I believe the engine changes began with the Cutwater boats.
The Yanmar engines (150hp and up) used in the Ranger Tugs are BMW engines; marinized (sp?) by Yanmar. BMW is always expensive. So are their parts. Most of their engines have good records regarding operations after a period of time around the 1600 and the 2000 tii models (late70s?), where some of them (mostly 6 cylinder) had some problems with overheating. But, BMW worked past that. Very good, dependable engines. But, expensive.
Volvo engines and parts aren't cheap either. My experience with their engines has been good, both cars and diesel boat engines. What I always appreciated was their simplicity and ease of maintenance. They have always been dependable; diesel engines installed in Albin 25s (early 1970s) are still running like swiss watch works. Parts are expensive! Can't speak about their new engines. If they're common rail electronic, they won't be simple.
I spent 12 years working with jet engines, aircraft, liquid and solid rocket engines and missiles, along with related support equipment. Even spent a couple of years teaching maintenance and a year teaching aircraft accident investigation. All that left me with an interest with all engines. 😱
gene