In the movie "The Hunt for Red October", the propulsion system of the titular submarine is described as a magneto-hydrodynamic (MHD) drive. I wondered how you could implement such a drive without modifying the pressure hull of the boat, stumbling upon the idea of using two halbach magnet arrays face-to-face to create alternating magnetic fields. Coupled with electrodes having alternating polarities, this allows to stack numerous spiral-shaped drive channels in parallel.