If you want to get the best performance out of your boat, you need to select the propeller and gearing that will suit your particular boat, engine and speed range. Below you will find a brief description of how propeller systems are designed. It is not just the engine capacity which determines the speed of the boat; it depends just as much on the efficiency of the reverse gear and the propeller system. Using the right propeller system will not only give you good fuel economy, higher speed; you will also experience greater comfort, with less noise and vibration.
Planing boats
In planing boats over 20 knots, the size
of the propeller depends on the engine
power. To transfer the power from the
engine to the water, you need approximately
7–8 cm2 propeller blade surface per
kW shaft power. If the shaft is at an angle
in relation to the flow of the water, this
requirement may be considerably greater:
8–15 cm2/kW is reasonable, depending
on the angle of the shaft and the water
flow.
At a shaft power of 400 kW, therefore,
the propeller blade surface may need to
be 400 kW x 9 cm2/kW = 3 600 cm2.
This surface may be divided over three,
four or five blades.
The efficiency of a propeller blade diminishes
when it becomes far too wide in
relation to its length. This means that if
the propeller diameter is limited in size (as
is often the case), it is better to select several
narrower blades (four or five) rather
than three wide ones, for example.
The angle of the propeller shaft should be
as small as possible. Shaft angles of less
than 12° do not usually cause any major
problems, but shaft angles of more than
14–15° should be avoided.
The distance between the bottom of the
boat and the propeller blades should be
at least 12–13% of the diameter of the
propeller.
When you have selected the diameter of
the propeller, you are ready to go on to
select the pitch.
Propeller blades should no travel faster
than 60–70 knots through the water at
70% of the maximum propeller diameter.
This means that the speed of the propeller
revolutions must be reduced when the
engine capacity is greater, which requires
a larger blade surface and therefore a
greater diameter.
The relations between pitch and diameter
should be:
P/D=pitch/diameter
0.9–1.15 at 20 knots
1.0–1.3 at 30 knots
1.05–1.35 at 35 knots
Generally, a larger propeller with narrow
blades and low revolutions is more efficient
than a small, high-speed revolving
propeller. When the boat’s speed goes
above 24–28 knots, the resistance of the
shafts, rudders and propeller supports
starts to become so great that the greater
efficiency of the propeller is not beneficial.
The resistance on the propeller system
can be reduced by reducing the shaft
diameter, selecting stronger materials and
reducing the rudders and surfaces of the
propeller supports. Lower gear ratios also
mean thinner shafts. It is necessary to find
a balance between propeller efficiency,
water resistance on the shaft, etc.
Displacement and semiplaning boats
Boats of less than 15 knots need propellers
which 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 diameter by 50% and reducing
the propeller speed by 40%.
The blade surface of the propeller is designed
according to the minimum of 0.17
m2 per ton of thrust.
As described above, a large, slow-moving
propeller is preferable. At a speed of 12
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:
T Newton=propeller effiency x shaft output in kW x1944
/ speed of boat
can be used to calculate the thrust and,
therefore, the blade surface.
Three-blade propellers are often more efficient
for large, slow-moving propellers
than four-blade or five-blade propellers.
However, four-blade propellers usually
produce less vibration, which is often
preferable. In general, there is a tendency
towards four-blade propellers.
A suitable pitch ratio at 10 knots is 0.7–
0.9 and at 15 knots 0.8–1.05.
As the best pitch ratio varies according to
the speed of the boat, it is necessary to
decide whether the propeller 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 (kp) ¢ 13– 14 x hp
Fixed propeller (kp) ¢ 11–12 x hp
An adjustable propeller fitted to “the right
boat” (up to 10 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/cm2 and m2/ton of
tractive force.
The above description is very general and
describes only superficially how propellers
are designed. The propeller manual “Propellers – Marine Engines, 60 to 120
Series, Part A – Calculation and Propeller
Selection” contains a more in-depth description
of propellers.
Volvo Penta also have various catalogs
which contain proposals for propellers
for each type of engine and reverse gear,“Propellers, Part B”.
Over the last year, Volvo Penta has been
developing computer programs for calculating
speed, gear ratios and propellers.
This is excellent for calculating 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.
It is often better to calculate
the speed from your own experience:
propeller manual A and the like simply
calculate the propellers using the computer
program.
Volvo Penta is constantly developing
new, powerful, more refined computer
programs which will enhance both speed
and propeller calculations. However, these
calculations will never improve upon
the values which have been built up over
many years of experience and programmed
into the computer.
Volvo Penta manufactures its own very
efficient reverse gears and collaborates
with leading suppliers of reverse gears
and manufacturers of propellers, shafts,
stern tubes etc. If you buy the entire“package”, from engine to propellers, from
us at Volvo Penta, you can be sure that all
your components will fit together properly.