<<We're actually spinning a 28" by 24" four-blade, left-handed propeller. The

transmission is a Twin Disk MG-5050 with a three-to-one reduction gear. We

use a 2" Aquamet propeller shaft.>>

The reported N40 fuel consumption is approximately twice as high as that
predicted by the BC tank tests and featured in Nordhavn advertizing. Part of
that is undoubtedly due to weather and to deployment of roll damping aids.
However a significant part may be due to the lowered efficiency of the prop
when running at reduced speed and output. The BC tests estimated 50% prop
efficiency. Our calculations show that the N40 prop is running slightly
greater than 35% efficiency at 1400 rpm. This makes a pretty good case for a
CPP on circumnavigations. Also a good case for actual sea trials before
starting the publicity engines.

Larry Z

<<Our calculations show that the N40 prop is
running slightly
greater than 35% efficiency at 1400 rpm. This makes a
pretty good case for a
CPP on circumnavigations>>

Larry,
I'd say this data makes a pretty good case that you
could do better than crossing oceans at 1400 rpm with
a fixed prop selected (as is common practice) for max
rpm (about 2500, I think, for the N40's Lugger).  To
maximize range at low rpm, you want to "overprop" the
engine to increase the load on the engine.  Otherwise,
you are developing (using) only a small fraction of
the engines available horsepower at the running rpm,
which is fuel innefficient and can be hard on the
engine.  Engine makers don't want you to overprop the
engine, because of the potential to damage the engine
by trying to run over 100% of the available power, but
they must assume that the boat operator is an idiot
(because some are).

=====
Mark Richter, M.E., aboard M/V Winnie the Pooh,
custom Morgan 46 Pilothouse Efficiency Trawler.
"Mark's Mobile Marine" electrical systems repair & consulting.  Homeport Stuart, FL

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