Here is my experience. I live in Mich. but boat in Canada on the Great Lakes. With fuel $5.00/gal I thought I would see how good I could do on fuel.
Twin diesels, top speed about 22, cruise nice at 16. At 16 I get about 1 mpg.
Running on one engine the boat handles very well and the auto pilot holds a straight course.
I ran 401 miles on one engine as much as possible. This was over a period of several days with many anchorages and some docking. So there was some idle and some use of both engines. I ran them at wot for 2 short periods.
In 401 miles I used 95.4 gal. This was running at 1000 to 1100 rpms which, depending on sea conditions was about 5.2 to 6 mph with a guess at 5.4 mph average. I also towed a 16.5 aluminum Sea Nymph with a 70 Evinrude and a small trollling motor. Usually about 30 gal gas so it was fairly heavy. From the little bit the previous year without towing I think I lost about 1/2 mph towing the boat. A rough estimate here.
Without towing perhaps as much as 10% better.
When I bought the boat last year the valves had not been adjusted in the 1300 hours on it. Should be every 500 hours.
The engines really smoked and when run at slow speeds then run wot, it would fog smoke so bad you couldn't see through it.
I adjusted the valves and the smoke went away. Not better but gone. Very clean with no soot on the boat.
Running at 1000/1100 rpms I ran it at wot after about 200 miles. Each time it smoked a little for about 15 seconds, then quit.
That told me it wasn't loading up.
Could be wrong but that was my take.
Even the marina operator when I took it for haulout remarked how clean they burned and I had just run 50 miles at slow speeds.
The engines were twin Hino's, 175 hp in a 3870 Bayliner Motoryacht.
Miles were recorded on a GPS and gallons was the old fashioned way, a fuel pump.
That's my story and I am sticking to it.
We were looking at trawlers before we purchased this boat. We fell in love with the style and room. I feel we have the economy of a trawler with the option of a decent cruise speed.
Hope this helps.