Fig. 1 shows a submerged experimental electric generator of high power (above 500 kW) having an exterior rotor and an internal stator. For testing purposes this generator has to be rotated a long period of time by a 1500-RPM, 1000-HP electric engine.

The question is: Will a (stainless steel) underwater gear, like in Fig. 2, formed by a 1500 RPM pinion and a ring cogged wheel 20 time its diameter (bolted to the rotor), work?

The speed of the pinion will be 25 rotations per second and that of the ring just 1.25 RPS. The outer diameter of the generator is of the order of 1 meter. In some tests the water will be mixed with small quantities of sand.

enter image description here Fig. 1. Experimental submerged electric generator rotated by an electric motor.

enter image description here

Fig. 2. A pinion and a ring cogged wheel forming a gear.

  • $\begingroup$ I don't think stainless steel that is continuously scratched will survive under water. THe scratching constantly removes the passivation later that protects the stainless steel from rusting. Also, I think galling might be a concern as stainless steel is known for galling. $\endgroup$
    – DKNguyen
    Commented May 29, 2022 at 1:34
  • $\begingroup$ A 1000Hp drive through a single tooth? may work, but not for long. $\endgroup$
    – Solar Mike
    Commented May 29, 2022 at 6:55
  • $\begingroup$ Just to have couple of numbers, 1.25RPS on 1m dia is approx 1.25s–¹×π×1m=4m/s and then 500kW/4m/s=125kN, seems really tough. $\endgroup$
    – carloc
    Commented May 29, 2022 at 8:46
  • $\begingroup$ @carloc , You mean 125kN = the force with which a tooth of the pinion presses against a tooth of the ring gear? For example, as an alternative, the pinion can be a worm and the ring cogged wheel can have teeth that are suitable for worm gears. I guess, there should exist 1000 HP gearboxes with a 20:1 reduction ratio that experience the same force 125 kN between the teeth of two of their cogged wheels so an ensemble like the one in Fig. 2 (see my question) could be feasible, at least for a non-underwater configuration where the two gears rotate in air. $\endgroup$ Commented May 29, 2022 at 14:57
  • $\begingroup$ Yes exactly the tangential force, I don't know if it's going to be feasible, I just like trying to somehow visualise the quantities involved. 125kN turns into about 10 small cars somehow hanging on one ring gear tooth. $\endgroup$
    – carloc
    Commented May 29, 2022 at 15:11

1 Answer 1


You want something like this, located above the surface and shafted to the underwater generator, presumably the two are connected with a torque tube. They are pricey and high maintenance.

enter image description here

This is what low speed MW scale looks like in an e-drive. Forget the transmission, just shaft one of these to the generator. enter image description here

The outer diameter of the generator is of the order of 1 meter.

Nonsense. Nothing that small can handle that torque. The shaft driving it might be 0.5 meter in diameter. Look at the shaft in the picture. (I suppose 5 meters is still on the order of one meter, isn't it?)

Here's a video of ABB's shaft generator that operates at similar speed and is available in 1-6 MW. The latter part shows assembly.


  • $\begingroup$ Take the diameter of the submerged generator as exactly 1 meter and its length 2 meters. The real dimensions can be 1 m +/- 10 cm and 2 m +/- 20 cm, respectively. The electric motor looks like this ( caseyusa.com/products/spec_sheet/… ) having roughly the same size as the underwater generator. In your picture you show a Siemens submarine motor of 4 MW (see: assets.siemens-energy.com/siemens/assets/api/… ). $\endgroup$ Commented May 29, 2022 at 14:16
  • $\begingroup$ How is the 20 X torque increase handled? The reason that ships direct-drive @ 80 RPM is that gearboxes don't make sense at that speed. They cost more than just building the motor like in the picture. You won't make a generator smaller than the ABB units unless you switch to cryogenic superconductors. And at 90% efficiency, you need to provide 75kW of cooling. How are you going to transfer that out of 2 cubic meter device? app-therm.com/chiller-range/… $\endgroup$
    – Phil Sweet
    Commented May 29, 2022 at 14:38
  • $\begingroup$ In the final configuration, a water turbine will be attached to the outer surface of the generator. The ring gear, the pinion, the 1000 HP motor will disappear. The generator, with its rotor spun by the turbine, is supposed to be submerged in an ocean with high tides or in a river. However, as the generator is still experimental, something has to replace the turbine in order to test the generator somewhere inside a factory, to see whether it really works for a long time and produces above 500 kW of electricity, which will be delivered to the grid. (The water in the tank is replaced by a pump.) $\endgroup$ Commented May 29, 2022 at 15:29
  • $\begingroup$ This is the size of the motor and generator I am talking about: youtube.com/watch?v=W92_kHR9-7k . None of them has a size anywhere near that of the generator posted by you. $\endgroup$ Commented May 29, 2022 at 22:33

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