You are kidding yourself if you think you will save significant fuel by shutting down one engine and feathering/braking the other one.
Take a look at the attached graph. The Alaskan based data was done by Bob Lowe and I know he reported it correctly. FWIW he didn't find significant difference between freewheeling and braking a fixed pitch prop.
David
Regarding there not be a significant difference between freewheeling and braking a fixed pitched prop, here is my anecdotal experience suggesting otherwise. Before leaving on the Great Loop last May I was fully aware that the cutlass bearings were very tight, so tight that it was extremely difficult with the boat out of the water to rotate the shafts even using the 25-inch props as a lever. Inside, using an 18-inch lever arm at the shaft coupling it was equally difficult. Still, the boat was that way when purchased five years earlier and we had no excessive vibration so off we went.
Fast forward to last January. In Pelican Cove at Cayo Costa in Florida, I managed to go aground in some sand exiting the cove at very low tide. No problem I thought, I'll just back off and retrace my course in-bound. I applied a fair amount of power, but too much as it turned out because I apparently exceeded the torque rating of my Drive Savers. The connecting bolts ripped out of the hard rubber and I was left with one-shaft propulsion. When I was able to get underway it was 40 miles on one engine to Fort Myers. Later, we had the boat hauled, eight cutless bearings replaced, the shafts trued, props reconditioned, and the engines aligned (in the water).
That's the background. Now for the anecdotal evidence. On the 40-mile run up to Forts Myers with the shaft not rotating keeping the boat on a straight course required TWENTY degrees of rudder. After the work was done, running on one engine required just four degrees of correction with the non-operating shaft happily spinning in the water AND I was able to use the autopilot. The transmissions are Borg Warner Velvet Drives which can be free-wheeled without damaging them. Before repair, on the one engine the fastest I could travel was barely 6 MPH at 1,600 RPM (Lehman 120). After repair, I got over 7 MPH at the same RPM.
Draw your own conclusions but, im my opinion, their is indeed a profound difference in running on one engine with a locked shaft versus a free-wheeling shaft. I do recognize that some brands of transmissions can be damaged if free-wheeled.
As to whether there is any fuel savings, my guess is perhaps some small amount but I don't run with just one engine now except in special situations and not to save fuel. For example, I ran on one engine when recently transiting the Dismal Swamp which pretty much requires speeds of four-to-five MPH. Keeping our boat at those speeds on two engines is quite annoying as it means operating at not much above idle speed which makes for annoying vibrations at times. Running on one engine the length of the river and canal made for a pleasant, quiet transit. I also ran on one engine going through Norfolk with its speed restrictions.
I think there are some folks who are convinced that running on one engine derives significant savings. I don't know that they are incorrect. I only know that I can't be bothered except in those special situations. One last note, I mentioned earlier that we had no vibrations before the repairs. Well, we just knew no different. We found significantly reduced vibration as perceived when standing on either side of our aft berth where the shafts exit the hull. The boat overall runs so much more smoothly now. We wish we had undertaken these repairs before on the Loop.
Oh, one last note on fuel consumption before and after the repairs. Before the repair we averaged 3.7 GPH. After we got down to 3.4 GPH. That is a huge difference over the course of a 6,000 mile trip. I calculate that I wasted four or five hundred dollars in fuel, perhaps more. Our boat weighs 56,000 pounds with a full load.