Volvo Penta 5.7 Gil-B, Руководство оператора

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5
Introduction
Power Ratings
A great number of environmental factors, such as baro-
metric 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 manufactur-
ers 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 Value
Sea water temperature . . . . . . . . . . . . . . . . . . 25°C (77°F)
Fuel temperature . . . . . . . . . . . . . . . . . . . . . . 40°C (104°C)
Air temperature . . . . . . . . . . . . . . . . . . . . . . . . 25°C (77°F)
Exhaust back pressure . . . . . . . . . . . . . 10 kPa (1.45 PSI)
Barometric pressure . . . . . . . . . . . . 100 kPa (14.504 PSI)
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 com-
bustion.
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 remem-
bering 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.