mheslep said:
I don't mean the old diesel-electrics. The Zumwalt class will (has?) have the propellers driven only by powerful electric motors (46 thousand HP), with the energy for the e-motors coming from combustion driven generators.
http://www.naval-technology.com/projects/dd21/
In the early years of the 20th century, steam turbines were directly connected to the propeller in large ships. This made the turbines into low-speed machines in order to prevent cavitation of the propeller. When combined with the low steam pressures of that era (less than 250 psi), a main propulsion steam turbine for a ship was a huge machine.
Then, someone hit on the idea of putting a reduction gear between the turbine and the propeller, so that the turbine could run at high RPMs without overloading the propeller. The problem was, cutting such large, high powered gears took a lot of machining resources and pushed the engineering envelope of the time.
General Electric, among others, hit on the idea to use the turbines in the ship to drive an alternator, which would supply current to a large AC motor connected to the propeller shaft. The turbine could not only be run at high speed, but it could also be run at a constant speed as well. The AC motor could be designed to turn at low RPMs which the propeller liked, and, by switching the polarity of the motor, could provide full backing power, unlike geared turbine vessels, which can only provide a fraction of ahead power when going astern.
The US Navy like the concept so much, they built a class of three colliers (coal carrying vessels) which were powered with the new turbo-electric power plants. Satisfied with these ships' performance, soon all large US naval vessels, like battleships and battle cruisers, were being designed and built with these new power plants.
http://en.wikipedia.org/wiki/Turbo-electric_transmission
After WWI, there was a cessation of large naval ship construction as the various disarmament treaties took effect. This 'holiday' allowed for more time to develop other improvements in naval ship propulsion, like high pressure boilers and double-reduction gears.
The turbo-electric concept was revived during WW II, when faced with constructing an enormous number of merchant and naval vessels, the US authorities had to make some hard choices about allocating the manufacturing resources available. The decision was to use turbine reduction plants as much as possible in new warship construction. Other types of plant were to be utilized for constructing the new merchant vessel designs.
In the Liberty ship class (over 2700 vessels constructed), a reciprocating steam engine was used to simplify construction. For its new tankers, the USN chose the T2 design (almost 500 built) which revived the turbo-electric power plant. These vessels could use a more modern steam turbine, but did not need to have reduction gears fabricated.
http://en.wikipedia.org/wiki/Liberty_ship
http://en.wikipedia.org/wiki/T2_tanker
Like the Liberty ships, the T2s remained popular after the war for new companies entering the shipping business. The vessels could be purchased rather cheaply, and were economical to operate. Many T2s were converted later in life, with the owners keeping the stern section where the power plant was located, and building and attaching new forebodies to the stern to extend the working life of these vessels.
