Azimuth Thruster
Steerable Z drive and L drive thrusters: Schottel SRP, Kongsberg US series, ducted and open, fixed and controllable pitch, and thruster interaction.
An azimuth thruster is a propulsion unit that rotates about a vertical axis so that thrust can be directed through any heading without a rudder. Drive is transmitted through a Z drive or an L drive gear train, and the propeller may be open or ducted and of fixed or controllable pitch. Named units in tug service include the Schottel SRP series and the Kongsberg US series, formerly Rolls-Royce Commercial Marine until its acquisition in 2019.
In tug applications the thrusters are almost always ducted, and an azimuth stern drive tug with ducted units produces 15 to 17 tonnes-force of bollard pull per 1,000 kW of installed propulsion power. Class rules take account of the geometry rather than the interaction: DNV requires the angular position of turnable propulsion devices to be recorded on a bollard pull trial, which is how a certificate handles the orientation question without quantifying thruster to thruster interference.
The full article will cover gear train arrangements, ducted and open configurations, steering torque and hydraulics, interaction between adjacent units, the underwater mountable variant and its docking implications, and the comparison against vertical axis propulsion.