I'll try.

The answer will be found in conservation of energy. The energy books
have to balance in every system. The amount if drag of the water
flowing over the prop will exactly equal the energy being produced.
With the prop stopped, it is simply pull aft on the shaft. The prop
isn't very efficient that way. Try propelling a boat by putting the
prop out ahead on a long line and then pulling it in quickly. When
the prop is freewheeling, it is producing energy that is absorbed by
heating up the bearings and lube oil in the transmission, (or in
Larry's case by charging batteries).

It's slightly counterintuitive but, whatever energy is produced has to
have an exact mirror image in drag on the boat. Since the prop is
making energy more efficiently while turning, there has to be more
drag.

--

Roger Long