I would think that more engineering would be involved than most people think. Sprkplug us right about wheels fed from the top are more powerful than just passing beneath. Once the buckets fill with water, you have gravity to assist turning the wheel by the weight of the water. I would think the size of the wheel, size of each individual bucket would be determined by the amount/volume of water flow. I'd seek out a water wheel manufacturer and see what size wheel they recommend for XX gph of water flow. Google water wheel builders/manufactures to contact them.
