Operation Manual

14
Diesel fuel flow sensor kit Installation and Operation Manual
NAVMAN
Understanding your fuel
consumption curve
Notice that while the speed is constantly
increasing as the engine RPMs increase, the
economy figure usually has a dip in its curve,
as shown above. The high consumption before
the dip is caused by the boat trying to climb up
over its own bow wave. This is a very inefficient
speed to try to operate the boat at.
The drop in consumption at the dip occurs as
the boat gets up onto the plane. Thereafter, as
you would expect, the faster you go the more
fuel is consumed.
If your graph shows such a dip, then running
the engine at the corresponding RPMs (1300
in the example above) will give the best speed
for the least fuel consumption.
Note on fuel usage
A significant feature of diesel engines is
3-2 Understanding the fuel consumption curve
A typical fuel consumption curve
Boats which plane will produce curves similar
to the example below. The curve will change
with boat and sea conditions.
that engine RPM alone does not dictate the
amount of fuel the engine is demanding. A
diesel engine’s governor will hold the engine
at the RPM you set. The amount of fuel it uses
at that RPM is determined by the load on the
engine.
If you are sliding down the surface of a big wave
like a surfer then very little power is required,
so to hold the RPM you have selected requires
very little fuel. Conversely, if the load increases
because you are battering into a head sea,
have a build-up of growth on the hull, or your
propeller is damaged or covered in barnacles
it is going to require a lot more fuel to maintain
that same RPM. SO, an engine’s thirst is
determined by load, not the RPM which it is
being asked for.
Understanding that point will be necessary
when we discuss propeller and load curves
later.
Speed - knots
Engine RPM
Economy - L / nautical mile
600 800 1000 1200 1400 1600 1800
24
20
16
12
8
4
0
12
10
8
6
4
2
0
Note: If the boat does not plane, there will be
no dip in economy as illustrated.
Speed
Economy