Another way of looking at this issue is the analogy of turning a drill bit through a piece of wood
vs. pushing it through. If the speed of the freewheeling prop can match that of a 'screw' going
through the water, it seems like that would have the
minimum drag. If the speed is considerably different, it would have the effect of
pulling an almost flat object, the diameter of the prop, through the water, causing
appreciable drag. Seems like self-feathering props are designed to fix this problem,
and outboards taken out of the water while under sail, both try to minimize drag.

Sherwin D.