Is Everyone Talking About MHD?

It just seems that there's a lot of buzz around the subject of "just one moving part and that's water" technology. Ricky explains. 

That's the presenter of 2 Bit da Vinci talking about MHD (MagnetoHydroDynamics) in an over-simplified "explainer" kind of way. And, as always, dumping a bit on DIYers (and yeah they were the "Fabulous Fakes" that always seem to want to promulgate bullshit for the few pennies in views but there are also a few ) and urging us to leave it to the "experts" aka corporately-financed projects. 

Here's a video of a con artist doing nothing like MHD, for several reasons. And just for balance, here's a video of a citizen scientist showing the right way a very basic MHD drive can be made

Concept (and Concerns): 

Electric motors work a conductor carrying a current, and placed in a magnetic field that's at right angles to the current flowing in theconductor, expeiences a force in the third right angle direction. In an electric motor we generally arrange things so that the force the conductor experiences acts to turn the armature of the motor around the axle in a circular motion. But you can also lay the wires of a motor flat and place the magnet on top and then the magnet will move along in a straight line over the conductors. 

Or, you could use water (which unless it's perfectly pure, contains impurities that allow it to conduct current) and apply a magnetic field horizontally - say, from left to right, pass an electric current through it verticallv (i.e. at right angles to the magnetic field) and the water itself will experience a force in the direction fore or aft and start pushing along in that direction. Or, to put that another way, the arrangement of current injecting plates and magnets will move in the opposite direction to the water.

If you attach the magnetic and electric bits to the underside of a boat, the boat will start moving one way while the water will move the other way. You get "water jet" propulsion, at a rate determined by the voltage applied, magnetic field, and how much current is able to pass through the water.

Do this in distilled pure water and you get - nothing. Introduce some conductive impurities (such as salt) and you get some oxygen bubbling off one electrode, hydrogen off the other, and some chlorine. But you get movement, too. 

So that's the principle of MHD. But you also get some other things:

As anyone that's ever done an electroplating experiment knows, metal will be expelled off the anode and deposited onto the cathode. And if you've ever done that experiment, you'll also remember that a load of "crud" falls down, too. I'm not too familiar with the exact chemistry and anyway I believe it depends on the electrolyte (the water and whatever impurities are in it) and the metals used. The point to me isn't so much what it is as that there IS some crud produced. Where does it go? Into the water that you're travelling through. And from there, into the creatures living in that water. 

If you're the only ship on the ocean using a powerful MHD drive, the effects would probably be negligible. But there are already thousands of ship movements across the oceans every day, and the effects would soon add up. 

On top of that there'd be an electric and magnetic field generated, and quite large ones at that. These might begin to affect marine life that uses electric and magnetic fields to navigate or perhaps even communicate. So yay us. We stop polluting the air and oceans with fossil fuel burning and carbon particles dropping and the noise of the engines disturbing animal communication and senses, and instead pollute the oceans with electrolysis crud and huge warps in the natural magnetic and electric fields.

Could It Be Done Though?

Yes. Sort of. Instead of permanent magnets, let's put electromagnets either side of the drive channel. Now we can reverse the direction of the magnetic field and the electric current really really quickly, maybe thousands of times a second. The overall effects of the magnetic field cancel out and won't be felt as far away. There'll still be a very loud and annoying howl in the magnetosphere and electrosphere around the craft, which will maybe cancel out.

But more importantly, think about the metal ablation from the electrodes. With a high enough frequency there's a chance that the particles won't be able to travel far from the electrode before the next current reversal drags them back onto the electrode. There may well be some particles lost and floating away into the sea but it would be a lot less than if direct current was used. 

Because the magnetic field and the electric fields will both reverse at the same time, the physical force on the water will remain going in the same direction all the time, so the one thing we want from an MHD drive will still take place and the boat will move in one direction consistently. By properly designing the way the drive works, it may be possible to confine much of the electric and magnetic mayhem inside some kind of "drive tunnel" under the hull, and by providing a place for electroplating crud to attach to (some kind of sacrificial material the length of the drive tunnel) one could probably also prevent most of the chemical byproducts from escaping.

For use in freshwater, one could perhaps use a continuous chain arrangement to add some ionising agent to the water at the beginning of the drive tunnel and extract most of it again at the other end somehow before recirculating it. 

Things like this might minimise the pollution and damage that even something as clean-running as an MHD drive. But...

One additional problem would be that it'll require a lot of electricity to move a larger ship, and it would probably be impossible to store enough energy in batteries for any kind of useful shipping range. So you'd need to generate power on board somehow, and that would mean a fossil fuel or nuclear  generator.