For loads of information on the Mainship 40, go to Steve Cyr's website:
Steven Cyr's Stella Blue Site He has the single Yanmar and says that it is a hull speed cruiser with that engine.
The MS 40 weighs 24,000 lbs dry and maybe as much as 28,000 loaded with people, gear and fluids. I don't think that even the twin Yanmars at 480 hp total will push it up to the high teens at wot, maybe 16 or so and cruise at 12.
David