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Factors that affect engine
performance
Power ratings
A great number of environmental factors, such as baromet-
ric pressure, ambient temperature, humidity, the quality of
fuel, and exhaust back pressure can affect engine perfor-
mance. When it comes to quoting and comparing ratings, it
is important that there is a unified set of standards for mea-
surement.
In September 1989, all major marine engine manufacturers
agreed to quote engine power output according to a com-
mon set of conditions. These conditions are referred to as
ISO 8665. All Volvo Penta engines meet the ISO 8665 stan-
dard. This ISO standard outlines the following fixed values
or common conditions for determining the rating of the en-
gine.
Condition .................................................................... Value
Sea Water ........................................................ 77°F (25°C)
Fuel Temperature .......................................... 104°F (40°C)
Air Temperature ............................................... 77°F (25°C)
Exhaust back-pressure ............................ 1.45 PSI (10kPa)
Barometric pressure .......................... 14.504 PSI (100 kPa)
Relative humidity .......................................................... 30%
A gasoline engine operates with very little surplus air. When
conditions deviate from the standard values, the result can
be a loss of power at full load. It can also cause a rise in
exhaust emissions due to incomplete fuel combustion.
Marine engines can be rated according to one of several
power standards, but power output itself is quoted in kilo-
watts (kW) or horsepower (HP), for a given engine speed,
usually at maximum revolutions per minute (RPM).
8
How load conditions affect the speed of
a planing hull
The overall weight of the boat is another important factor in
performance. Any increase in boat weight will slow down the
boat speed, particularly on boats with planing and semi-plan-
ing hulls.
For example, a new boat tested with fuel and water tanks
only half filled, and without any load, can easily drop 2 to 3
knots in speed when tested fully fuelled and loaded with all
normal equipment and supplies for safe and comfortable
cruising. This is because the propeller installed originally is
frequently one that is designed to give maximum speed when
the boat is new. For this reason it is often advisable to re-
duce the propeller pitch by as much as an inch or more in
order to counter the effects of the increase in overall weight
encountered in normal cruising, particularly in hotter climates.
Although this will reduce top speed somewhat, overall ride
conditions will improve and you should achieve greatly en-
hanced acceleration.
In considering the influence of weight, it is worth remember-
ing that fiberglass boats absorb a significant amount of wa-
ter into their hulls while left afloat for any length of time and
so become progressively heavier. Another negative influence
on boat performance is marine growth beneath the water-
line - a problem that is often overlooked.
"Wagon-back" effect
As long as we continue to use combustion technology for
engines, there will be exhaust emissions. Despite the sub-
stantial reductions achieved in modern engines, as fuel burns
exhaust emissions and fumes will always be given off.
But there is also another effect. Any shear-bodied object
moving forward will create a phenomenon we call
"wagon-back effect." Due to a difference in air pressure, the
airflow behind such an object will have a tendency to draw
dirt and exhaust emissions back into the object. (You'll fre-
quently see evidence of this in the excess of road grime that
is drawn back onto the rear windows of cars.) A boat with a
sheer, broad transom and high superstructure creates its own
wagon-back effect. This recirculating air has a tendency to
draw exhaust fumes and water spray up toward the after-
deck. Often people will make the mistake of opening a ven-
tilator or port hole in order to remove the fumes, but it has
the opposite effect — more fumes are drawn in, and condi-
tions merely become worse. In high concentrations these
fumes may be hazardous. If you suspect that your boat ex-
hibits this "wagon-back" effect, please contact your Volvo
Penta dealer.
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