> What about using a rotorless system like in
> the movie Red October?? If you apply electricity

If you apply electricity (DC) to anything in sea water the anything will
disappear very fast. Also the efficiency of a MHD thruster is directly
related to the conductivity of the liquid and the latter is very poor. You
will end up electrolyzing more water than what you can move (even with
ac).

The oil-filled small off the shelf motor has been used serveral times by
amateurs. It is not without problems but you can take almost any off the
shelf motor, seal it off properly, provide a membrane and O-ring seal on
the axle, and use it (within reason). The only trick is to use the motor
at slow rpm because the oil moving inside eats a lot of power at higher
rpms. This solution requires a lot of meintenance but amateurs have the
time ...

VCR motors seldomly reach 20W which is low power for a decent sized sub,
and they have very poor efficiency (due to the large magnetic circuit gap
and sheet steel pole under the stator). They deliberately do not reduce
eddy currents as this helps them run smooth in the intended application.

It is also very hard to over-energize them using a different coil set and
drivers as the magnets can't take the punishment and separate
mechanically.

However, I think that large size steppers could be promoted to submarine
thrusters. They have the necessary parts in them and the stator could be
potted. They could be oil filled as above or open to the sea.

One thing Steve Baldwin did not mention about their (Imetrix's)
magnetically coupled thrusters (great idea) is the slip clutch function of
the magnetic coupling. If something stops the prop the motor would stall.
With the magnetic clutch it does not.

Peter

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