The MD22 is basically the Austin Maestro diesel marinised, which in itself was the BL O series petrol (used in the Ital and Princess). As such is has a low compression ration and a short stroke, so is a high-revving engine relative to most diesel engines. It only really puts out anywhere near 50hp at 3,500rpm. Assuming you cruise at about 2,200-2,400rpm, it will be producing about 20hp.
I went through this a couple of years ago with my own boat, and actually wrote an article on it which is in the last BOA yearbook.
Personally, I decided to fir a Beta engine rather than a Volvo, for a number of reasons, namely:-
1./ Volvo parts are really expensive
2./ The Volvo warranty is only valid if the engine in fitted and serviced by a VP dealer
3./ The Volvo engines use an MDI unit to electronically control the engine (which have known issues of failure)
4./ The Beta engines are based on Kubota engines, which you can get spares for anywhere in the world
5./ Beta engines are better value than Volvo engines.
Have a read of the article (you can download it from the website), and come to your own conclusions. I would have thought that a Beta 43 would be an excellent choice, and will sit directly on the same engine bearers.
My boat (a 49) was originally fitted with the turbo version of the MD22 (rated at 105hp), and I now have a Beta 50. The old engine used to cruise at 7kts at 2,400rpm. The new one cruises at 7kts at 1,900rpm, using about half the fuel. Bigger is not always better when it comes to engines.
