I think that the aspect ratio of the prop and propeller
are very significant factors in this discussion. The drag (caused by fixed blades) to swept area (the area of the turning circle) ratio is very different between propellers and props. High aspect ratio propellers and low aspect props are oranges and apples.
A boat prop presents a large portion of the turning circle to the water
stream. I'll call this the "felt" cross section.
I suspect that a turning boat prop in effect reduces the "felt" cross section. Water
rather than hitting the the fixed blades head
on is swept past the rotating blades at an angle to the boats motion (thus seeing a reduced cross section).
The test (outstanding by the way) done shows that for a given sized outboard
the rotation prop presents less drag and thus allows for a faster motion.
The difference between fixed and freewheeling is non-linear so I would be cautious of giving a force difference. If the test were rerun and the force measured at a specific speed then we could compare the drag force. This would yield a set of curves (speed vs drag) for fixed and freewheeling that would be interesting. (surely this has been done in somewhere back in the '30s?)
My boat has a velvet drive that makes a low growl when freewheeling. You can see where the prior owners have clamped the prop shaft with vice grips. It is scarfed up.
We were at dock
the other day and the current
of the Columbia
river was great enough to cause the prop to freewheel. The river is above flood stage. I ended up clamping the shaft as well so as to get a better nights sleep.
Who knows, someday I expect to put a zinc on the shaft. Drill a whole in the zinc and insert a shear pin in the whole in such a way to stop rotation but still break away if needed. (Oh, I think that I will patent that idea.... remember you saw it here)