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Comment by cocoablazing | Hacker News Reader

If the impeller is capable of developing sufficient force to move a submarine at operational speeds, it will require serious axial thrust bearings.

The impeller pushes against the housing with the same force that a conventional shaft develops against a shaft-mounted axial thrust bearing would for the same speed and vessel geometry.

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What do you have to use in this case? A large needle bearing along the motor ring? How much noise does that make?

Is it possible to have a water-bearing with injected high-pressure water in a small area, or will that use as much power as the propulsion?

Large axial thrust => tapered roller bearing, usually.

Large vessels use much more complex and specialized axial thrust bearings for propulsion shaft loads. What works for an ROV won't necessarily do the job at the required scale.

An ROV-sized unit uses "water-lubricated slide bearings".[1] Some use ceramic bearings. There's been experimentation with magnetic bearings. Good overview paper [2].

Hydrodynamic bearings running in water seem to be popular. Those have a design which self-pumps water between the bearing surfaces. The load is carried by the fluid, not the surfaces.

The neat thing about this design is that most of the headaches of operating in water become advantages. Water provides lubrication and motor cooling. No oil, no shafts, and no seals, the usual headaches of things which operate in water. There's still corrosion and general marine crud to fight, of course.

[1] https://www.schottel.de/fileadmin/data/pdf/eng/SRT_Flyer_A4_... [2] http://www.elkraft.ntnu.no/eno/Papers%202004/ICEM04-Krovel-N...

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