Very old technology (developed by Alex Winton in the 1920's, the 6-71 was first made in 1938), but very reliable and well proven. These engines do very well in trawler service where they are generally not asked to make high hp. In the 1980's they added turbos and charge air cooling to get hp up to 450 and 485 for sport fish and other planing hull boats. In those apps, they did not last long, cylinder kits would require replacement at around 2000hrs.
Post what hp rating these are and whether they have turbos. Also post what the owner cruised at regarding rpm, boat speed and gph. That will help determine if the engines are being run in a "sweet spot". If run at say 1400rpm and under 5gph each, they can run dang near forever, 10k hrs and running perfect is not unusual.
They are a little different than four strokes. Exhaust noise can be louder if mufflers are not carefully designed. They sound a little different, some like it, some don't.
They are not quite as fuel efficient as the modern four strokes, but at low rated hp and modest power settings they are very close. Most around 18hp/gph compared to modern four stroke at 19-20hp/gph. The 450 and 485 are a good bit worse at 16-17, but I doubt those are what you are looking at.
But a Jimmy in good shape can be one of the most reliable powerplants on the planet. While out of production now for like 20yrs, there is no problem at all getting the basic engine parts. Some marine package specific parts may be a challenge, but since there are thousands still in service, there is always a way to get what you need.
Prior to buying, you want someone well familiar with these to check them out. The weak spots are well known by these techs, and the symptoms are not the same as on a four stroke. A good DD guy can tell you basic engine health in a half hour. Then a sea trial gets you the whole story.
An absolute classic gem of an engine.
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