Magnetohydrodynamic Propulsion

MHD propulsion is a revolutionary type of propulsion system that doesn't use propellers to power ships and submarines but instead uses a magnetic force on the current. The method eliminates motors, drive shafts, gears as well as propellers, so it proposes to be a low-noise system with great reliability at low cost. 

YAMATO1 was the first such boat ever made to make use of this revolutionary technology. It was built in the early 1990s by Mitsubishi heavy industries Ltd. and was driven by liquid helium-cooled superconductors and could travel at 15 km/h or 8 knots.

The next diagram shows the schematic side view of one of the two MHD propulsion units that are mounted underneath the vessel.Seawater enters from the front of the unit and is expelled from the rear. A jet engine uses air in a similar fashion taking in air in the front and expelling it from the back to propel the plane forward.

Working

The following diagram illustrates the MHD unit in greater details:

An electromagnet within the vehicle pushes the semiconducting wire to produce a strong magnetic field. Electrodes mounted on either side of the field are attached to a d.c generator. Thus the generator generates current which traverses from one electrode to another thorugh the seawater, perpendicular to the magnetic field.

At this situation the Fleming's right hand rule says that a force F will be exerted in a direction perpendicular to the magnetic field and the electric field as well.

The course the force takes is thus the direction in which seawater is expelled. The force aids in the drawing of water as well as it helps in expelling it out with greater force. The magnetic force pushes the seawater out. As a result the water is expelled from the tube. Since the MHD unit exerts a magnetic force on the seawater, the seawater also exerts a force on the unit. This is the reaction force according to 3rd law of newton and it is equal in magnitude but opposite in direction. This reaction force is the one responsible on pushing the boat forward and thus the speed which the boat takes is directly proportional to the force exerted.

The speed of the boat hence can be increased by increasing:

1. value of current through the electrodes 
2. value of magnetic flux.

This is how the cost effective , noise efficient magnetohydrodynamic propulsion system works.

Bibliography

1. Wikipedia Page on Yamato 1.
2. Cutnell and Johnson physics