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Factors that affect engine performance __________________________________________
Power ratings
A great number of environmental factors, such as barometric pressure, ambient temperature, humidity, the quality of
fuel, and exhaust back pressure can affect engine performance. When it comes to quoting and comparing ratings, it is
important that there is a unified set of standards for measurement.
In September 1989, all major marine engine manufacturers agreed to quote engine power output according to a
common set of conditions. These conditions are referred to as ISO 8665. All Volvo Penta engines meet the ISO 8665
standard. This ISO standard outlines the following fixed values or common conditions for determining the rating of the
engine.
Condition
Sea water temperature
Fuel temperature
Air temperature
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 kilowatts
(KW) or horsepower (HP), for a given engine speed, usually at maximum revolutions per minute (RPM).
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 semiplaning 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 reduce 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 enhanced acceleration.
In considering the influence of weight, it is worth remembering that fiberglass boats absorb a significant amount of
water 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 waterline - a problem that is often overlooked.
"Wagon-back" effect
to a difference in air pressure, the airflow behind such an object will have a tendency to draw dirt and exhaust emis-
sions back into the object. (You'll frequently 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 afterdeck. Often
people will make the mistake of opening a ventilator or port hole in order to remove the fumes, but it has the opposite
effect — more fumes are drawn in, and conditions merely become worse. In high concentrations these fumes may be
hazardous. If you suspect that your boat exhibits this "wagon-back" effect. please contact your Volvo Penta dealer.
10
Va lue
Condition
77° F (25° C)
Exhaust back pressure
104° (40° C)
Barometric pressure
77° F (25° C)
Relative humidity
As long as we continue to use combustion technology for engines,
there will be exhaust emissions. Despite the substantial 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
Va lue
1.45 PSI (10 kPa)
14.504 PSI (100 kPa)
30%
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