Okay, that’s a little premature.  Still, these guys at OpenROV had the brilliant idea of using a bilge pump motor to propel their ROV and it looks like it’ll work really well.

I bought a $25 Johnson Pump 500 gallon per hour cartridge bilge pump at Canadian Tire.  The casing pops right off, leaving a nice little waterproof 12 V DC motor with a 3 mm shaft.  I attached a propeller blade and tried running it underwater:

It worked pretty well.  It draws 2.3 A at 3.5 V.  I’ll use a smaller propeller when I find one, which should reduce the power consumption and output.

One strange thing is that the pump has a greased seal between the pump chamber and the top.  Are they trying to waterproof the top for some reason?  Is it just to keep the water in the chamber so that it doesn’t lose pressure?  The label says the pump is submersible, so hopefully it actually is.  I haven’t worked up the nerve to submerge the whole thing, and I might just seal it up with some potting compound before trying.

I have a feeling I will appropriate a lot of ideas from OpenROV over the next year.