Whenever building an aquatic bot, we always have to take care to prevent leakages, for obvious reasons. Now, holes for wires can be made watertight easily--but what about motors? We can easily seal the casing in place (and fill in any holes in the casing), but the part where the axle meets the casing is still left unprotected.
Water leaking into the motor is still quite bad. I doubt there's any way to seal up this area properly, since any solid seal will not let the axle move, and any liquid seal (or something like grease) will rub off eventually.
I was thinking of putting a second casing around the motor, maybe with a custom rubber orifice for the shaft. Something like (forgive the bad drawing, not used to GIMP):
This would probably stop leakage, but would reduce the torque/rpm significantly via friction.
So, how does one prevent water from leaking into a motor without significantly affecting the motor's performance?
(To clarify, I don't want to buy a special underwater motor, I'd prefer a way to make my own motors watertight)