Propeller
Modern ships use screw propellers for
propulsion. When we talk about single screw, this means
that the ship has only one propeller at the stern of the
ship. This is the most common arrangement for cargo ships.
There are also twin screw, triple
screw or even quadruple screw ships. Twin screws are
relatively common while the higher numbered screws are
quite rare, the latter used mostly in high speed warships.
Special arrangements for supporting these propellers are
needed because of the curvature of the ship's hull. The
supporting structures are called "spectacle frames"
because of their shape.
The power to drive the ship is from
the main engine. This can either be a diesel engine or a
steam turbine. The speed of rotation of the propeller must
be quite slow (90 to 150 rpm) in order to avoid cavitation
damage to the propeller blades. The diesel engine is
capable of such low speeds while the steam turbine uses
reduction gears to achieve them.
The propeller blades are shaped in
such a way as to give a smooth flow of water through its
blades. In principle, it behaves like a screw - screwing
ahead or astern as if the water is a solid material like
wood.
Incidentally, the ship moves ahead
or astern by reversing its main engine rotation.
Axial Thrust
The axial thrust produced by the
propeller against the water acts on the thrust bearings of
the intermediate shaft which then transmits the thrust
against the ship's structures to move the ship.
Axial thrust, or fore and aft thrust
is the force which causes a ship to move ahead or astern
through the water. The propeller blades are shaped to give
the most efficiency when moving the ship ahead and less efficiency when going astern.
The reduction in
efficiency in particular turbine powered ships where the
astern power to the shaft may be only 60% of the ahead
power.
Stopping Distance
Stopping distance from full ahead depends largely on axial
thrust, and is important especially in case of emergency
maneuvers. It might be about 6 ship lengths for a 10000
tonne cargo ship but will depend on the type and size of
the ship, the power available and also on factors like
draught and trim. Turbine powered ships take longer to stop
because of the lack of astern power and delay in full
astern revolutions are built up.
The way to stop a ship in the shortest distance from full
ahead in not to put the engines immediately to full
astern. The engine and propeller are strained, the
propeller races without gripping the water, so it is
largely ineffective.
The propeller exerts a greater braking effect if the
engines are first put to slow ahead, then to slow astern
and then to full astern as the ship progressively loses
speed.
Transverse Thrust
The depth of immersion of a
propeller has also an effect on working of the propeller.
Transverse thrust is the sideways thrust of the propeller
blades as they rotate. The upper blades work near the
surface and their transverse effect is not sufficient to
cancel out the opposite effect of the lower blades. The
effect is for right-handed propellers resultant thrust
tends to cant a vessel's stern to the starboard and her
bow to port when the engines are put ahead. When going
astern, the stern cant to port and the bow cants to
starboard. This action cannot be controlled as the rudder
is ineffective when going astern.
Left hand screws will have the opposite action to that
described above.
For controllable pitch propellers the canting effect of
transverse thrust will always be in the same direction,
whether the pitch is set to ahead or astern, because the
shaft always rotates in the same direction.
