A propulsor is a mechanical device that gives propulsion.
A propulsor, as can be seen in the accompanying picture has a shroud, which cuts down on blade-tip cavitation and radiated noise. It also has a rotor element and a stator. 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 & stator will typically be odd-numbered, and different prime numbers to avoid standing 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 stage Axial-flow compressor.