What distinguishes MHD submarines from ordinary ones?

Answer 1

MagnetoHydroDynamic propulsion (MHD) uses electromagnetism without moving parts to move water and propel the submarine forward. Though believed to be silent and stealthy (in large part due to the Clancy novel "Hunt for Red October", which featured an MHD Type drive) in reality the gases and electromagnetic signature would be detectable by a range of ASW sensors commonly in use. In other words, sounds great, but in practical application for blue water ops, it's not realistic. It could be good for coastal defense though.

Its primary disadvantage is speed - it's slow compared to boats of Diesel Electric or Nuclear propulsion with conventional screws. In any combat situation, speed is critical, especially when you've got a 55 knot torpedo aimed up your stern. Speed is one of the primary reasons that the U.S. and other major countries moved to Nuclear Power for their submarine fleets. Speed equals overall tactical advantage in any engagement, or when time is a factor in getting to a global hot spot. Only nations without real blue water navies, save for Russia, still operate DE boats. Most who use them are for coastal defense only, though modern technology gives them comparable speed to older nuclear submarines. However, top speed is still limited by battery consumption, and at some point they still need to recharge their batteries. Running a diesel is an instant target acquisition for any nuclear submarine Sonar gang on the ball.