Power Ratings; Load Condition (Speed Of Planing Hull) - Volvo 5.0OSiE-J Operator's Manual

Power Ratings

A great number of environmental factors, such as bar-
ometric 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 manufac-
turers 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 deter-
mining the rating of the engine.
Condition
Air temperature ......................... 25°C (77°F)
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 combustion.
Marine engines can be rated according to one of sev-
eral power standards, but power output itself is quoted
in kilowatts (KW) or horsepower (HP), for a given en-
gine speed, usually at maximum revolutions per
minute (RPM).
VPA 7748131 English 09-2007
Value

Load Condition (Speed of 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-planing 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 some-
what, overall ride conditions will improve and you
should achieve greatly enhanced acceleration.
In considering the influence of weight, it is worth re-
membering 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 water line–a problem that
is often overlooked.
Introduction
15