<<By increasing the load on the engine, you're
increasing the fuel needed.
If you had the prop set for 1400 rpm (overpropped),
you'd be using much more
throttle to achieve those revs wouldn't you?
So to achieve the same level of fuel consumption,
you'd be running the
engine at, say, 500rpm.>>

You're right that if this boat was overpropped, it
would achieve the same low cruise speed at even lower
than 1400 rpm.  This is not a bad thing.  It is low
power loading that is hard on diesels, not low rpm.

<<I've always understood that the boat should
be so propped that the engine is just able to reach
it's recommended maximum
rpm (and thus max power) with a clean bottom. This
enables the operator to
throttle back to max continuous, or whatever engine
speed is chosen for the
GPH required.
Or am I missing something?>>

You are correct, for most boats, for most uses.
Crossing oceans in a small boat that is over-engined
for the job is not normal useage.  If the N40 is
burning 2 gph at about 6 kts, it's using about 36 hp
from its 130 hp engine.  If the chosen prop is
optimized for 2500 rpm, it will be far from optimum at
1400 rpm.  One (impractical) solution would be to
replace the engine for this voyage, with something
smaller and therefore more efficient at this low
cruise speed (low power requirement).  Another
solution would be to replace the prop with a
controllable pitch propeller (a good choice if a
long-term mix of coastal cruising and passagemaking is
contemplated).  The low-cost solution is to over-prop
the engine for the voyage, limiting top rpm to perhaps
1800, instead of the engine's capability of 2500.  Now
the boat will achieve the same 6 kts at perhaps 1200,
instead of 1400, the prop is running at a more
efficient point, and we can easily put it all back to
normal with a prop change when we arrive at our
long-term cruising destination (or complete the
circumnavigation).  The engine will be happier
producing 50% of available power at 1200 rpm than
producing 30% of available power at 1400 rpm. (Note
that all figures here are approximate, for
illustration only.  I don't have the prop curves,
engine curves, or my scientific calculator with me.)

Overpropping is a way to effectively reduce a boat's
installed engine size when the engine is too large for
the intended use.  If you find that in your useage of
your boat, you never approach to within 200 rpm of
your engines max, you may be a candidate for
overproping (if fuel economy or increased range are
important to you).  In my opinion, many full
displacement trawleryachts in the US are severely
over-engined, and could benefit by a little judicious
overpropping.

=====
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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<<Now I'm confused.  I thought the goal with fooling
with prop size was to
decrease fuel flow at the desired cruise speed, or
alternately increase
speed for the same fuel flow.  But it seems to me that

this tactic would
tend to increase overall efficiency, resulting in a
lighter engine power
loading.>>

Hal,
You reasoning is correct, except for thelast line.
Terminology may be our only problem here.  I'll define
"engine power loading" as the ratio of power used by
the propeller (at a given rpm) to power available from
the engine at that same rpm.  Most diesels have their
best efficiency (usually given as specific fuel
consumption) in the range around 70-80% power loading.
At max rpm, this is 70-80% of the engine's rated
power.  At half rpm (1250 for our 130 hp engine
capable of 2500 rpm) the available engine power may be
about half the rated engine power (check the engine
power curve), but the power used by the propeller will
be very much less.  The propeller power curve law
indicates that the power used by the propeller is
proportional to the rpm raised to the power 2.8 (some
authorities use 2.7 or 2.9; it depends somewhat on the
propeller design).  This difference between the engine
power curve, and the propeller power curve explains
why one fixed pitch prop can be fairly efficient only
over a relatively narrow rpm range, which is the
problem which the CPP overcomes (at considerable cost
in complexity and price).

Most full displacement trawler yachts have a rather
small range of speeds at which they usually travel, so
a single fixed-pitch prop works OK.  Tugs use
controllable pitch props because their displacement
varies by at least an order of magnitude, with barges
added or absent.  Also a small percentage fuel saving
on a 12000 hp tug is worthwhile, while the same
percentage fuel saving on our little boats is not so
important.

Mark Richter, Winnie the Pooh

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<<I think I see.  So another way of looking it is
trying to minimize engine
RPM for a given horsepower output, or actually for a
given speed through the water.>>

Yes, although minimizing the rpm would be going too
far.  The minimum rpm for a given power output would
occur at 100% power loading, which is not a good point
at which to operate a diesel.  80% would seem to be a
good target for best fuel economy, but even this power
loading for normal cruise speed would leave little
reserve power for adverse conditions (dirty bottom,
barnacles on prop, head wind & seas).  I might choose
60-70%, to leave a little something in reserve.

Mark Richter

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