My understanding of what is going on and why differs with that assumption of "less effciency". The "Auto" in AutoProp gives a fantastic Bite when needed and 'de-pitches' when not.... such as when you have sail up to aid in propelling the boat by its unique design. So I carry that forward to assume that when "big waves are stopping the boat" it goes to max pitch and bites hard - just like when you are starting from a stop - in either direction (the blades switch around to always face the same way into the resisting water)..
No, the specific situation that I was talking about happened at Fournells in Mallorca. Furnells is a large cove with a small entrance. It is a very good shelter. Me and lot's of boats were inside while the Mistral was blowing outside at more than 40 k. It take almost a week for the wind to lower to about 25K. The seas raised by that storm were hudge (for the med) with very steep 5 or 6 meters waves that come (as usual in the med) with a very short period.
The wind was just blowing dead on the passage and we atempted to sail out knowing that if he could motor out of that narrow passage we could turn the boat and sail fast out of that hole.
Everything went find except that we take 3/4 of an hour to clear that small passage, making less than a knot with the engine at 3/4 of max Rpm.
The boat took a steep big wave and the autoproop ajusted for power, the boat went down the same wave and the propeller adjusted for speed. Almost imediately it started to climb another one and the propeller tried to ajust for bite, but with so much fast changes in pitch, de overall bite was not good.
As jrd22 as said: "You do need to use more throttle from a stop compared to a fixed blade" and as the boat was always trying to gain speed, just to be stoped by the next have, I believe that in these conditions a fixed prop woud have been more efficient.