Wow TG, devious minds think alike. I was thinking of exactly the same thing, but mostly for surface aeration. Trolling motor motors are about the most efficient you can get. Screw pumps (look a lot like propellers)are also the most efficient at moving large volumes of water at very low pressure. Since I have no electricity at my place, I was thinking of going solar. I think a 55w panel would really spin the motor up. But Theo's idea about a battery charger is excellent. It does need to be fairly heavy duty though. I'll be following your idea with interest.

Shrimp