Volvo D11 series, Installation Manual

Page: 33 / 212

General arrangement and planning
3
Propeller theory
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 propel-
ler systems are designed. It is not just the engine ca-
pacity 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 and higher
speed but you will also experience greater comfort,
with less noise and vibration.
The following description is very general and de-
scribes only superficially how propellers are de-
signed. The propeller manual
Propellers gives more
detailed information.
Planing boats
In planing boats over 20 knots, the size of the pro-
peller depends on the engine power. To transfer the
power from the engine to the water, you need approx-
imately 7–8 cm
2
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: 8–5 cm
2 /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 cm
2
/kW =
3 600 cm
2
.
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 2° do not usu-
ally cause any major problems, but shaft angles of
more than 4–5° should be avoided.
The distance between the bottom of the boat and the
propeller blades should be at least 0% of the diam-
eter of the propeller.
When you have selected the diameter of the propel-
ler, 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 en-
gine capacity is greater, which requires a larger blade
surface and therefore a greater diameter.
The relations between pitch and diameter should be:
P/D =
0.90–.5 at 20 knots
.00–.30 at 30 knots
.05–.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 exceeds 24–28 knots, the re-
sistance of the shafts, rudders and propeller supports
increase to a level where the improved 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.
Pitch
Diameter