Z-drive

Note the ducts surrounding the propellers

The Z-drive is a technology that both propels and steers a vessel. Most ships rely on one or more propellers, which are connected to the vessel's engine(s) by a long propeller shaft, and steered by rudder(s) mounted aft of the propeller(s)

A Z-drive has engines, and propeller shafts, but the propeller shafts drives a gear box, that takes a 90 degree turn, through the hull, and enters a tear-drop shaped pod, with another gear box, with another 90 degree turn, that powers a propeller. This propeller is contained with a duct, or shroud. Finally, the tear-drop shaped pod can be turned 360 degrees, for steering the vessel.

Azipods are a technology with a similar appearance and functionality, except the propellers inside their tear-drop shaped pod, are electrically powered, not mechanically powered.