Although it sounds complicated, a simple MHD system can be built at home or as a classroom project, using a couple of ordinary magnets, some 9-volt batteries, and other common materials. An article in the October 2004 issue of The Physics Teacher will show you how. You can find it online just Google “magnetohydrodynamic propulsion.”
As you might imagine, building a real working system is a bit more complicated. The first (and to date, only) full-scale ship with MHD propulsion is the Yamato-1, a prototype built by a consortium of researchers in Japan that was sea trialed in June 1992. The 30-meter (98-foot) catamaran weighs about 185 tons; comparable in length and weight to a 100-foot full-displacement motoryacht. Her design speed was a modest 8 knots.
Because of the powerful magnetic fields that are required, the hull of an MHD-powered craft should be built of nonferrous material. Yamato-1 was built with an aluminum hull, but a military vessel might well be built of stainless steel or titanium. But the bigger complications arise from the large amount of electrical current needed to generate the magnetic field; the magnetic coils must operate in a state of superconductivity, meaning they can pass thousands of amperes of current with virtually no resistance. To achieve superconductivity, the coils must be encased in a cryostat (think of a giant thermos bottle) and cooled with liquid helium at a temperature of -269C, or about -452F.
The other major issue relates to the electrical current that must flow to the seawater ducts to create the Lorentz force. The system designed for Yamato-1 requires up to 2,000 amperes to each of the electrodes. This large current flow through sea water results in electrolysis, generating chlorine and oxygen gas at the anode and hydrogen gas at the cathode. By judicious selection of materials for the electrodes (Yamato-1 used iridium-oxide), production of chlorine can be minimized, but the hydrogen and oxygen bubbles are not so easy to eliminate. Although they’re not harmful, they are a source of noise that tends to defeat the purpose of using MHD in the first place.
Last but not least, there’s the matter of efficiency. Yamato-1 requires a pair of 2,000-kW diesel gensets to approach her design speed of just 8 knots; that power is equivalent to a pair of 2,700-hp diesel engines. An expedition yacht of similar size with conventional propulsion should make about 12 knots with twin 800-hp diesels. Clearly, improvements in technology will be required before the MHD concept can achieve a place in the commercial market.
Somewhere between the set of science-class magnets wired to a cluster of 9-volt batteries and the energy gobbling, helium-cooled behemoth installed on Yamato-1, there must be a commercially viable technology waiting to be found. Maybe the helium produced by electrolysis can be captured to fuel part of the MHD’s energy needs?
Comparing the Wright Brothers’ venture at Kitty Hawk with the jumbo jets of today, and recognizing the rate at which our technology now improves, it’s plausible that we might see a viable MHD propulsion system in the next decade or two. But while we may see it, we probably won’t hear it.
This article originally appeared in the November 2006 issue of Power & Motoryacht magazine.