The Z-drive is a specialized form of marine propulsion. With a Z drive a vessel doesn't have conventional propellers at the end of fixed propeller shafts. Instead they mount propellers on pods that emerge from the bottom of the vessel. The pods can be oriented through a full 360 degrees, allowing rapid changes in the direction of thrust. Z-drive equipped vessels don't need a conventional rudder.
This form of propulsion is called a Z-drive because, when the propeller is mechanically powered, the rotary motion has to make two right angle turns -- thus looking a bit like the letter "Z". In contrast the propulsion system known as an azipod also mounts a propeller at the end of pod that can be oriented through 360 degrees, but the propeller is powered by an electric motor in the pod itself.
The maneuverability provided to vessels by the Z drive system can be particularly useful to ferries, harbor tugs and fireboats.
The Voith-Schneider marine propulsion system can also quickly change the direction of thrust.
Vessels equipped with these kinds of propulsion systems don't need conventional rudders.