Trans RINA, Vol 154, Part A2, Intl J Maritime Eng, Apr-Jun 2012
condition. Consequently, the propeller is much closer to the surface, and the tip of the propeller may be above the waterline. The lower pressure due to the smaller hydrostatic head is likely to cause be significantly more cavitation for a vessel in ballast than in full load. In addition, when a ship is in ballast it is usually trimmed by the stern. This generally has a significant detrimental effect
on the wake field to the propeller, further
worsening its cavitation performance. Hence it is likely that a tanker or bulk carrier in ballast will generate more hydro-acoustic noise than one in full load.
2.5 EFFECT OF SPEED
When ships are operating below CIS then the hydro- acoustic noise levels will be reduced considerably. However, this speed is likely to be around 10 knots, or lower, and for many merchant ships operation at such speeds is impracticable. Therefore, merchant ships will be exhibiting some level of cavitation, and so in this paper the effect of speed is only considered above CIS.
Although there is limited detailed information about the effect of speed on the hydro-acoustic noise generated by merchant ships, it is clear that in general for a ship fitted with a fixed pitch propeller, reducing the speed reduces the overall noise [21, 32]. However, levels may not necessarily decrease across all frequency bands. Quantifying the relationship between speed and noise is complex and the limited data available do not always indicate a consistent
relationship. A model giving
generalised relative expected spectrum levels (S) in dB in terms of speed and length of the ship relative to a reference speed V0 and reference length L0 (equation 1) has been suggested [35]
SS VL
L
D 60log V DD 20log
(1)
One study found no relationship between speed and noise levels for assemblages of
relationship suggested in equation 1 may still hold for individual vessels measured at
source level (Sw) in dB and speed (V) [36]
SV w 61.9log 117.9
ships but noted that the different speeds [19].
Comprehensive experiments conducted on a military coal carrier fitted with a fixed pitch propeller gave a significant
(2)
This would appear to be consistent with the relationship suggested in equation 1, at least for this one ship. Earlier measurements made on small craft also showed a linear relationship between the noise level in dB and the log of the speed [31].
In the absence of direct noise measurements, the relationship between speed and power can provide a qualitative indication of how noise output may be affected by changes that result in small increases in
A-82 linear relationship between the wideband 3.2 PROPELLER BLADE SURFACE
Propeller blades are subject to impact damage and other defects during their lifetime. Small imperfections, particularly in the leading edge, can reduce the efficiency of a propeller by the order of 2%, depending on the damage [38] which should be repaired during routine dry dockings. In addition, a certain amount of polishing can be conducted afloat, which will ensure the propeller remains as efficient as possible.
Imperfections can
significantly affect local cavitation, resulting in increased hydro-acoustic noise.
In addition, it has been shown that
improving the general surface of a propeller from that typically specified for normal merchant ship use by applying a modern non-toxic antifouling system referred to as a Foul Release system can increase the efficiency for a medium sized tanker (100,000 dwt) by up to 6% [39,41]. There have been some
reports
coatings can also reduce the noise. 3.3
Propellers are designed for that these
OPTIMISED CONVENTIONAL PROPELLER DESIGN
predicted operating
conditions, which rarely occur in practice. Firstly, the design is often optimised for the full power condition, whereas it is likely in practice that the machinery will be typically be operated at 80 – 90% of the maximum continuous rating.
3.1
efficiency due to cavitation reduction for fixed pitch propellers. The situation is not so clear for ships fitted with controllable pitch propellers. Whilst results from tests on a cruise ship fitted with controllable pitch propellers generally show an increase in noise with increasing speed [21], this is not always the case. In all cases full scale measurements are needed for quantitative analyses of the relationships between speed and noise.
3.
PRACTICAL TECHNOLOGIES FOR REDUCING NOISE ON MERCHANT SHIPS
GENERAL
There are a range of technologies that can be used to reduce the hydro-acoustic noise generated by ships. For example, warships and research vessels make use of specialised propellers which are designed to increase the CIS. Unfortunately, many of these noise reducing technologies result in propellers which are less efficient than the existing conventional propellers normally used in merchant ships. These noise reducing technologies will not be dealt with here, as their use would increase the carbon footprint of the vessel, increase the operating costs, and are unlikely to be embraced by commercial ship designers and owners.
Instead, the noise reducing
technologies reviewed are those which claim to increase the efficiency, and thereby reduce the running costs.
Secondly, the propeller is designed
©2012: The Royal Institution of Naval Architects
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