Volvo D11 series, Руководство по установке

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General arrangement and planning
32
Displacement and semi-planing boats
Boats of less than 5 knots need propellers that are
as large as possible. For example, in a trawler it is
possible to save 20–30% fuel or to gain 20% greater
thrust when trawling by increasing the propeller di-
ameter by 50% and reducing the propeller speed by
40%.
The blade surface of the propeller is designed ac-
cording to the minimum of 0.7 m
2
(0.26 in
2
) per ton
of thrust.
As described above, a large, slow-moving propeller
is preferable. At a speed of 2 knots, for example,
a three-blade propeller with a 50% blade area will
achieve an efficiency rate of approximately 57% if the
propeller blade cuts through the water at 50 knots
with 70% of its diameter. At a blade speed of 70
knots, approximately only 47% efficiency is achieved.
The formula:
propeller efficiency x shaft output (kW) x 944
T (Newton) =
speed of boat (knots)
can be used to calculate the thrust.
Three-blade propellers are often more efficient for
large, slow-moving propellers than four-blade or five-
blade propellers. However, four-blade propellers usu-
ally produce less vibration, which is often preferable.
In general, there is a tendency towards four-blade
propellers. A suitable pitch ratio at 0 knots is 0.7–0.9
and at 5 knots 0.8–.05.
As the best pitch ratio varies according to the speed
of the boat, it is necessary to decide whether the pro-
peller should be at its best when trawling, e.g. with a
pitch ratio of 0.7, or whether it should be better when
not trawling with a slightly higher pitch ratio.
Adjustable propellers are an excellent solution for
trawlers, tugs and freighters.
As a very rough estimate, the bollard pull thrust can
be calculated using the formula
Adjustable propeller (N) 95–05 x kW
Fixed propeller (N) 80–90 x kW
An adjustable propeller fitted to "the right boat" (up to
0 knots) can therefore save a lot of fuel.
Speed range between 15 and 20 knots
Within this speed range, a large slow propeller is
preferable to a small, fast one. The blade surface is
designed as a compromise between kW/cm
2
and m
2
/
ton of tractive force.
Propeller and performance computer
program
Over the last year, Volvo Penta has been developing
computer programs for calculating speed, gear ratios
and propellers. This is excellent for predicting speed
and propellers simply and safely.
The estimated speed in the individual computer
programs is based on the experience gained from a
number of installations.
Propeller calculations
Theoretical speed and propeller calculations are
made using well-established methods and a number
of practical test results, but are still a result of ap-
proximations and estimations. We believe that for a
standard type of boat they can give you a reasonable
good estimation, provided that correct and complete
input is available. However the Volvo Penta organisa-
tion can not take any responsibility for the final result
which only can be found out during a sea trial.