Hi, Paul. No, if the prop self feathers or folds when the shaft is not being turned by the motor, and you have no way to prevent it, then you cannot get any regen charging at all. Period. A controllable pitch prop would work of course. There must be some power source for the shaft to turn. It will either be the resistance of the water acting on the prop, or the electrical power from the controller into the motor. Whichever end is causing the shaft to turn, something on the other end is consuming that power. Either the prop, pushing on the water, or the regenerative load on the motor acting as an alternator or generator. When the shaft is stopped, the prop is acting as a brake on the hull. Unless it is a folder.
For racing, your rules probably prohibit this, but when cruising you can simply turn the prop very slowly with the motor so that there is no drag from the prop but no thrust, either. Energy expended when doing this is minimal.
Regen is something that a lot of businesses use to convince you to go electric. The fact of the matter is, though, that charging through regeneration is not significant at the typical sailing speeds of small sailboats. So don't let regen influence your prop selection overly much. I will say this, though... when motoring, most folding props are not particularly efficient. They are not meant to be. They are pimarily meant to offer minimal drag when not in use, and enough power to maneuver in and out of a marina. A three blade fixed prop properly pitched and sized for your boat will be much more efficient. Like I said, you can simply turn the prop very slowly to reduce drag. Or let it turn in trail shaft mode and get a token amount of regen. Or boost the power slightly to motor-sail. For long ocean passages you might even consider removing your prop, especially if you have an outboard for backup in case some maneuvering might be necessary.
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