There are two major variants, based on the location of the motor:[citation needed] The most powerful podded thrusters in use are the four 21.5 MW Rolls-Royce Mermaid units fitted to RMS Queen Mary 2.
Underwater-mountable thrusters are used as dynamic positioning propulsion for very large vessels such as semi-submersible drilling rigs and drillships.
[citation needed] English inventor Francis Ronalds described what he called a propelling rudder in 1859 that combined the propulsion and steering mechanisms of a boat in a single apparatus.
[2] The modern azimuth thruster using the Z-drive transmission was invented in 1951 by Joseph Becker, the founder of Schottel in Germany, and marketed as the Ruderpropeller.
[4] In the late 1980s Wärtsilä Marine, Strömberg and the Finnish National Board of Navigation developed the Azipod thruster with the motor located in the pod itself.