Rereading the thread, I think you've been too quick to dismiss outboards. Boats like the Rosborough would be quite happy with a modern 60-90 hp outboard. I have a friend with one that has a mid-size Honda and it's a lovely slow cruiser.
Agreed. For the record I do indeed love my 18hp Kubota diesel. I am grateful and like the reliability. Every problem has been resolved. She starts every time and cruising takes little fuel. I do know that fuel burn will increase with an outboard, so that can come into play.
Diesel is easy. It is also LOUD. Which is okay -- however as my hearing deteriorates, the VHF becomes almost impossible to hear when underway.
Anyway, back when shopping I turned my nose up at outboards. In retrospect, that may have been the wrong decision. Outboards are quieter.
That said, NEVER EVER an inboard/outboard. Those things can sink your boat (the boots are seldom maintained) and when those boots develop a crack and fail the boat becomes a rock.
So though diesel is indeed great, an outboard can be a viable option. Times change. Remember when tractor engines were modified for boats? Then diesel. Next turbo charged, and now... it appears to me at least around here that there are quite a few 40-plus footers with three or four outboards across the transom.
From the boat show: (admittedly flashy!) and IF I'm reading that sticker correctly 1,700,000. Could that be possible?!?
Two MUCH SMALLER outboards would work better for a slower journey, and with less fuel burn. And outboards bought new are costly... outboard experts can provide better information than I. However, if my engine finally is not repairable, i would seriously consider an outboard for the transom.
A white 9.9 Suzuki long-shaft was selling at the boat show last year for less than $3k. I have spent wayyyy more than that on my diesel. Fuel costs would I suspect balance out the "deal" that the outboard appears to offer....
Just rambling however I do agree with Jeff F. Definitely consider an outboard driven vessel. I suspect this will become more and more common in the coming years.