MHD II: Make a magnetohydrodynamic propulsion boat

The craft is not designed for performance; it's designed to demonstrate a functional MHD craft made with inexpensive, easily available materials. While it may move slowly, hey, it moves, and you can make it in about five minutes.

• 9V batteries. Plural, because they will get used up very quickly. Rechargables would be nice.
• Two quarters (or similar coins)
• Two short cables with alligator clips on the ends
• A sturdy foam tray, like the kind that meat or vegetables are sometimes packed in
• a strong neodymium magnet (I suggest one about an inch in diameter by half an inch thick)
• Regular transparent tape.

  Tools: a small knife.
  Optional: Double-sided tape.

In the photo, you can see that my half-inch thick "magnet" is actually a stack of two 1/4" thick magnets.

If you have double-sided tape, use a piece below the magnet to help hold it in place.

Danger: You must exercise extreme caution when handling a sharp knife near a strong magnet like this. The safe way to cut these slits is to first mark where the magnet goes, take the magnet off, cut the slits, and then tape the magnet on.

Begin wiring up the craft: Hook one end of each of the two alligator cables to the battery terminals. (Don't hook the terminals to each other.)

If you don't already have any alligator clip cables, this is a great excuse to make or buy some.

When you float the craft in salt water, the water completes the electric circuit, allowing electricity to flow between the two exposed half-quarters on the bottom side of the tray. As in the case of the simple MHD demonstration, the electric current flows perpendicularly to the magnetic field, so a force is produced, which will push the craft along.

Avoid getting salt water in the top of the tray; it will short out the circuit and drain your battery without giving you any propulsion.

We used this clear storage bin to test out our little boat. The water is a couple of inches deep, so that the quarters won't touch bottom. We emptied that container of salt into it, but I'm not convinced that we needed quite that much.

We had an early shipwreck with this design when we tried to test it out in our bathtub. It turns out that our bathtub is magnetic. The boat was almost instantly yanked to the bottom. Oops. A more subtle problem is that your craft may slowly float towards the nearest giant chunk of steel, so don't try this right next to your car. Once you eliminate major magnetic interference, you may find that your craft still acts as a weak compass. (If your magnet pointed horizontally instead of vertically, it would make a very good compass.) The compass effect will only mess with your steering, not the thrust direction. If you're careful, you should then be able to tell compass steering from attraction towards metal objects.

To check for magnetic interference without eating up batteries, disconnect one of the alligator clips from the battery and set the craft in the basin of water. If it starts drifting (not turning), then you need to put your basin somewhere else. Once you've found an interference-free test site, reconnect your battery.

To prolong battery life, always take the craft out of the water as soon as it hits one of the walls. Disconnect the battery if it's going to spend any significant time sitting out before the next run.

If you have trouble getting your craft to work, the first suspect should be the battery; try a new one. There's very little to go wrong in this circuit, so it should (in principle) be easy to debug. If the battery is hot and the craft doesn't move, then you may have a short circuit. Check to make sure that you don't have any water leaks (remember that salt water is a conductor!) and make sure that the insulating wrap around your magnet is intact.

This might be a good starting point for a larger vehicle, as a classroom demonstration, or a science fair project. That's up to you. Right now, I'm making radio controlled versions. How will it turn out? Time will tell!