The Z-drive is a specialized form of marine propulsion . With a Z drive a vessel doesn't have conventional propellors 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 aimed 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". However some have an electric motor in the pod.
The Voith-Schneider marine propulsion system can also quickly change the direction of thrust.