Trans RINA, Vol 154, Part A2, Intl J Maritime Eng, Apr-Jun 2012


where noise can be generated by vibration of rudders and other appendages.


For new ships, the wake flow can be improved by more careful design, which will require an increased design effort, including careful model testing and computational fluid dynamics analysis. For ships which spend time in ballast, this work should be extended to include optimisation of the propeller design and wake flow in that condition. This extra effort will cost more, however it is likely to result in improved propulsive efficiency as well as reduced hydro-acoustic noise.


The main challenge in ensuring that efficiency measures also optimise their potential for noise reduction is in moving from theoretical predictions and model tests to full scale at-sea measurements of noise from ships under typical


operating conditions. The development For


Figure 1 outlines the possible activities required to reduce the


noise


propagated into the water by


conventional merchant ships. Many of these could follow from efficiency initiatives related to the introduction of the EEDI. Central to most of these activities is the need for more measurements at both model and full scale, requiring the adoption of international standards for such measurements. The increased understanding that will then arise based on the results will input into theoretical models, and improved propeller and hull designs. In turn, noise measurements may also indicate energy efficiency issues that can be addressed within a vessel’s SEEMP.


6. of


standards for such measurements by ISO and ANSI/ASA has highlighted some of the difficulties in obtaining sufficiently precise, comparable measurements.


example, even under calm conditions fundamental blade rate sound of a medium sized merchant vessel can exhibit a standard deviation of about 5 dB with most of the variations roughly correlating to the pitch period of the ship [36]. A 5dB difference corresponds to about a factor of 5 in acoustic footprint and is greater than the 3dB target reduction for overall shipping noise within 10 years, yet is difficult to reliably measure. Thus quite subtle differences in operating conditions may confound results. Reliable conclusions on the most effective noise reduction techniques will require extensive data sets.


Although model scale measurements cannot replace those made at sea, they could nevertheless give useful indications, and important insights would be gained if tank testing facilities adopted measurement of noise as a matter of routine. Despite the IMO recommendation in 2009 that member states should review their fleets to identify the noisiest vessels, the lack of sufficient measurements from ships at sea continues to hamper a full understanding of the most effective noise quieting methods.


5. WAY AHEAD


While attempts to mitigate impacts of other noise sources such as sonar and seismic surveys on marine life have so far proven difficult and costly, substantial reductions in shipping noise appear technologically and economically feasible.


There is increasing evidence that there are


definite limits to the resilience of marine ecosystems [54], and this will apply to the introduction of noise from shipping. Although it may not be possible to determine how close to these limits some species or ecosystems have been pushed by elevated noise from shipping, the relatively small costs but


potential benefits make addressing the ship noise problem a high priority. CONCLUDING REMARKS


It appears that there is considerable difference in the noise


propagated by the noisiest and the quietest


conventional merchant ships (excluding those designed specifically for low noise).


It is reasonable to develop a cautious note of optimism that the noisiest ships can be quietened using existing technology without reducing their propulsive efficiency.


There is little doubt that the dominant feature of these noisiest merchant ships is cavitation associated with the propeller. The two major aspects that influence the level of cavitation are propeller design and wake flow into the propeller.


As ships often operate in different conditions to those predicted at the design stage, it is quite likely that if the propeller were redesigned to suit the actual operating conditions this would result in an improved propulsive efficiency, as well as reduced hydro-acoustic noise. In addition, there are a number of different propeller design concepts


that have been developed pressure pulses and associated hull vibration.


There is the potential to improve the wake flow into the propeller for existing ships by fitting appropriately designed appendages such as wake equalising ducts, vortex generators or spoilers.


For new ships the wake flow can be improved by increased design effort, including careful model testing and computational fluid dynamics analysis.


For ships


which spend time in ballast, this work should include optimisation of the propeller design and wake flow in that condition.


The way ahead to reduce the noise propagated into the water by conventional merchant ships includes the need for more measurements at both model and full scale, requiring the adoption of international standards for full scale measurements.


will then arise based on the results from these by


proponents, normally with the express purpose of increasing propulsive


efficiency and/or of reducing


various


The increased understanding that


©2012: The Royal Institution of Naval Architects


A-85


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62