A propeller is the most common propulsor on ships, imparting momentum to a fluid which causes a force to act on the ship.
The blades are attached to a boss (hub), which should be as small as the needs of strength allow - with fixed pitch propellers the blades and boss are usually a single casting.
An alternative design is the controllable pitch propeller (CPP), where the blades are rotated normal to the drive shaft by additional machinery - usually hydraulics - at the hub and control linkages running down the shaft. This allows the drive machinery to operate at a constant speed while the propeller loading is changed to match operating conditions. It also eliminates the need for a reversing gear and allows for more rapid change to thrust, as the revolutions are constant. This type of propeller is most common on ships such as tugs where there can be enormous differences in propeller loading when towing compared to running free, a change which could cause conventional propellers to lock up as insufficient torque is generated.
An Azimuth thruster is a configuration of ship propellers placed in pods that can be rotated in any horizontal direction, making a rudder unnecessary. These give ships better maneuverability than a fixed propeller and rudder system. A Z-drive thruster has an horizontal input shaft, vertical shaft in the azimuthing column and a horizontal output shaft with two right-angle gears.
Pneu‐Grip clutches can be found on ships and vessels in marine drives, bow thrusters, main propulsion and propeller shaft brakes as well as dredgers.