The dinghy outboard thought experiment is not valid because the whole
system is coming to a stop and the prop is slowing as the boat is
slowing. Do it while towing the dinghy behind the big boat at a steady
speed with a scale on the tow rope and you may get a different result.

Under some circumstances, a freewheeling prop may have less drag than
a fixed one. Props on normal transmissions are not freewheeling
however. There is enough drag in the shaftline, bearings, and
transmission to upset things. Outboards, with their clutches right in
the lower unit are closer to freewheeling.

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Roger Long