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I just finished a competition for underwater robots. The water is about 40-50cm in depth and 90% of the designs just exposed the thrust motors completely in the water similar to the picture below and nothing happened during the competition. BTW, the water is tap water, not salt water.

exposed thrust motor

I saw many questions and how-to articles about sealing the motor shaft for underwater ROV. I am not sure I understand the reason, especially after I found this reply to Preventing leaks in motor shafts for underwater bots. Is it because of the concern of short circuit or wear and tear if water goes into the bearings? Or will things be very different when the robot has to go deeper?

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    $\begingroup$ you do not want water inside the motor ... if you are wondering why, then try spinning a bicycle wheel underwater $\endgroup$
    – jsotola
    Feb 7, 2020 at 22:32
  • $\begingroup$ @jsotola, is it because of the rust that could happen on rotor or stator? $\endgroup$
    – roTor-roTor
    Feb 8, 2020 at 8:29
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    $\begingroup$ do an experiment ... what happens to a spinning bicycle wheel when it is placed in water? $\endgroup$
    – jsotola
    Feb 8, 2020 at 17:59

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Brushed motors are functionally waterproof not by design but by principle (brushes sending power to a stator, spins the motor) and will run under water, albeit inefficiently. Most brushed motors have an internal fan for cooling so if the motor does not have that and uses bushings or sealed bearings for holding the rotor, it is at least acceptable for submersion. Most radio control brushed motors are actually recommended to be “broken in” to wear down the carbon enough for good efficiency by being run underwater and oiled/cleaned afterwards, and in most cases, it increases life and power of the motor.

Rust however is still an issue but if you aren’t running it 24/7 and oil it afterwards, it’ll be fine.

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  • $\begingroup$ Thanks for the explanation. Would you please elaborate a little more why it is inefficient to run the brushed motor underwater? Besides the water resistance on propeller which is considered as the load for the motor, is there any extra drag or resistance the motor needs to overcome underwater? $\endgroup$
    – roTor-roTor
    Feb 9, 2020 at 13:29
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    $\begingroup$ Hint: do the experiment suggested by jsotola on Feb 8 '20 at 17:59. If you don't have a bicycle wheel, then just imagine the various intricate windings, laminate stampings, etc., trying to push the water around in turbulent flow inside the motor case. $\endgroup$
    – r-bryan
    Dec 23, 2021 at 5:06
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The only major reason for doing this is the outcome of the interaction of water & current!!!

Besides, as the depth increases, the pressure increases & the probability of the contact(water & current) as well increases.

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