Few large-scale marine prototypes have been built, limited by the low electrical conductivity of seawater. Increasing current density is limited by Joule heating and water electrolysis in the vicinity of electrodes, and increasing the magnetic field strength is limited by the cost, size and weight (as well as technological limitations) of electromagnets and the power available to feed them.[8][9] In 2023 DARPA launched the PUMP program to build a marine engine using superconducting magnets expected to reach a field strength of 20 Tesla.[10]
Stronger technical limitations apply to air-breathing MHD propulsion (where ambient air is ionized) that is still limited to theoretical concepts and early experiments.[11][12][13]
^Dane, Abe (August 1990). "100 mph Jet Ships"(PDF). Popular Mechanics. pp. 60–62. Retrieved 2018-04-04.
^Normile, Dennis (November 1992). "Superconductivity goes to sea"(PDF). Popular Science. Bonnier Corporation. pp. 80–85. Retrieved 2018-04-04.
^Way, S. (15 October 1958). Examination of Bipolar Electric and Magnetic Fields for Submarine Propulsion (Report). US Navy Bureau of Ships. Preliminary Memorandum Communication.
^US patent 2997013, Warren A. Rice, "Propulsion System", issued 1961-08-22, assigned to Carl E. Grebe
^Phillips, O.M. (1962). "The prospects for magnetohydrodynamic ship propulsion". Journal of Ship Research. 43: 43–51.
^Doragh, R.A. (November 1963). "Magnetohydrodynamic Ship Propulsion using Superconducting Magnets". Transactions of the Society of Naval Architects and Marine Engineers (SNAME). 71: 370–386.
^Pope, Gregory T. (September 1995). "Fly by microwaves"(PDF). Popular Mechanics. pp. 44–45.
^Weier, Tom; Shatrov, Victor; Gerbeth, Gunter (2007). "Flow Control and Propulsion in Poor Conductors". In Molokov, Sergei S.; Moreau, R.; Moffatt, H. Keith (eds.). Magnetohydrodynamics: Historical Evolution and Trends. Springer Science+Business Media. pp. 295–312. doi:10.1007/978-1-4020-4833-3. ISBN978-1-4020-4832-6.