The amount of regenerated power, as Peter suggests, would be approximately the NET of the amount generated less the amount input. If you've tried turning the motor shaft by hand, you've observe that it seems to want to "step" through its rotation. This is the effect of the alternating resistance and attraction felt as the permanent magnets on the rotor pass by the stator windings. To free-wheel a sailboat propeller, power is sent to the motor in order to energize the stator and overcome the latent resistance. Once the resistance if equaled, the additional force of the water passing by the prop will turn the motor faster than it would otherwise be turning given the low power input, and -- voila -- the motor becomes a generator, transforming the surplus of energy into electricity flowing back into the battery.
I see this same effect in my battery-electric Chevy Bolt when coasting, and especially when going downhill at moderate speeds.
Although I haven't seen any scientifically oriented research to support it, my thought is that in an electric boat scenario, it makes more sense to have a three or four-bladed prop, as large as will comfortably fit the boat and with moderate pitch to allow some amount of regeneration with the ability to make the prop "transparent" as Matt stated earlier (clever metaphor, Matt!) when sailing at slower speeds. Two-bladed, variable pitch, and folding props simply won't work as well *because* they're lower drag!
Have I tested these thoughts and theories out in practice ... NO, not yet!! But I'll be back with you on the topic later in the Great Lakes sailing season!
[-tv]
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