Feature 3 | Noise aNd vibratioN
The case of the singing propeller
Singing propellers – why they happen and how to eliminate them. A report
by Raymond Fischer, of Noise Control Engineering*.
FIGURE 1: Propeller In-Water Natural Frequencies
A
‘s
inging propeller’ occurs when the
Vibration Response of Port Propeller in Water
blade’s natural frequency of vibration
0
is excited by a vortex shed from Blade 1
the trailing edge. This vortex has the same
Blade 2
-10
Blade 3a
‘shedding’ frequency as the blade mode, which
Blade 3b
Blade 4
causes a resonant response. The result can be a
Blade 5
G
-20
high underwater noise level or a high noise or r
e
1
vibration onboard the ship. The worst case is -30
structural damage to the propeller itself.
t
T
i
p
,
L
a
,
d
B
There are prediction methods [1] that can
n
a
o -40
a
t
i
be used to determine when and if a singing
c
c
e
l
e
r
propeller is likely to occur. The controlling
A -50
factors are the thickness of the trailing edge
-60
and the flow speed over this edge. The general
approach to eliminating this phenomenon
-70
once it occurs is to change the geometry of the
0 500 1000 1500 2000 2500
Frequency, Hz
trailing edge. It is possible to determine during
the initial design phase the likelihood a propeller Figure 1: Propeller in-water natural frequencies.
Figure 2: Underwater Radiated Noise, Singing tone denoted by red arrow
will sing and take effective steps to ensure this
phenomenon is avoided.
A ‘singing’ propeller typically has a tone
generated by the interaction between a blade’s
10 Knots
9 Knots
natural frequency and a Karman vortex 8 Knots
shedding mechanism from the trailing edge
7 knots
r
5.7 Knots
of the blade. The vortex sheets shed from
10 dB per division
5.4 Knots
e
t
e
the trailing edge cause an oscillating force
1
m
component perpendicular to the direction of
a
@
flow. The ‘Strouhal’ frequency of this oscillation
o
-
P
i
c
r
is determined by the Reynolds number, flow
m
1
velocity, and trailing edge thickness. This excites
r
e
B
the ‘lightly-damped’ propeller blade into chord-
,
d
L
p
wise vibration, a cantilever mode of vibration
or a torsional mode when the blade’s natural
frequency matches the vortex frequency. An
example of the multiple resonances of a propeller
blade is shown in Figure 1. At some ship
1
0
1
6
2
5
4
0
6
3
1
0
0
1
6
0
2
5
0
4
0
0
6
3
0
1
,
0
0
0
1
,
6
0
0
2
,
5
0
0
4
,
0
0
0
6
,
3
0
0
operating point, a ‘singing’ propeller is created
1
0
,
0
0
0
1
6
,
0
0
0
2
5
,
0
0
0
4
0
,
0
0
0
1/3 Octave Band Center Frequency, Hz
when the frequency of the vortex shedding Figure 2: Underwater radiated noise, singing tone denoted by red arrow.
formation correlates or locks onto a blade modal
frequency of vibration. Lock-in occurs when
FIGURE 3 Propeller being tested in air.
the system of vortices is strengthened and made with the blade’s mass per unit area and stiffness For any resonant condition, small forces
orderly by the vibration of the trailing edge. varying along its span. The blade’s natural
3
can create large displacement. As a result of
The blade’s natural frequencies are controlled frequencies can be accurately computed by this large blade displacement, the propeller can
by the stiffness, shape, and mass of the blade, Finite Element methods, taking into account radiate high underwater levels (see Figure 2),
the frequency dependant effects of entrained which can couple to high airborne noise within
water loading. Water loading reduces the ‘in-air’ after compartments of the vessel. Similarly, high
*Raymond Fischer, Noise Control Engineering, Inc, 799 natural frequency by a factor of approximately displacements on the blade can translate to
Middlesex Tnpk, Billerica MA 01821, 978-670-5339 0.6. Alternatively a blade’s natural frequency can high shaft vibration and actually cause fatigue
nonoise@noise-control.com be approximated by empirical methods [1]. failures of the blade. This resonant condition
70 The Naval Architect March 2008
NA Mar 08 - p70+
71.indd 70 10/03/2008 14:19:18
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