Hi Cal:
I have thought of this problem also, of not having enough load on
a diesel to have it run efficiently, and have come to think that a Diesel-
Electric system would be the way to go. You can have a small generator
running for light loads, and slow running, have a larger generator for
running at cruising speed, and run both generators for War Emergency
Power!
Don't know if this is an option in your situation, but here are some
links that you might find interesting reading.

OSSA:
http://www.ossapowerlite.com/index.htm
Fast Electric Yatch System:
http://www.feys.org/
--

SeeYaa Harbin Osteen KG6URO

When American Citizens with dual citizenship pledges allegiance
to the flag, to which flag do they pledge allegiance too?

-
"Cal Vanize" wrote in message ...


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!!