I am on my 3rd Mainship, my first being a Mainship 390. As a trawler platform, the boat was great. Honestly, the only reason we went to the Mainship Pilot's was due a bridge that I have go under when leaving our dock.
My 390 had a single Yanmar 315, which moved us along at about 10-12 knots cruising speed. With the standard bow thruster, backing into a slip was quite easy, even with crosswind. Thoughts on a single diesel is that it is reliable, and most failures are due to fuel issues. Another thought comes from an old article from Capt Pike, where it is noted that when you have one engine you treat it well, with two it is double the expense. Full economy was excellent (something I miss with the Pilot 43). The 390 moves well with a single, and I believe that it was designed that way.
My wife and I have covered the northeast with the 390. It served us well. Enjoy!