As we continue to look at trawlers and motor yachts, they seem to come
in a wide range of diesel engines. We've seen some 44' - 47' boats that
use engines ranging in horsepower from a single 125 to twin 450s. They
seem to have only small "relative" differences in LWL (to me meaning
similar hull speeds). They have differences in displacement, but the
differences don't seem to track very closely to engine HP.

If these boats are operated below planing speed as displacement hulls,
then shouldn't they require around the same amount of power to push them
through the water?

In reading engine specifications (power and fuel consumption) it appears
that large diesel engines running at a relatively low RPMs use around
the same of less fuel than smaller diesels that are run harder. Or at
least the horsepower to the prop seems to cost about the same fuel
consumption regardless of engine size. The engine RPMs for a given HP
seems to fall at different points on the torque curve for larger HP
engines than for smaller ones, especially for turbocharged engines. (I
don't know if this makes any difference or not.)


(OK, thanks for following. This is what I'm leading up to....)


So is it more economical to operate large diesel engines at low RPM or
small diesel engines at higher RPMs?

Is there a long term problem with either running a large diesel under
gentle RPMs or a smaller diesel closer to its continuous duty capacity?

Is there a maintenance advantage (extended oil, belt, filter, etc.,
longer engine life) to running a diesel slower?


Are these questions even meaningful to boat ownership and maintenance?

TIA and happy New Year to all!!