Operation Manual

18
Diesel fuel flow sensor kit Installation and Operation Manual
NAVMAN
and ahead of the propeller, the degree of fouling
of the hull, and the amount of weight aboard.
You will probably not be able to find the exact
propeller load curve for your boat, but knowing
the general shape of the curve will help you
understand how your boat performs.
The important thing to notice is that, for a
correctly sized propeller, the engine is loaded to
the maximum only at the maximum RPM. (that
is where the two curves meet) At all RPM less
than maximum, the engine, if it were asked to,
could actually generate more power than the
boat requires.
For example, at 1200 rpm this engine is capable
of generating 250 horsepower, but at those
engine revs the boat only needs about 125
horsepower transmitted through the propeller.
This is where the governor on the marine
engine comes into play. The throttle on a boat
behaves quite differently from the accelerator
pedal in a car. It does not directly control the
amount of fuel the engine burns. The boat
throttle levers just set the rpms at which you
want the engine to turn - that is all. The injector
and governor system of the diesel engine then
works out how much fuel it needs to feed into
the engine to keep the engine turning at that
speed. If you ask for 1500 rpm with your boat
heavily loaded and with a lot of fouling on the
hull, the engine will do 1500 rpm, but use a lot
more fuel compared to how much it would use
if lightly laden and clean.
The theoretical propeller load
curve
For those interested, the theoretical propeller
load curve for a correctly chosen propeller
follows the form of:
Propeller load = K x RPM
y
Where:
y is a number that is determined by the
type of boat. The value of y can range
from around 2.2 for ducted propellers
to around 3 for slow, heavy commercial
boats. The most common value of y is
around 2.7, and for medium to high speed
pleasure boats, y is usually around 2.7.
K is deliberately chosen to make the curve
pass through the point on the engine
power curve that is max revs/max power.
To work out K for a pleasure craft use:
Some engine manufacturers will plot an ideal
propeller load curve on their engine data
sheets, using an exponent of 2.7.
K =
Maximum engine power
Maximum engine RPM
2.7