Great thread, lots of good observations. I have spent a lifetime of operating boats with 271, 371, 471, 671 generators and 671N, 871N, 1271TI and a few 1671's for propulsion. My current boat a 47 Bertram M/Y that has 871TI's. At a young age I began rebuilding the 671's from the boats in the family business. I said all of this to openly admit that I am a bit biased because I am comfortable with the 71's.
If money were not a factor would I buy a new boat with DD? No way! But as stated in an earlier post the DD provided a pretty good aft cabin M/Y for around a 50K investment. I never run above 1000 to 1200 rpm (displacement speed) and obtain about 1 mpg. Over the 1500 miles traveled this year.
Yes they are old, they smoke when cold and have a few oil leaks. But, when I rebuild over the next couple years it will be cheap and yes 99% of the oil leaks will stop with the rebuild. After market parts are available everywhere. Just compare the cost of a cylinder kit for a Cat to a DD and you understand why when on a budget I rebuild the DD rather than repower.
Because at displacement speed I do not need the current HP, when rebuilding I will remove the turbo and after coolers, use N style pistons over the current turbo piston and step down to 45 or 55 injectors. This will now produce an 871N that will run long past my lifetime at a fraction of repower cost. I've seen similar 1200 rpm generator packages run 24-7 for 3-4 years and then only need basic service.
Love the 4 stroke John Deere, Cat's and Cummins but financially not a sound $ investment in an old boat.