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


principal aim of the asymmetrical afterbody is to take this into account, and reduce the power required by improving the flow into the propeller. Claims of reduction in power of up to 9% have been made [53].


3.9 CHANGES TO OPERATING PROCEDURE


The most general relationship between overall wide-band hydro-acoustic noise and ship speed for merchant ships with fixed pitch propellers travelling above CIS, seems to be that noise expressed in dB will increase according to 60log(speed). Although this relationship will not hold for all vessels, it does provide a useful indication of the likely reduction in acoustic footprint associated with reduced speed. In particular, although slower steaming will require more ships to be operated to carry the same quantities of cargo there should be a large reduction in total acoustic footprint associated with slow steaming. For example, the acoustic footprint for an individual ship at 12 knots would be 10% of that ship at 16 knots, but the number of ships required would increase by 33% for the same quantity of cargo carried. Thus the total acoustic footprint at 12 knots would be 13% of that for the same cargo transported at 16 knots. Similarly, the total acoustic footprint at 12 knots would be 34% of that at 14 knots. Compared to typical container ships travelling at 25 knots,


total acoustic footprint would be reduced to


21% for slow steaming at 20 knots and 7% for extra slow steaming at 17 knots, allowing for the extra vessels required to transport the same cargo.


Where slow steaming is used, as noted above, it is particularly important


to consider a redesign of the


propeller(s), especially for ships fitted with controllable pitch propellers.


Noise from shipping that enters the deep sound channel will propagate more efficiently than noise in a homogenous water column and can contribute to raised ambient noise levels across an entire ocean basin. Noise generated at the surface may enter the deep sound channel where the sound channel intersects bathymetric features such as the continental slope or at high latitudes where it is very close to the surface [7].


Hence the


contribution of shipping noise to ambient noise may be reduced by minimising the time spent in locations where sound will propagate into the deep sound channel. In some areas this may be achieved by transiting further off- shore, although the implications of any increase in distance travelled or increase in speed would need to be carefully considered.


4. DISCUSSION


Although there are only limited data on the propagated hydro-acoustic noise for merchant


ships, the large


measured differences between the noisiest and quietest across merchant fleets indicates the potential to reduce the noise generated by the noisiest ships. Based on


existing technology it is reasonable to be cautiously optimistic that the noisiest ships can be quietened without


reducing their propulsive efficiency.


The


greatest improvements are likely to be achievable for ships operating at sub-optimal efficiency and these are also likely to be the noisiest.


It is almost certain that these noisiest ships suffer from greater


cavitation than other merchant ships. For


merchant ships it is necessary to accept a certain level of cavitation, as this gives a more efficient propeller than one designed to eliminate it altogether.


Reducing the noise generated by cavitation is not


currently the main focus of the extremely quiet ships developed for military purposes as they concentrate on reducing the noise at speeds below CIS, and on raising the CIS as high as possible.


Thus the technologies


developed by the military cannot be directly transferred to merchant fleets. However, a number of technologies can improve efficiency and are likely to reduce hydro- acoustic noise.


The two critical aspects influencing cavitation performance are the propeller design itself, and the wake into the propeller, which is determined by the presence of the hull. Therefore, careful propeller and hull design are essential to improving the cavitation performance.


In addition, as ships often operate in different conditions to those predicted at the design stage, it is also 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.


There are a number of different propellers design


concepts that have been developed in order to increase propulsive efficiency and/or reduce pressure pulses and associated hull vibration.


In most cases, it is not known


how these concepts will influence hydro-acoustic noise, however available data suggest noise is likely to be reduced. To most effectively address the noise problem, detailed measurements of noise associated with different propeller design concepts are needed.


There is also 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. The technology exists to do this, and although there is some understanding of the improvement that these devices will have on propulsive efficiency, there is little knowledge about how they will reduce the hydro-acoustic noise.


As with propeller


designs that improve efficiency, it does seem very likely that improved wake flow will also reduce noise, but noise measurements are also required for these concepts. While some noise measurements may be made at model scale facilities, there is also likely to be a need for full scale measurements


particularly at low frequencies


A-84


©2012: The Royal Institution of Naval Architects


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