I have been thinking about how a totally penetration-free ROV could be built. While it's probably not practical to build an ROV with no pressure-vessel penetrations of any kind, it's interesting.

Since most ROVs (and submarines for that matter) are tubes, I was wondering if it would be possible to integrate permanent magnets and propeller blades onto a ring that spins around the outside of the tube, and glue the coils/electromagnets to the inside.

I know that motor efficiency decreases pretty drastically with larger stator gap, so perhaps this would be a really terrible motor, I'm not sure. It would also be a very girthy (sorry) motor given that the tube would be 80 or 100mm ( a few inches ) in diameter.

Perhaps it's a lot more practical, relatively speaking, to use a standard brushless motor connected to a spinning array of permanent magnets. This would have no efficiency loss whateversoever, I believe, up until the point where it stopped working entirely because of overwhelming the magnets with torque. That's what this fellow has done with his lego/tupperware submarine. https://www.youtube.com/watch?v=KLEH8RJsYgI

This thread talks about stator gap and effiency: How does the gap between coils and magnets affect BLDCs

  • \$\begingroup\$ sounds like something that could be built \$\endgroup\$
    – jsotola
    Sep 20, 2022 at 0:43
  • 1
    \$\begingroup\$ Usual technique is a magnetically coupled motor shaft to propeller shaft. \$\endgroup\$
    – qrk
    Sep 20, 2022 at 2:11
  • \$\begingroup\$ Don’t those motors already exist? \$\endgroup\$
    – Solar Mike
    Sep 20, 2022 at 4:52
  • \$\begingroup\$ fish tank, and desktop water feature, pumps use motors something like this \$\endgroup\$ Sep 21, 2022 at 21:21
  • \$\begingroup\$ @Jasen I'd love to see a picture of that, it really does make more sense \$\endgroup\$ Oct 19, 2022 at 21:11

2 Answers 2


I don't see any problem whatsoever with that idea. From time to time I see motors e.g. fan motors on PCBs where the windings are on the PCB with a bearing in the middle then windings, and the fan spins around that.

Further, it could be an induction motor by putting the aluminum "squirrel cage" just outside the windings.

The biggest trick, really, would be bearing placement.


I’ve seen (and repaired) an induction motor that worked exactly that way in a dishwasher. The limiting factor is that the tube necessarily requires a fairly large gap between the rotor and stator, especially if you’re wanting to operate at significant depth.

  • 1
    \$\begingroup\$ For any readers, the reason depth comes into play is that the tube wall/stator gap will be bigger because the tube needs to be stronger. \$\endgroup\$ Sep 20, 2022 at 18:13

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