hi fungus,
i had the same thoughts last summer, my pal has an ruined mill, and had been building up the pond. I put a 2inch pipe from the farm dump into his sliuce gate, with a fertilizer dispencer to make a nice jet about 30mm by 15, 2m head.
Got an old dyno hub bike wheel, with a flat bar hoop where the tyre went.
flat bar welded on to the hoop, radially, then the beer tins cut nearly in half and bent round and rivetted on.
it looked really good, went round nice, but pretty slow, tho good torque.
only made a few volts, and the cans tore at the rivets, and the fold.
now i'm trying smaller wheels, and stepper motors from printers.
reckon you need rpm for electric,