This has been going on for a long time this argument. And I don't mean just
in this forum for the last few days.

In the late fifties (of last century), my father was building himself a
steel ocean racing yacht. The prop drag controversy was alive and well then

There was a handicap allowance at the time for prop based on diameter, and
some silly ineffectual props that looked likesomething off a model airplane
were being tried. Sometimes two in tandem, and they were of course locked in
line with the deadwood. Safety regulations required that the boat be able to
demonstrate performance under power. In still air and calm water, of speed
in knots equal to the square root of the wateline length in feet. I remember
a 58ft 10 Meter Class yacht I was crewing on at the time being unable to
make headway into a 25 Knot breeze. She had about 50 SHP, but a small prop
that was able to transmit only a small fraction of the engine power into
thrust.

Dad didn't want any of this nonsense, if his boat needed an engine for
safety reasons, then it was going to be able to use the power of the engine
and go to windward if necessary despite the weather. But he wanted it to be
competitive.

He got a piece of steel tube about 3 inches in diameter and 18 inches long
and machined inside each end to take the outer ring of a tapered roller
bearing. The inners of the bearings were installed on the shaft with
shoulders so that the bearings were opposed and transmitted the forward and
reverse thrust to the outer rings. Outside of the bearings at each end of
the tube was a normal oil seal, installed the right way round so as to keep
oil inside the tube. The oil inside the tube was pressurised by a header
tank mounted about 3 ft above the waterline.

The tube was mounted at the trailing edge of the keel aperture, with the
prop sized to the 40 HP diesel engine.

Inside the boat between the shaft and the engine gearbox there was a dog
clutch, so that the prop and shaft could be completely disconnected from the
gearbox.

The unit had so little friction that on the slip on a windy day the prop
would revolve in the breeze.

Driven off the shaft by a small chain was an aircraft tachometer generator,
and in the cockpit the tachometer, which was calibrated to read knots. It
was about as accurate as anything I've ever used. I once ran a DR plot based
on it that was 10 nm out at the end of a 600 nm Sydney Hobart race. 1.5%?

Dad ran into the third or fourth owner of the boat in 1980, and when told
that the stern tube assemble had just required repair took delight in being
able say what a pity it was, as the guarantee had just run out.

Going back to the argument about locking or freewheeling: this subject
occupied numerous off watch race hours without as I remember any consensus.
Had we had a GPS or paddle wheel log then it would have been easy to set the
boat up with the prop locked, and then unlock it to see if the speed
increased. Though I doubt we would have done this during a race, and in
light winds (when the effect will be greatest) we never wanted to sail if it
wasn't a race. We had this beaught engine with a big prop.