There are literally hundreds of small trawlers, up to 36 feet which were built with 40 or less HP diesel engines. For example the standard engine in the Willard's was the Perkins 4-107 or 108. This engine is closer to 35 useful HP--and it pushes the boats up to about 7.5 knots. Thousands of sailboats use the 2 and 3 cyl Yanmars, as well as the Perkins.
I was a guest aboard a 55 foot trawler--10 foot beam--which made the transatalantic crossing with a Perkins 4 107, Idlewild, a 57 trawler, which circumnavigated, via the Northwest passage, had a single 55 hp diesel. Check out "Voyaging under power" by Beebe and Leishman--as well as "The Nature of Boats" by Gerr, for discussions of low power boats.
Both of my motor sailors had 80 hp tractor engines--one was 46 feet and weight of 36,000 lbs, the other was 62 feet and over 60,000 lbs--both got about 6 miles a gallon and cruised at slightly over 6 knots--max about 9 knots. It doesn't take much hp to move a boat. The boat I built had 18 hp diesel, with a controlable pitch prop and would do 7 knots easily days on end.