Singing Propellers
A HydroComp Technical Report
Report 138
Product link: NavCad,
SwiftCraft,
PropExpert
| Overview
Some propellers
in service produce a high-pitched noise, often referred to
as singing. This sound typically is a clear harmonic tone
much like that of a ringing wine glass. (Other sounds which
have been attributed to singing have been characterized as
"grunting" or "sawing". These sounds are most likely different
in nature than the ringing tone considered here.)
More of
an annoyance than anything harmful, the causes of singing
are not completely understood. This report will try to introduce
what - in our opinion - is the likely cause of singing, as
well as to offer some guidance regarding ways to eliminate
singing from a propeller.
Fluid
circulation and vortex-shedding
Have you
ever been driving in an automobile and its vertical radio
antennae begins to vibrate and produce a sound? If so, the
tone you heard was caused by the way fluid (air, in this case)
will circulate around the rod and set up alternating eddies.
The pitch of the tone is the frequency of these alternating
vortices.
Consider
the accompanying graphics which illustrate the development
of the tone. Fluid flow starts to curl around the body (A).
Eddies (vortices) are created behind the body (B). Any asymmetry
in the flow direction or in the shape of the body will cause
the separating vortices to set up sequential eddies. The force
of the unbalanced vortex on the body will impart a sideways
force on the body - further promoting flow and shape asymmetry,
and the development of alternating vortices (C). Finally,
a well-behaved system of alternating eddies and forces is
established, resulting in the audible tone of singing that
we hear (D).
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| The propeller trailing-edge
as the singing body
The basis of using this
model for propeller singing is that a rounded trailing-edge
corresponds to the circular body - an "equivalent cylinder"
of sorts. This is illustrated in the lower graphic (E). |
 |
Prediction
of possible singing
The vortex-trail frequency
- the sound you hear from a singing body - can be predicted using
a non-dimensional coefficient called the Strouhal number.
 
where,
Sn = Strouhal number
f = audible frequency
d = effective diameter of rounded trailing
edge
v = kinematic viscosity of water
Strouhal number can be related
to the local Reynolds number of the trailing edge effective diameter
[Saunders, 1957]. This relationship was presented as a graph in
the reference, but we have prepared the following numerical formula
of the plot.
where,
Sn = Strouhal number
k = log10(Rnd)
Rnd = vblade * d
/ v
vblade = [vship2
+ (0.7*D*n)2]0.5
D = propeller diameter
n = propeller revolutions
It has been suggested that
the frequency range of audible singing is approximately from 10
to 1200 hz [Saunders, 1957]. Therefore, we can use the Reynolds
number of the effective edge diameter to find the Strouhal number,
which in turn is used to predict frequency. If the frequency is
between 10 and 1200 hz, then singing can be anticipated.
Mitigation of singing
These relationships also tell
us that singing is a function of propeller diameter and rpm, boat
speed, and trailing-edge size (thickness) and "roundness". We cannot
do much about diameter, rpm or speed, but we can modify the edge
geometry. This has been the strategy for all efforts to eliminate
singing.
Most propeller professionals
(and others) are familiar with the "anti-singing edge" - a chamfering
of the trailing-edge, typically on the suction side. The intent
of this shape is to avoid the creation of curving flow eddies by
cleanly separating the flow off of the blade. The following graphic
illustrates the desired geometry of an anti-singing edge, where
points of flow separation are spaced both in thickness and in flow-stream
position [Saunders, 1957].
| Many
sources recommend that the anti-singing edge be applied
from the 40% radius (0.4R) fully to the tip, or even slightly
beyond [Carlton, 1993]. It has also been noted that erosion
of the blade edge is a risk if the new edge were made too
thin. There is also some evidence that cup can be an effective
anti-singing technique. Cupping, however, changes the thrust
and power characteristics of the propeller, where an "anti-singing
edge" would not measurably alter performance. |
 |
References
Carlton, J.S. and Fitzsimmons,
P.A.; "Hydrodynamic aspects of ship propulsion - results of service
experience", Transactions IME, Vol. 105, Part 4, 1993.
Saunders, H.E,
Hydrodynamics in Ship Design, SNAME, 1957
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