Students are generally impressed with the demonstration, especially when an external power source is connected and a vigorous water jet is produced. The question that is immediately voiced is the following: "When is the Navy going to make one of these?" In the process of building and operating this boat, all of the data needed to answer that question have been collected. Unfortunately, the data debunks several myths. The first myth is that this propulsion system is silent. As the students can clearly observe, the boat produces many bubbles and the associated noise. The bubbles are hydrogen and oxygen being liberated through electrolysis as the current passes through the water. This propulsion system is anything but silent. The second myth is that it is nondetectable. Setting sound considerations aside, the system produces a jet of chlorine ions and metal chlorides that pour out the back of the boat. While this is useful for flow visualization in the classroom, it is doubtful that a stealthy submarine would want to leave such a trail pointing back at itself.

The metal chloride stream also points to one of the greatest engineering difficulties (monsters) with this technology. The chlorine ions are very reactive and furiously corrode most metals. The prototype MHD boat for this study lost 1 cm of 12-gauge wire in just 15 minutes of operation. Thus, an operational ship would have to find a material that would not be consumed by the chlorine ions or else resign itself to replacing its electrodes on a regular basis. The amount of salt needed is also of concern. Since pure water does not conduct electricity to an appreciable degree, this propulsion system would not work in freshwater lakes or near coastal rivers. The final monster that must be examined is the current and magnetic field requirements. A Trident-class submarine has a mass of 18750 tons (17.0 × 10⁶ kg) and a power of 90,000 hp (67 MW)available from its propulsion system.³ If this drives it at about 20 knots (10.3 m/s), a quick calculation (P = FV) shows that it must be exerting a thrust force of about 1.5 × 10⁶ lb (6.5 × 10⁶ N).

Now, let's attempt to drive a submarine of this size with a magnetohydrodynamic propulsion system. Assume the water channel through the submarine is about 3 m in diameter. This will be our electric current path length. A typical power plant produces a current on the order of thousands of amps. Let us assume that we can generate 5000 A with our 67-MW nuclear reactor. In order to create the 6 × 10⁶ N force necessary to drive the submarine at 20 knots, a 435-T magnetic field is needed [Eq. (2)]. The best field-portable, superconducting electromagnet used on a full-scale Japanese experimental MHD ship only developed about 4 T.^4,5 Therefore, the magnet technology is not yet available. Conversely, if we limit ourselves to 4.0 T and the same geometry, in order to drive our Trident-class submarine, a current of 540,000 A is needed. This is clearly an unreasonable requirement, at least in the near future. Incidentally, this makes for an excellent eye-opening example in class.