Retractable Azimuth Thruster

Propeller and nozzle

Usually a fixed-pitch propeller in a Kort nozzle, chosen for thrust efficiency at low ship speed rather than for open-water propulsion performance.

Seals and hull well

A watertight seal arrangement between the strut and the hull well is critical, since this is the joint that keeps the sea out whether the unit is deployed or stowed.

Selection / Sizing

• Required thrust in kN, set by the vessel's DP capability plot and the environmental conditions it must hold position in.
• Drive power, matched to whether the unit is diesel-electric or hydraulically driven from a shared power pack.
• Retraction time and hull well depth, which affect both the general arrangement and how quickly the unit can be brought into service.
• Redundancy requirement, since DP class notations dictate how many independent thrusters the vessel must carry.

Regulations / Class

Vessels relying on retractable azimuth thrusters for dynamic positioning are assigned a DP class notation by the classification society, which sets redundancy and failure-mode requirements for the thruster system as a whole. Class also surveys the retraction mechanism and seals on the same interval as other steering and propulsion machinery, since a seized retraction system can leave a unit stuck in the deployed position.

Typical faults

• Seal wear at the hull well — slow ingress of water into the strut compartment, often found late because the leak rate is small.
• Hydraulic ram failure on the retraction system — unit stuck deployed, adding drag and vibration at transit speed.
• Slew ring or bearing wear — increased play in the azimuthing motion, degrading DP thrust accuracy.
• Debris ingestion into the nozzle while deployed near the seabed — propeller damage or imbalance.

What to look for in a supplier

• Documented redundancy and failure mode analysis matching the vessel's intended DP class notation.
• Access arrangements for seal replacement without full drydocking, since this is the most frequent maintenance item.
• Track record of the specific retraction mechanism in the sea states the vessel will actually operate in.

Cycle the retraction mechanism on a fixed schedule even when the thruster itself is not needed; a ram that has not moved in months is the one that will not move when DP operations start.