[new_bug]

I'm looking to replace a 1970 (design originally 1938ish!) Detroit diesel 4-71 (~115-125hp set up @1600-1800rpm max, non turbo) with estimated 50,000hours on it and only one rebuild, in a commercial workboat.

It's a low speed 6-7mph boat, you could think of it more like a heavy sailboat hull than a typical passenger/yacht & it's already optimized for great efficiency for the size (40-50ft), with large prop and currently run most of the time around say 2 gph at 1200-1400rpm. (Yes the full ~100+hp at 1800rpm is certainly occasionally needed so I can't go much smaller). In (displacement) boats, large as possible diameter slow rpm prop is always best for efficiency. I am, ideally- looking for more of a stationary/generator "type" engine not a 2600-3200rpm screamer. But also the 4-71 weighs 2000lbs so something lighter wouldn't be a bad thing, and the gearbox will be changed out at the same time ,,,,so that could all be dialed in on a higher rpm engine with gearing.

I'm not looking to go electric or hybrid diesel electric at this time.

I know boats probably don't get much attention here but I believe they could still be considered "vehicles"- and thought I'd post it here for the most varied different sets of eyes on the "problem" - moderators feel free to move if there is a more appropriate forum.

I'd like to choose a "COMMON, readily available" replacement 4cyl diesel with;
1) fuel economy first
2) simplicity & reliability second

I have many of my own opinions & preconceptions which I will post more of later. But to get the discussion started/ for example:

-A lot of the higher end commercial boats and mechanics these days will say to look at john deere turbo diesels

-Sailboats and Yachts of all sizes often lean towards Yanmar, but I might lean away from that route after dealing with their prices and dealers in the past. I would consider the previous generations.

-Kubota (marinized under various names, Universal, Beta Marine etc) is more common on the smaller end of the spectrum, but I've seen good examples of reliability and economy. Affordability gets a couple bonus points here overall. Average or better longetivity, maybe not the top of the list though?

-Some of the old school mechanic types will say there is NO other solution (in the say ~130hp range) than a 5.9l cummins 6bt, and if you must- maybe 4bt (the 4cyl version of the same engine, probably closer to 80-105hp) could be considered despite the prices etc

-Isuzu is also a strong contender.

-I've had good luck with marinized 4cyl Volkswagen but that's closer to the 60hp range as far as I know

-Mercedes is an option, if I was in Europe I think that might be a no brainer, but their "industrial" engines are NOT common or cheap in north america.

Lower RPM options optimized closer to the 1600-2200rpm range (for peak fuel economy) are not as common , but there are some still out there. Most or nearly all are older designs. It's time to upgrade, despite the longetivity of the current setup- rebuild is not a real consideration.


Thanks in advance for any specific model/brand recommendations, your thoughts, or other input. Overall I'd like to stay with a "bigger" 4cyl for efficiency "generally speaking" and NOT consider 5 or 6cyl options/etc.