Hi,
The smaller the diesel engine the harder it has to work to achieve the
hull speed of the boat.

(Square root of the waterline length X 1.34)

You need the minimum size diesel that will achieve this. If 50
horsepower does it then that is what you need.

Diesels must be worked hard or there are several issues with glazing and
smoking. A hard worked diesel engine is a happy engine.

If you want a planing fast boat then you must buy more horsepower.
However a Trawler type 'Slow Boat' only needs minimum power.

They are cheaper to run per hour as well.

Oliver Fleming



"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!!