Magnetic Propulsion in Salt Water

[Hermanator]

Could someone direct me to research on using magnetic fields to move salt water through a nozzle for slow propulsion in salt water.

 

[Astronuc]

The concept is called "magentohydrodynamic" propulsion.

http://en.wikipedia.org/wiki/Magnetohydrodynamic_drive - not much there, but general info and some links.

http://www.sanu.ac.yu/English/Shipbuilding/Tema4.htm - better, some figures.

Reasonably good paper - http://www.unipv.it/fis/fisica2mn/barca_a_prop_MHD.pdf pdf file - click on link or use 'save target as'.

 

[russ_watters]

From Wik:

In the 1990s, Mitsubishi built several prototypes of ships propelled by an MHD system. These ships were able to reach speeds of 15 km/h despite projections of higher ones;

"Japan began sea trials of a prototype magnetic ship. Yamato 1 is propelled by two MHD (magnetohydrodynamic) thrusters that run without any moving parts. When completed, the MHD ship should be able to attain speeds of more than 100 knots (125 miles or 200 kilometers per hour), with little noise. This is several times the top speed of today’s ships, which are slowed down by turbulence created by the ship’s propellers. MHD works by applying a magnetic field to an electrically conducting fluid. The electrically conducting fluid used in the MHD thruster of the Yamoto 1 is seawater. [1]" [quoted from a Penn State Delaware County Campus Newsletter]

Wow, swing and a miss by Wik and by whoever wrote the blurb for that newsletter. I'm reasonably certain the "Yamoto 1" was a two knott ferry prototype featured in an article by Popular Science in 2001. Regardless, 100 knots is one of those random numbers people throw around because they sound neat - it has no meaning.

They are also wrong about the limiting factor of a ship's speed: it isn't the propeller, its the length of the hull (think about it: how is a propeller going to cause drag and still move a ship forward). A ship at its top speed sits in a trough between the bow and stern waves - to go faster requires riding over its own bow wave, literally lifting the ship out of the water. So a ship that goes 100 knots either needs to be a hydrofoil, be a planing hull, or be slightly more than a mile long.

Anyway, this type of drive is neat, but it requires an enormous amount of electricity and probably superconducting magnets. I'm not sure it'll ever make it to prime time.

 

[Grim_Savage]

@russ_watters

First of all...Wiki quoted the manufacturer's expectations for Yamoto's top speed, not just some random rolled number...and yes, they were greatly as disappointed as you...

Second, everything in the Earth creates drag, unless you're a ghost, therefore ships do create both drag and thrust, yet ƩF would suggest a movement because of the greater thrust (BTW: drag is affected by thrust, and is always lesser)...

And about MHD...

...its probably more useful in nuclear subs where energy is probably limitless, or just insanely huge, and maybe not YET for commercial ferries as the Japanese thought...

 

[jedishrfu]

as an aside, this was the drive the Russians had in the movie and book: Hunt for Red October

 

[Subductionzon]

as an aside, this was the drive the Russians had in the movie and book: Hunt for Red October

In the movie, yes, in the book, no. From the Wiki article cited earlier:

In the novel, of which the movie was an adaptation, the caterpillar was a pumpjet.

ETA: Typical of movies to make fuzzy science even fuzzier.

 

[Danger]

In the movie, yes, in the book, no. From the Wiki article cited earlier:

I've always pictured that as a garbageload of SeaDoos with a pressure hull in the middle. Score one for Quebec.