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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.
=== Diesel-Electric ===
Diesel-electric propulsion systems are not new. They have been used in submarines, trains and buses. In a diesel-electric system, the engine drives a high output DC generator. The current from this drives one or more electric motors connected to the propeller shaft. The chief advantage of an electrical drive is increased performance throughout the speed range. This results because the engine can be run at its optimum RPM, while the speed of the boat is regulated by the electrical drive.
Much is said about the higher efficiency of mechanical drives, but electric drives are more or less as efficient as mechanical ones. The efficiency of an electric motor is measured in Watts-out/Watts-in, where Watts out are measured in hp x 746. Generally, modern electric motors (NEMA standard) have an efficiency between 87 and 97%, with larger motors being more efficient.
Relevant standards are IEEE 112, JEC 37, and IEC 34-2. The main energy loss in electric drives is through energy transformed to heat.<ref>AC Motor Efficiency Guide, Rockwell Automation, http://www.reliance.com/mtr/b7087_5/b7087_5_3.htm</ref> <ref>Advanced Energy, http://www.advancedenergy.org/progressenergy/motor_efficiency.html</ref>
Diesel-electric drives seem to have evolved at different times in different sectors. Rudolf Diesel, a refrigeration engineer, invented and developed the diesel engine between 1893-97.<ref>About, http://inventors.about.com/library/inventors/bldiesel.htm</ref> In World War I, British C-Class submarines in 1906 were gasoline-electric. The D-Class, in 1908, introduced diesel-electric.<ref>Royal Navy Submarines (D-Class 1908 to 1911), http://members.iinet.net.au/~eadej/dclass.html</ref> In the railroad sector, in the United States, General Electric introduced gasoline-electric locomotives in 1913, switching to diesel in 1917.<ref>The Evolution of the Diesel Locomotive in the United States, Benn Coifman, 1994, the yardlimit</ref>
Electric drives are very reliable, with a MTBF of 25,000 hours (about 2.8 years of continuous running). Brushless (permanent magnet) motors are more robust and are becoming cheaper as patents expire. Commercial trawler systems such as FEYS are warranted for 10,000 hours (1.1 continuous years). Some other factors to consider are lower noise and fuel consumption, and costs.<ref>FAST Electric Yacht Systems, Inc., http://www.feys.org/</ref>
=== Continuously Variable Transmission ===
Continuously Variable Transmissions (CVT) are mentioned here only for completeness. A CVT has a nearly infinite range of gear ratios. Like a controllable-pitch propeller, a CVT allows the engine to operate at its optimum rpm regardless of the speed. Until recently, CVTs were too expensive and unreliable for use in cars. They are unlikely to be adapted for marine use.
== References ==
