Hi John, basically you need to have the paddles turning at approx 90 to 120 rpm so a gearbox is a must. Have a look around on fleabay for motor /gearbox combinations, they are ex industrial motors and are usually well made, the only drawback is they need 12v but will be quite happy on 6 to 12. I connect mine up with chain drive but belt drives are also used. I try to avoid exposed gears as they tend to be quite noisy.
Have fun, Chris M.