Pod Drives and Rotating Propeller Housings

An electric pod drive places the propulsion motor in a housing beneath the ship’s hull. The motor sits directly beside the propeller; on a steerable pod, the housing can rotate around the vertical axis to direct the propeller thrust. This combines propulsion and steering in a single unit. Power generation remains separate: onboard generators supply the electricity.

How a pod drive is built

The Queen Mary 2 illustrates how propulsion units and the power plant work together. In its 2001 construction description, Wärtsilä specified four electrically driven pods for the ship: two fixed and two steerable. A combined diesel and gas turbine power plant with an output of 115.5 megawatts was to supply power for propulsion and onboard systems. For redundancy, the diesel generators are divided between two separate engine rooms. The pods produce the propeller thrust; the generators provide the electrical power required for it.

How it differs from shaft drive

In a conventional shaft system, a long shaft transmits the rotation of a motor inside the ship to the propeller; a separate rudder steers the ship. The shaft carries the power along the ship’s longitudinal axis to the stern. A steerable pod, by contrast, directs the propeller thrust itself. ABB describes its Azipod system as an electric drive with a motor pod that rotates through 360 degrees and a pulling propeller. In ABB’s Azipod system, the motor in the housing drives the propeller directly, eliminating the need for a long shaft to transmit power from the engine room to the stern. According to the manufacturer, the pulling propeller reduces water resistance and allows the motor to operate at an advantageous load. This design is one of the different electric marine propulsion systems that shape the design of modern cruise ships.

Uses and limitations

The ability to direct the thrust can make maneuvering easier. For Azipod, ABB cites tighter turning circles for cruise ships, pulling propellers on icebreakers, and precise positioning for offshore vessels. ABB says Azipod can save up to 20 percent in energy compared with conventional shaft systems. On cruise ships, the choice of propulsion system depends on the ship’s design and intended use. The number, arrangement, and power of the pods, together with the hull shape and power plant configuration, determine how a particular ship can maneuver and be propelled.