Really? Perhaps someone should tell Grand Banks as they and the original company, American Marine, have been doing exactly this with staggering success on a worldwide basis since 1966
No question , the REASON for the "staggering success"is very simple ,
the engines installed, Ford Econo-Power marinizations or Perkins which have an remarkably high rated power compared to their actual output capability.
No one in their right mind would consider using a tractor or taxi or light truck engine suitable for a constant rated load.
Like your car , it might say 455hp on the air cleaner , but in an earth mover it wouldn't last the day. In your car 200,000+ miles .
Same with tractor or lawn implement takeouts . FAB! for an hour or two on the pin, followed by weeks of plodding at 3 GPH.
A real industrial engine IS useful at 100% load factor , DD's and others do it for tens of thousands of hours in the oil patch hooked to big pumps.24/7/365
For lower noise and good service life , most pleasure boat assemblers use engines that are comfortable at about 75% power , or less.
Genset builders prefer 80% load at 90% RPM, again for longer service life.
Single or twin if 70 hp moves the boat , 135 rated making 70 great , 5 gph and not too much noise , with 2 engines is only 40hp each , not great but still at 3 gph ,not that all that far from what they were built for.
The method of success was engine selection , of an engine with great low power ability to run almost forever.