"For their price
I'm sure I could put in a new more efficient smaller diesel engine
and still come out way ahead"
I'm sure you can scrounge a used engine
for free. But if you had to BUY one, or do a major refit
, and hire some help, build a new engine bed
, whatever, more typically you could drop $5000 to $10,000 before you turned around. Even at $5/gallon, that's a lot of any kind of fuel
you could buy instead.
And of course, if you put in an odd-ball non-standard engine...you may have an issue when you eventually try to go sell the boat. Another major expense.
Just saying, consider these things before you make a "bespoke" boat for your own purposes.