Fox news has been running a piece on the newly commissioned USS Makin Island, a big assault carrier. Fox describes a "new" propulsion system using electric motors. It is so efficient that the ship's radius of action is doubled and the cost of fuel is cut in half. Wow. A sea going Prius.
Well, it doesn't work that way. Makin Island's propulsion system is diesel, augmented with gas turbines. The gas turbines are cut in only when full speed is required, the much more economical diesels furnish power for regular cruising. This rig is claimed to be more efficient than regular steam turbines. Probably is, diesels are very efficient. Both diesels and gas turbines are small and light and I would expect the new power plant takes less room below than boilers, stacks, turbines, propeller shafts and reduction gears.
So where does the electric motor come in? This is a retro idea going back to the roaring 1920's. Back then a number of big US warships were built with electric propulsion. Steam turbines turned generators that powered electric motors to drive the propellers. This design eliminated the propeller shafts, the shaft tunnels, and solved the reduction gear and reverse problem. Turbines run at thousands of RPM, whereas big propellers want to run at a hundred RPM or so. Gear boxes doing 10:1 reduction and not breaking while handling 25000 horsepower are expensive and hard to make. Turbines only run in one direction. Electric motors can go from full ahead to full reverse at the flip of a switch, which is very attractive to skippers easing a big ship along side a dock.
The down side to electric drive is salt water. Should battle damage let salt water into the electrical gear bad things happen. Because of this, and improvements in gear boxes, the Navy dropped electric drive by the 1930's, and all the WWII warships had straight mechanical drives. Reverse was handled by an auxiliary reverse turbine. This didn't give full reverse, it gave a reverse thrust only about 25% of full ahead thrust. Skippers were told to dock gently or use tugboats.
Sometime in the 1990's the Navy revived the electric drive idea. I worked on an early project to do this. The motors were huge, fancy, brushless, solid state controlled with the semiconductors handling really massive currents. I asked what the purpose of the electric drive was. Someone told me the Navy had been watching too much Star Trek. They wanted to be able to divert full power from the propellers to the shields or the lasers or the radar or whatever. I got off that project long before it left the laboratory and came anywhere near salt water.
Looks like the electric drive project panned out, and USS Makin Island is equipped with it.