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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)

÷

RPM

Or

Torque, KGM = (975.175 x kW)

÷

RPM

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

RPM.

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

÷

18.

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.

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.