Motor for an RC Submarine

Magnetic drive. There'll be no direct contact between the water and the motor. Instead, the motor will turn a magnet that in turn will turn your turbine or whatever.

A 4" DC submersible with a suction pipe might just do what you want.

Try Franklin pumps for one.

So, if it doesn't have a prop, is the "rotary motor" the impeller?

What kind of rotary motor is it?

Submersible DC powered Bilge pumps. Found in any marine supply store.

I imagine a gas charged cylinder might work- like those used to propel BBs through a barrel. Then I recalled an idea I had many years back- for the same thing (propelling BBs/bullets, etc.)- using pressure from an electrically heated chamber. The chamber has distilled water & the expansion of molecules can be regulated via proper feedback/monitoring. Then released according to your needs (to propel a sub or a slug!?. Funny huh?) I'm sure this will work for many applications. ... & while on the subject of bullets(?) I apologize to those who might find this subject matter offensive- never the less- what about a bullet that is made to unwind (not mushroom or splatter on impact)? That would be a slug that is made out of wire. There could be many configurations within that basic design. But if the object is to eliminate (kill) the target- the (technical) sky is the limit I would think. Regards Carlos

"The choices you make might be mistakes but it is never too late to turn around." Tony Lang- musician

I have another approach for you! My daughter has a pool toy shaped a bit like a fat torpedo with fins that derives thrust from hydrodynamic forces during surfacing. You hold the thing under water and release it with out applying any movement at all and it will "glide" kind of like an airplane for quite a distance before surfacing. All you need is to replicate that shape or something similar and by doing nothing more that pumping ballast in and out achieve propulsion from gliding up then ballast negative and glide down... Yeah I know it sounds crazy but the thing will shoot 20 to thirty ft across the pool if allowed to rise from 4 or 5 ft down.

Yeah, my kids have one too. They tried it from the bottom (approx. 8 ft) and the darn thing shot out of the water and hit their mom. I though it was funny, but she didn't think so. I think the method of propulsion depends upon the use (i.e. exact replica of an actual submarine or as a ROV) of the model. I have two models, one a WWII U-boat that has two electric motors and one is an Ohio class submarine that I have been trying to modify to launch a missle.