Apropulsor is a mechanical device that gives propulsion. The word is commonly used in the marine vernacular, and implies a mechanical assembly that is more complicated than apropeller. TheKort nozzle,pump-jet andrim-driven thruster are examples.
An example propulsor is shown in the accompanying picture. It has a shroud which cuts down on blade-tipcavitation and radiated noise. It also has a rotor element and astator. The stator concentrates the thrust in axial direction and reduces energy wasted in the tangential flow (therefore eliminating torque on the hull). The number of blades in the rotor and stator will typically be two different prime numbers to avoidstanding waves. The blades in the rotor or the stator may be angled to further reduce noise. The physical design and layout is very much similar to a single-stageaxial-flow compressor.
TheVoith Schneider Propeller, also known as acycloidal drive is a specializedmarine propulsion system. It is highly maneuverable, being able to change the direction of itsthrust almost instantaneously. It is widely used ontugs andferries.
From a circular plate, rotating around a vertical axis, a circular array of vertical blades (in the shape ofhydrofoils) protrude out of the bottom of the ship. Each blade can rotate itself around a vertical axis. The internal gear changes theangle of attack of the blades in sync with the rotation of the plate, so that each blade can provide thrust in any direction, very similar to thecollective pitch control andcyclic in ahelicopter.
Anazimuth thruster is a configuration ofship propellers placed in pods that can be rotated on the horizontal plane, making arudder unnecessary. These give ships better maneuverability than a fixedpropeller and rudder system.
Amagnetohydrodynamic drive orMHD propulsor is a method for propelling seagoing vessels using only electric and magnetic fields with nomoving parts, usingmagnetohydrodynamics. The working principle involves electrification of the propellant (gas or water) which can then be directed by a magnetic field, pushing the vehicle in the opposite direction. Although some working prototypes exist, MHD drives remain impractical and exist mostly in the world of science fiction. A magnetohydrodynamic drive was featured in the 1990 filmThe Hunt for Red October, differing from the pump jet propulsion system featured in the Tom Clancy novelThe Hunt for Red October upon which the movie was based.