Hellosailor, you raised an interesting question about prop drag when a boat uses propellers to generate electricity. This subject seems complex to me. I have read diametrically opposed opinions about prop drag: assuming a fixed-blade prop, is the drag more or less when it is spinning/freewheeing compared to locked and stationary? Some claim that cavitation causes a stationary prop to lose drag with increasing boat speed. While it's intuitively obvious that a folding/feathering prop has less drag, there are probably more boats on the water with fixed props. How much loss in boat speed would there be for those boats if allowing the props to turn a generator? I'm sure there must be some, or everyone would have prop shaft generators!