On Dec 31 2006, 9:18 pm, Cal Vanize
wrote:
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!!

=====

Cal: Some points to keep in mind:

1. Despite the countless variations in diesel engine designs and sizes,
there are common principles and fundamentals that apply. For example,
every diesel engine likes to run hard and long at its engineered
"cruising RPM", otherwise known as its "sweet spot." At cruising RPM,
the engine vibrates less, is quieter, and is most economical in terms
of fuel burn. That's why it's called the sweet spot. There's nothing to
be gained, and sometimes much harm is done, by "going easy" on a
diesel. Long-term, a diesel designed to run at high RPM will strangle
itself at low RPM -- carbon up the exhaust system, clog injectors with
unburned fuel, etc. So, as you ask, there is no maintenance advantage
to running a diesel slower than its designed cruising RPM.

2. Follow the mfr's recommendations regarding lube oil changes, filter
changes, etc. The biggest single killer of marine diesel engines is
unchanged lube oil and lube oil filters, and paradoxically, this is one
of the easiest maintenance chores to perform.

3. In my diesel engine maintenance workshops ("Diesels for Dummies"),
and aboard boats where I train new owners, I encourage familiarity with
their diesel engines. Routine preventive maintenance, which is neither
difficult nor time-consuming, ensures reliable performance and long
engine life. Presuming the engine is properly spec'ed for the boat, if
you give it clean filtered lube oil and clean filtered fuel, plus
abundant clean filtered air, it will be there to serve you every time
you need it.

Hope this helps,


Captain Bernie Weiss
www.AtlanticYachtDelivery.clm