Because the engines did not have to be placed in line with the motors actually driving the propellers, they could be spread out in such a way that the ship could not be disabled by a single hit. For example, the power plant could be divided (at least in theory) between a portion in the conventional below-water position (where it was reasonably well protected against above-water attack) and a portion above water, hence better protected against underwater attack. Such dispersion did entail a larger ship and some problems of routing uptakes and downtakes, and it has not yet been attempted – but it is probably in the cards if navies become more interested in survivability. Electric power could be routed to the motors along several paths, further improving the ship’s ability to survive damage. This seems to have been done in the new Zumwalt. Quite aside from that, eliminating a long propeller shaft would eliminate an important vulnerability, in that the shaft itself could be distorted by underwater damage (such as shock). If it kept turning, it would tear up the ship’s bottom. If that seems a remote consideration, remember that a bent shaft that kept turning helped considerably to sink the British battleship Prince of Wales in 1941 by opening up the ship’s hull like a sardine can.

At the least, the generator-motor combination eliminates the gearing standard in gas turbine ships. Gearing is essential because a gas turbine spins at high speed, far above the speed at which a propeller can turn (the U.S. Navy initially adopted turbo-electric drive for this reason, as an alternative to the gearing that other navies tried). It is also inherently noisy. That noise can be eliminated by isolating the gearing from the hull, using the sort of raft that submarine power plants have, but any such installation is expensive and space-consuming. The French adopted electric drive for their nuclear attack submarines specifically to avoid the size (hence cost) penalty associated with silenced geared turbines. For that matter, the U.S. Navy built the unfortunate Glenard P. Lipscomb for silencing, but she turned out larger, rather than smaller, than geared turbine submarines of similar type (and also far less reliable, hence the prejudice against electric drive in the 1980s).

Without the usual propeller shafts, there would be less reason to locate all of a ship’s propellers at the stern. Thus it would be possible to spread out the ship’s propulsion so that, for example, she could keep running even if her stern were blown off.  This was a serious idea in the late 1980s, when survivability in the face of Soviet attack was considered essential, but no navy has taken so radical a step since then. For example, the Zumwalt design shows conventional propeller shafts driven by motors in more or less the positions typically occupied by a ship’s prime movers.

Electric drive also simplifies the ship’s internal arrangement. When the Spruance-class destroyers were fitted with vertical missile launchers, they received cells only forward of their bridges. The considerable deck space aft could not be used because the bottoms of the launch cells would have blocked the ship’s propeller shafts.

Protection was why the U.S. Navy became interested in electric drive just before World War I. Instead of placing a ship’s steam turbines in tandem with her propeller shafts, the Navy was able to place them on the centerline, as far as possible from any underwater hit. They were surrounded by spaces containing the boilers, and then by layers of underwater protection. The result was probably the best degree of underwater protection in the world, although it was vulnerable to shock, which might cause circuit breakers to jump (the carrier Saratoga was once disabled that way).

Electric drive also simplifies the ship’s internal arrangement. When the Spruance-class destroyers were fitted with vertical missile launchers, they received cells only forward of their bridges. The considerable deck space aft could not be used because the bottoms of the launch cells would have blocked the ship’s propeller shafts. The ship received about half as many missile launchers as a Burke-class destroyer of very similar size. This consideration does not apply to the Zumwalt, which carries its missile cells along the sides of its hull, but it was an important argument when the idea of electric drive was first being pressed within the U.S. Navy in the late 1980s.

Electric drive also offers a subtler advantage. By disconnecting the prime mover from the propeller or propulsor, it much simplifies the replacement of the prime mover. For example, from time to time, it is suggested that fuel cells can be far more efficient than gas turbines; they also seem not to entail the same sort of thermal signature. It would be relatively simple to replace gas turbines driving generators with fuel cells, assuming that the latter could provide a similar output.

In a submarine, the seal between hull and propeller shaft is difficult to design, particularly if the submarine is to operate at greater and greater depths. At least in theory, an electric motor could be designed that could be mounted entirely outside the submarine’s pressure hull. Instead of a penetration for a propeller shaft, cables could be passed through the hull from generator to motor. Non-nuclear submarines have used electric motors for years for underwater propulsion, but not in this way; the great bulk of the world’s nuclear submarines are driven by geared turbines (the French seem to be the only exception, and they use motors driving shafts passing through the hull in the usual way). If deeper diving is important in future, or even if it makes sense to simplify hull design by eliminating the penetration for the propeller shaft, then this kind of electric drive becomes attractive. It becomes even more interesting when it is pointed out that modern periscopes no longer penetrate the hull, since they send their data in via fiber optics. Again, this is not an academic point. In the late 1950s, the diving depth of U.S. submarines was set by the extent to which hull penetrations, particularly that for the propeller shaft, could be made to work at great depths (hence great pressures).