Feature 3
FIGURE 4: Trailing Edge Shapes
can occur through a fairly wide range of flow
velocity or propeller rotation rates. Multiple
tones can occur on one propeller as it is excited
by different vortex shedding from varying radial
positions along the propeller or at different
operating speeds.
For a given design, not all propellers in a
group will ‘sing’. Variations associated with
surface roughness, marine growth, trailing
edge finish, etc, can cause one propeller to sing
whereas the others of the nominal identical
design do not.
diagnostics and treatments
The presence of a singing propeller can be
confirmed by computing the expected Strou- Figure 3: trailing edge shapes.
hal frequency and blade natural frequencies to
determine if a resonance condition exists. Bear
in mind the trailing edge geometry and edge The propeller will not usually sing if the vessel on several case histories, is to provide a trailing
thickness vary with blade radius. As a follow up, is propelled solely on one shaft of a twin shaft edge with asymmetrical bevels and included
measurements of the blade vibration in-water vessel. Testing can also be conducted with the angles of 45degs or to use curved bevels or chisel
or in-air should be undertaken (see Figure 3). propeller ‘free wheeling’; the free spinning edges with included angles of 25degs or less.
If taken in-air, the frequency should be reduced propeller should start to sing at the appropriate These edges change the frequency of the vortex
by approximately 60% to account for water shaft rev/min that singing originally occurred shedding and de-tune the system.
loading. If this blade natural frequency is close while the powered. Given today’s modelling tools, one can
to the frequency of concern while operational, The use of an ‘anti-singing’ edge is the first predict or diagnose the occurrence of singing
then the likelihood of a singing propeller is line of defence. Examples of these edges are propellers. The best approach, in all cases, is to
high. shown in Figure 3. Tests have shown that a blunt place anti-singing trailing edges on the blades.
Underway tests should also be considered. trailing edge has an expected displacement These edge shapes do not affect the overall
For instance, a propeller will not typically almost three times that of a square trailing edge, performance of the propulsion system while
sing during a full acceleration run or while which provides sharply defined separation precluding a potential vibration or radiated
turning, when the propeller is fully loaded. points. The author’s recommendation, based noise problem. NA
advertiseMeNt
5
The Naval Architect March 2008 71
NA Mar 08 - p70+
71.indd 71 12/03/2008 11:40:37
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