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nor consideration. The reality is that the two upper power
curves aren’t what your propeller sees or uses. It’s the pro-
peller power curve that governs things. But you do want to
reach maximum rated engine RPMs, or pretty darn close.
The Torque Curve
Toque is the twisting force on your prop shaft. You could
have all the power in the world, but if the shaft didn’t spin
you’d have no torque and no go. Torque is actually defined
mathematically with the formula:
Torque, in pound-feet = (5252 x HP)
Torque, KGM = (975.175 x kW)
The torque curve shows the torque generated by this engine
at various RPMs. The interesting
thing is that maximum torque on
normal internal-combustion en-
gines doesn’t occur at maximum
engine rated power and RPM. In
fact, the torque curve of the 420
CX Yanmar is pretty typical. The
maximum torque occurs at
about 77 percent of maximum
RPM, or 2,100 RPM. Indeed, on
most engines maximum torque
falls somewhere between about
55 percent and 80 percent of
max RPM. (Light gas engines
tend to have peak torque at
lower engine RPMs and heavy
diesels at higher RPMs.) The
units for the torque curve are in
kgm (kilogram meters) and Nm
(Newton meters). This is the
metric equivalent of pound feet.
We’re not particularly concerned with the absolute numbers;
however, we’re simply interested in where torque is highest.
It simple terms, the 420 CX Yanmar is delivering the most
oomph per gallon of fuel consumed from 2,100 RPM. This
wouldn’t be a bad low-cruising speed, but you don’t unnec-
essarily want to limit operating speed this much. Let’s see
how fuel consumption fits into the picture.
Specific Fuel Consumption
The specific fuel consumption curve reads—as is often the
case—in rather inconvenient units. In this instance, in grams
per horsepower per hour (g/hp·h) and in grams per kilowatt
per hour (g/kW). For the moment, though, what we’re really
interested in is the shape of the curve and where fuel con-
sumption is lowest for the output power and torque. In other
words—just the opposite of the power and torque curves—
the best spot on the specific fuel consumption curve is
where it’s lowest. For the 420 CX Yanmar, this is at 2,000
Since we already know that the optimum torque is at 2,1
RPM, you could say that you’d get the most bang for the
buck out of this engine at 2,050 RPM — a combination of
best fuel efficiency and most oomph.
As for the curve of specific fuel consumption, this is incon
venient in more ways than simply converting grams per
horsepower hour to sensible units such as liters per kilow
per hour or gallons per horsepower hour. This isn’t becau
the conversion is difficult (it isn’t) but because almost eve
engine’s specific fuel consumption curve seems to under-
state the real-world, in-the-boat fuel consumption.
In practice, I’ve found that almost all diesel engines con-
sume approximately 0.054 gallons per horsepower per
hour. Or (restated to make things simpler still) simply divi
the propeller power curve
power at any RPM by 18.5 to
find fuel consumption in real
service in gallons per hour.
For the 420 CX Yanmar, prop
ler output power at 2,300 RP
is 250 HP, so fuel consumpti
at this RPM (assuming the pr
peller is properly matched) is
13.5 gal./hr. [250 HP
13.5 gal./hr.], while at maxi-
mum 2,700 RPM fuel consu
tion is 22.7 gal./hr. (The sha
of the specific fuel consumpt
curve and its point of maxim
and minimum consumption
usually quite accurate in ser-
vice, just not the absolute co
sumption numbers indicated
Determining Optimum Cruising Speed
So far, we’ve seen what the engine performance curves
mean and that maximum oomph and fuel efficiency occu
for this engine at between 2,000 and 2,100 RPM. Is this
proper cruising speed? Well, an argument can be made f
this in terms of sheer efficiency, but it’d be a bit slow. In-
stead, we can look at the performance curves to determi
where the best compromise between efficiency and spee
falls. In this case, 2,300 RPM looks like a good bet. At
2,300, torque is still high, specific fuel consumption is sti
low, and we’re getting a reasonable 250 HP at the propel
Low cruise, for quiet, efficiency, and maximum range wo
be around 2,000 RPM, and you’d only open her up to 2,7
from time to time to show off or outrun a storm.
Units of Torque
In the English system torque is measured in rather
consistent “pound-feet.” The Yanmar (being a Japa-
nese engine) has its torque measured in metric units.
For torque the engine graph reads in “kg·m” on one
side and in “Nm” on the other side. These are
(currently) the two standard metric units for torque.
kg·m is kilogram meters (the old metric torque meas-
ure commonly noted as kgm), while Nm is Newton
meters (the new metric torque measure). Kilograms
are units of weight or force and, to be absolutely ac-
curate, engineers should use the designation
“kgf” (kilograms of force) rather than just plain "kg."
Of course, it’s usually omitted because it’s obvious.
Newtons are the new metric measure of force only.
Newtons can’t ever be mass. This makes them tech-
nically “better.” The bottom line is that pound-feet,
kg·m, and Nm are all measures of torque.