propeller, propulsion, ship, screw, propeller blades, thrust bearings, shaft, thrusts, emergency maneuvers, stopping distances, ahead, astern, braking, port, starboard

Propeller

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propeller, propulsion, ship, screw, propeller blades, thrust bearings, shaft, thrusts, emergency maneuvers, stopping distances, ahead, astern, braking, port, starboard


Propeller of Tugboat


Intermediate Shaft that is connected to the propeller shaft

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.

 

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