| On this enigmatic graph (borrowed
from the Suzuki 140 sales brochure) I have superimposed an Engine RPM scale to facilitate comparisons.
The manufacturer intended for this graph to illustrate improvements in fuel
economy, using the new fuel-injection technology, vs their older "DT-140"
model. I have indicated "Optimum
Cruise" at a point where the fuel-consumption rate has flattened,
just prior to a rapid increase in gas guzzling. This appears to be
about 3450 RPM. The "SPEED (MPH)" values are worthless for any comparison
as they only pertain to one particular test boat. Conspicuous in its
absence is a torque curve. Gasoline engines do not develop high torque
at low RPMs therefore more throttle is required to realize acceptable thrust. |
|
| Cummins offers this detailed
graph relating engine speed to horsepower, torque and fuel consumption.
I applied a green vertical line (as above) to indicate the "Optimum Cruise" speed (2150 RPM)
and highlighted a "Normal Operating
Range" of acceptable engine speeds. between 2150 and 2750 RPM.
Sometimes additional speed is desirable, but cruising along at 1.3 GPH, without
the high-speed buzzing of an outboard, certainly seems more appealing.
Selecting propellers to optimize efficiency in the Normal Operating Range is next. |
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