A turning prop can be made to do work, charging a battery, and when
doing so creates more drag than a stalled one, so they say. If a
shroud were to streamline a stalled prop, would not the drag
decrease significantly?

Could a retractable shroud be allowed to slide down a prop shaft to
fair a stalled prop? How could it be reliably retracted to allow the
prop to be used after a voyage spent growing barnacles? Perhaps the
shroud could retract up into the hull with the prop stopped in a
certain position?

Or would such a problem not occur if the shroud retractor and prop
was used many days for a few hours to charge up the battery, etc?

Perhaps a proper variable pitch prop would be better efficiency
given no need for the weight of a gearbox?

Terry K