method of keyless propeller fitting is the oil injection system. The propeller bore is
machined with a series of axial and circumferential grooves. 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 fastened together.
The pilgrim nut is a patented device which pro vides a predetermined frictional grip between
the propeller and its shaft. With this arrangement the engine torque may be transmitted without
loading the key (where fitted). The pilgrim nut is, in effect, a threaded hydraulic jack which is
screwed onto the tailshaft (see Figure 5 .41). 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 pressured, the Propeller
is drawn off the taper. Assembly and withdrawal are shown in Figure 5.41.
Controllable-pitch propellers
A controllable-pitch propeller is made up of a boss with separate blades mounted into it. An
internal mechanism enables the blades to be moved simultaneously through an arc to change
the pitch angle and therefore the pitch. A typical arrangement is shown
in Figure 5.42.
When a pitch demand signal is received, a spool valve is operated which controls the supply of
low pressure ail ta the auxiliary servo-motor. This moves the sliding thrust block assembly to
position the valve rad which extends into the propeller hub. The valve rod admits high pressure
oil into one side or the other of the main servomotor cylinder. The cylinder movement is
transferred by a crankpin and ring to the propeller blades. The propeller blades rotate together
until the feed-back 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 from the
bridge. Varying the pitch will vary the thrust provided and since a zero-pitch position exists the
