diesel engine maintenance and powerboat seamanship 175
How to Make a Fuel Consumption Graph
In addition to making a speed/RPM
table, as described in Chapter 3, you
should also make a fuel consumption
table. In power vessels, you must know
the RPM boundary where your engine
or engines become extremely thirsty.
Th ink of this as a critical RPM setting.
Stay below this limit and you’ll get decent
mileage. Run above that limit and your fuel needs increase heavily. Always measure
fuel consumption in gallons per hour (GPH).
- Fuel tank gauges. Before you start, you must have an accurate way to
measure fuel consumption. See which of these methods apply to you.
In-line fuel fl owmeter. A fuel fl owmeter installed directly onto the fuel line
gives the most accurate readings.
Transfer to day tank. If you have an odd-shaped fuel tank, transfer the fuel to
a symmetrical day tank. Th e day tank should have square or round sides
and a fl at top and bottom. Th is avoids the problems with V-shaped bilge
fuel tanks (see next method).
Float gauge markings. Float gauges that measure fuel from a V-shaped
bilge tank tend to give inaccurate readings. When you top off the tank,
the fl oat sits in the tank’s widest area; consumption appears much slower.
Once you are past the halfway point in the tank, however, consumption
accelerates because you’re near the bottom of the V.
Sid Stapleton, author of Stapleton’s Powerboat Bible, recommends running
the tank down close to empty. Th en add 20 gallons to the tank and stop.
Mark the fuel gauge with a marker. Add another 20 gallons; stop and
mark the gauge again. Keep marking in this manner until your tanks are
topped off.
Refi ne each mark by dividing it in half. Place smaller marks between the
20-gallon marks. Th at way, each gauge shows marks at 10-gallon intervals.
- Set up your consumption graph. Set up the graph with fuel consumption
in tenths on the left side and RPMs in hundredths on the bottom. - Sea trial for fuel consumption. It takes a lot of time to make an accurate
fuel graph. If possible, make the graph in ideal conditions: fl at seas and
little wind or current. In less-than-ideal conditions, make each run headed
into the elements, or make a run into the elements, then turn and make a
reciprocal run with the elements, keeping the same RPM on both runs. You
need to make two graphs for this method (see below).
Supplies and Tools
Needed to Make a Fuel
Consumption Graph
X accurate fuel fl owmeter or fuel
gauge
X clipboard and graph paper