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Comment by Animats | Hacker News Reader

That's a nice application for brushless motors with variable-frequency AC drive. The motor is so simple - it's just windings and the propeller ring. No seals, no gears, and no bearings that carry heavy loads. All of those things give trouble underwater. The drive electronics is more complex, but drives for variable-frequency AC motors are well-debugged now. Every drone has several of them, and electric cars have big ones.

Some pictures of existing units: [1] They're mostly used as steerable thrusters for boats that need to be highly maneuverable, such as ferries. They're becoming very popular for ROVs, because they're much lower maintenance than existing thrusters used at depth.

[1] https://www.youtube.com/watch?v=Te_xejpriFM

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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.

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...

I don't understand the advantage of this. Surely it must exert the same forces as the usual shaft-mounted propeller, and therefore require bearings of the same strength? What's the advantage? Is it something to do with the load being distributed all around the rim instead of concentrated at the center?

Guessing E/M noise might be extremely significant though? I suspect there are other ways, besides microphones, to detect a sub.

Salt water dissipates EM very well.

E, yes. M, no.

And this thing seems like it would generate an enormous magnetic field.

Seawater is a pretty great absorber of EM energy at all frequencies above tens of kilohertz, which is probably around the switching frequency of the drive inverter.

The problem is the fancy new quiet drive is attached to a regular old noisy nuclear power plant.

Are nuclear reactors noisy? Isn't it just some hot rocks sitting in water? What's noisy?

You need to turn that water into steam and make it run a turbine attached to a generator, and they tend to be noisy.

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