Electromagnetic propulsion System

[monrosm@shrewsb]

I need help with a question that has been puzzling me.
It probably has a name, but there is a propulsion sytem that uses a copper bar which is fixed under a boat, a large current is passed through it which causes an electromagnet to produce a vertical magnetic field. Using flemmings right hand rule (first finger field, second finger current and thumb as thrust or force) this should propel the vessel through the water. However it doesnt work. Im trying to answer an A level advanced physics question which asks me why this does not work? Any ideas....have a hunch that it could be to do with water only having a small charge. But i could be way out....any suggestions are helpfull as i have none.
Once again any suggestions helpfull.
Thakyou
Stef

[yipster]

cant help you but trow another unusual in here since i recall
seeing a working prop - free from stern- on magnetic fields once

[CDK]

With enough current you may be able to lift magnetic objects from the seabed. And certainly all compass needles in the marina will point to you, but that's all that will happen. The formula for electromagnetic field strength contains the distance as a square root if I remember correctly. The nearest immobile magnetizable object, needed for a mechanical force, is the earth's core, several 100's miles away.......

[fredrosse]

The concept is similar to MHD power production (or MHD Pumping) where a magnetic field is at right angles to an electric current, producing a force on the conductive liquid which is in proximity to the current and field. This technology was well established several decades ago. Sea water has good enough electrical conductivity to allow MHD to work well.

Several experimenters have tried boat (and submarine) propulsion using MHD, virtually all have failed where there is no fluid venturi and diffuser in the system. The problem is getting the pumping force oriented in the right direction, and using the pumping force to produce pressure, then a nozzle to turn the pressure into kinetic energy which provides a jet force to propel the boat.

This is, in fact, exactly what ordinary jet drives do, except the pump is a common mechanical driven machine (centrifugal, axial, or mixed flow type pump). One could substitute an MHD pump and theoretically have the same efficiency as a mechanically driven type jet system. ala Jet Propulsion with a nozzle.

[Sternwheel Rat]

The simple reason it does not work, is FRICTION! Too much in one spot, not enough in the other.

[CDK]

To move water by magnetic force might be possible if you could create a field like in a MRI scanner. But you wouldn't only move water: everything else on board is less transparent for magnetism and would be drawn to the magnetic source with immense force.
This is a topic for the SF section, but sadly there isn't any.

[tom28571]

The principle uses what is known as the Lorentz electromagnetic force. It has been known for quite a while but no one has been able to make it work in a practical sense. This is the same "caterpillar" drive of the movie "Search For Red October". The idea is to induce a current in the water and have that working against a magnetic field to generate thrust. Beautiful idea. It does work but one basic difficulty is relatively low conductivity of sea water, among others. The Japanese actually built such a boat and had it moving under this power. Not very well though. The efficiency was very-very low of power-in vs power-out and the boat barely crawled along.

I could not find reference to an article on the Japanese system, but you can read more about this in a patent:

http://www.patentstorm.us/patents/5333444-description.html

[yipster]

link to an article on this force http://syd.mech.eng.osaka-u.ac.jp/papers/1994/05_IMDC.pdf
and vaque remember various other caterpillar systems ;)

[CDK]

The Lorentz force, named after its discoverer, has indeed been known for quite some time. The principle is widely used in earphones, microphones, loudspeakers, moving coil instruments etc., but always with copper as a conductor. The only exception I can think of is the old fashioned Kwh counter, where an aluminium disk rotates between the poles of an electromagnet. The stray currents in the disk create a weak magnetic field with just enough phase shift to make it rotate.
Although sea water does conduct electricity, a Lorentz based pump will be even less efficient. "Search for Red October" is a work of fiction and doesn't stand a chance in the real world. And a patent also is no guaranty for feasibility. I once searched for marine propulsion systems and found a russian owned patent based on 'a standard warp-4 engine'. The inventor clearly had confused reality with a popular science fiction series but was willing to pay the patent fees.

[yipster]

ah, always thought it took two standard warp-4 engines (http://en.wikipedia.org/wiki/Warp_drive) http://skins.hotbar.com/skins/mailskins/em/033102/033102beer_1_prv.gifhttp://discovery.scifi-art.com/texturing/titlepic.jpg

http://upload.wikimedia.org/wikipedia/en/thumb/1/12/Star_Trek_Warp_Field.png/250px-Star_Trek_Warp_Field.png http://upload.wikimedia.org/wikipedia/en/thumb/4/4b/Warptable.gif/450px-Warptable.gif

http://en.wikipedia.org/wiki/Warping