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→Hydraulic Z-Drive
There is a bias against hydraulic drives because of their additional complexity, and perceived lack of efficiency and robustness. Hydraulic systems lose energy mainly through torque losses, and are typically 80-85% efficient. This is not quite as efficient as a straight mechanical or an electrical drive. If you have doubts about robustness, go watch some heavy equipment in operation. Z-drives of 4,000-hp and up are routinely fitted to tugs. The only thing against hydraulic drives is their cost.
=== Azipod® ===
An Azipod®<ref>Azipod® is a registered trademark of the ABB Group</ref> (azimuth thruster) is a form of rudderless diesel-electric propulsion. They were developed in 1990 by [http://www.abb.com/ ABB Group], primarily to drive large cruise ships. A pod combines the main electric drive motor and propeller in a casing beneath the ship. Typically the propeller is on the front of the pod. The pod rotates through 360 degrees, providing steering, stern thrusting and reversing.
Like other forms of diesel-electric propulsion, pods reduce fuel consumption because the diesel generator runs at an optimum fixed speed. Turning circles are reduced by as much as 30%.
Pods have also been designed with contra-rotating propellers, one at the front and one at the rear of the pod, to further increase fuel efficiency. Pods have since been used on icebreakers like the [https://en.wikipedia.org/wiki/USCGC_Mackinaw_%28WLBB-30%29 USCG Mackinaw] and on cargo ships. Robust pods have been developed for icebreakers with strengthened propellers to chop the ice. And pods are starting to appear in larger yachts.
== References ==
