I hope this will fall under "fair use". I did some ocr and took
parts from the first part of this article by Stephen Olsen to help
professional fishermen evaluate the setup in their boats. It's meant
to supply cautionary information about adapting engines for marine
The performance curves you look at may be laid out differently.
From National Fisherman Magazine
Article by Stephen Olson
"Today most engine specification sheets include a chart called a
performance curve. In theory, it presents all the information you
need to evaluate the engine that you're considering buying. It is
also about the closest you're going to get to actual engineering data
on engines, instead of marketing hype and rosy scenarios.
The chart contains the performance of the engine as measured on a
dynamometer. Read with 'proper understanding', it shows you what the
engine will do when installed in your boat.
A performance curve like. the one accompanying this article has two
lines of reference, or axis, that apply to all three curves plotted
on the graph.
The engine speed, in revolutions per minute, is plotted on the
horizontal axis. At the left is 1,000 rpm, which is the slow end of
this engine's operating range. On the right is 2,400 rpm, which is
the highest speed the governor will let the engineturn. Power, fuel
and torque are set up on the vertical axis".
"Not shown on the chart is a high-output or intermittent curve that
would show the performance you'd get if you ran the engine at maximum
injection and full turbo boost. This is pretty much of interest to
One reason for this is that the engine cannot tolerate the heat
generated by operation at this power setting for very long. After a
short burst, it needs to be backed off so that the exhaust valves and
turbo can cool down.
This makes some sense in truck service, where a truck may need an
extra burst of power to climb a hill. With a planing hull this kind
of power is needed to get the hull up out of the water.
But with a displacement hull you choose your gear when you select the
propeller diameter and pitch. It's not like a truck, where you can
shift down and unload the engine a little.
The other explanation for "intermittent" ratings has to do
with marketing. Many buyers look at the horsepower number on the
brochure, and the bigger the number, the happier they are."
"So if the engine can be tricked into producing a higher output, it
allows advertising a bigger number than the competition. The
manufacturer isn't obligated to mention that when you're running at
the top of the chart the engine is swilling fuel like a drunken
sailor. And it's burning itself out in half the hours that could be
run at the lower medium duty or continuous settings.
In any-event, all comments on the various curves on these graphs will
be about the medium-duty curve.
The first graph shows torque output plotted on the vertical axis,
with the torque numbers on the right side of the chart. Torque is
stated in pounds-foot in America and Newton-meters in the rest of the
world. Sort of like Fahrenheit and centigrade, or horsepower and
kilowatts different terms for the same thing.
On the sample graph, the engine produces 330 pounds-foot of torque at
1,000 rpm. This means that if you could use the engine's power to
lift a weight at the end of a beam, it could lift a 33-pound weight
at the end of a lO-foot beam and hold it there. Put on more weight
and the engine would slow down, lose power and drop the weight.
Torque used to be very important, particularly in bulldozers. When
you lower the blade into the earth it's the torque that keeps the
engine from lugging down and stalling when you hit a tree root. Nowa
days almost all bulldozers have hydraulic transmissions, and the
hydraulics will bypass before the engine kills.
That's one of the reasons why companies like Caterpillar no longer
make the long-stroke slow-rolling rock crushers of yore. A boat's
propeller is a textbook example of a hydraulic coupling, which
cavitates and unloads itself, keeping you from killing the engine by
Looking at the torque curve, you see that torque starts peaking at
1,400 rpm. It reaches highest torque at 1,800 rpm and then starts to
drop. It is a fairly common rule of thumb that an engine should beset
up to run at a speed where torque is still near its peak.
Power output is plotted on the next graph down. Output is stated in
brake horsepower, with kilowatts given in parenthesis.
The numbers are on the left side of the chart. This keeps them
separate from the torque values for the chart above it. Notice that
these three graphs are all alike for engine revolutions on the
horizontal axis, while the vertical axis numbers are different.
The horsepower curve resembles the torque curve..It is low at low
speeds, then reaches a point where it rises quickly, then flattens
out. Looking at the engine speeds on the bottom line, you can see
that the rate of increase has slowed between 1,400 and 1,800 rpm.
After that it's nearly flat.
The engine produces about 130 hp at 1,400 rpm, and rises to about 175
at 1,800. From there to the maximum engine speed, horsepower doesn't
increase at all."