Hey--quit ruining threads by tossing in expert opinions. If someone tells me his Grandma's Hupmobile straight eight got 85MPG and he could suck on the tailpipe all day long without even a cough, that's enough for me.
#1 is because of weight and aerodynamics, #2 is because of displacement speeds. Very little to do with the engine . . .
When I started in the diesel engine biz (1977) .4 lbs of diesel per bhp/hr was respectable, now it is .3 . . .
I actually think longevity in similar modern diesels is the same or better. By similar I mean similar weight and engine speeds. There are a lot of very good lightweight and highspeed diesels these days. They may not live as long, but they aren't designed to either . . . We do a lot of business today with 7 liters that was once done by 14 liters . . .
BTW, back to your Roughwater, modernrocketry . . . Edwin Monk was the king of single screw, semi-displacement, super efficient hulls . . .