I'm sure locking a prop so that freewheeling does not hurt transmission can become PIA if done often and not having specialized, easy to actuate shaft-lock apparatus.
Luck for us locking is not needed. BW Velvet Transmissions our Tolly has is fine for freewheeling without hurting it; especially at the low speeds we travel when purposefully using just one engine. In addition, I alternate engine use on regular basis during the very slow cruise to keep use-hours similar as well as to not constantly let just one trany experience free wheel stress... no matter how OK it is according to BW experts.
And... because freewheeling is no problem... the freewheeling prop causes less drag than prop locked rigid in position. Which is another assistance to getting near 3 nmpg at 5 nmph, on single engine use, slow-cruise in a planing boat that can also easily cruise at 16 to 17 knots - or above.
Years ago, on this forum in its infancy or on the old Passagemaker Mag Forum, I don't recall which, we had a fellow in Europe with a GB42 who provided actual data to make a true comparison between his boat running at displacement speed on both of his twins OR on only one of them. IIRC his data was collected over a long enough time span to be meaningful.
He proved conclusively that trying to gain efficiency by running on one of a pair was a fool's errand. A single, pushing from the corner, lost efficiency through the drag of the still or freewheeling prop and the rudders having to be angled to keep the boat running straight.
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