Volvo D11 series, Installation Manual

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Marine engine environment
3
Other factors affecting performance
It is important to keep the exhaust backpressure at a
low level. The power losses caused by backpressure
are directly proportional to the increase of backpres-
sure, which also increases the exhaust temperature.
Thermal values differ between markets and influence
engine output. Environmental fuel, which is compul-
sory in some markets, has a low thermal value. En-
gine output may be reduced up to 8% compared with
fuel specified in the ISO standard.
The weight of the boat is another important factor
affecting boat speed. Increased boat weight has a
major effect on boat speed, especially on planing and
semi-planing hulls. A new boat tested with half filled
fuel and water tanks and without a payload easily
drops 2-3 knots in speed when tested fully loaded
with fuel, water and equipment for travelling comfort.
This situation arises because the propeller is often
selected to give maximum speed when the boat is
tested at the factory. It is therefore advisable to re-
duce propeller pitch by one or more inches when en-
countering hot climate and user load conditions. The
top speed will be somewhat reduced but the overall
conditions will improve and provide better accelera-
tion, even with a heavily loaded boat.
With this in mind it is important to remember that fi-
breglass boats absorb water when they rest in water,
making the boat heavier over time. Marine growth, an
often overlooked problem, also has a serious effect
on boat performance.
Propeller selection
Naval architects, marine engineers or other qualified
people should choose the propeller. The required en-
gine performance data to make the proper propeller
selection is available in technical literature.
With regard to the propeller selection it is important
to achieve correct engine RPM. For this purpose we
recommend Full Throttle Operating Range.
In order to achieve good all-round performance the
propeller should be selected within this range.
When the prototype and first production boat is built,
a Volvo Penta representative and a boat manufactur-
er should undertake a fully loaded trial of the vessel
as near as possible to the conditions that the boat will
meet in the field. The most important conditions are:
• Full fuel and water on board
• Ballast evenly distributed throughout the boat to
represent the owners’s equipment including such
things as outboards, inflatable dinghies etc.
• Genset/air conditioning equipment and all domes-
tic appliances fitted.
• Adequate number of people onboard.
Once the vessel is subjected to these conditions a
full engine/propeller trial should be undertaken where
all engine parameters are checked, i.e. engine rpm,
fuel consumption, rel. load, ref. rpm (EDC) boost
pressure, exhaust temperatures, engine space tem-
peratures etc.
When the correct propeller has been established
based on the tests, the engine rpm should be within
the " Full Throttle Operating Range" at full load.
However, it is advisable to reduce pitch some more
to handle varying weather conditions and marine
growth. For this reason boat manufacturers must fol-
low the actual situation of their differing markets.
Propeller (too big)
Propeller (OK)
Propeller (too small)
Rated
Governor
cut out
00% of full
output.
Full throttle
operating
range
Engine output, kW
rpm