I am working on a toy boat with a motor under water and I have a budget of $10
I've tried running a small DC hobby motor under water once and I found that it runs really slow then will cut out after a few seconds. I then tried it again with a different motor except I sealed it with tape and blue tack and it worked quite normally.You should try to also keep water off of at least one of the bear terminals to prevent it from conducting between terminals through the water.
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Fresh water is essentialy non-conductive.
Do you mean river/ lake fresh water because I did those tests in plain tap water?
Is your water hard ?
Yes, it contains ﬂuoride.
I meant carbonates. Fluoride is present in such small quantiities I doubt it makes much difference.
I believe it has very small amounts of calcium carbonate.
At low (battery) voltages clean fresh water hardly conducts at all.
You could use a motor from a small bilge pump, the type used to keep water out of boats. These are typically 12VDC powered, and are inside a plastic housing containing the motor and impeller for the pump. I've seen these used in underwater robot applications and competitions, where the pump jet is actually used like a low power thruster for manuevering. I don't know how large of a boat you are contemplating. You may be able to get one of these for $10 new in a marine shop or less online/ e bay.
You should find that in fresh water any small DC motor will still run even though it is under water and wet.. Keep the battery dry.