Trans RINA, Vol 154, Part A2, Intl J Maritime Eng, Apr-Jun 2012 for a predicted ship speed and wake distribution.


Although these may have been obtained from model experiments, there will always be some uncertainty in model to full scale correlation, and so the actual operating condition will be different to that assumed in the design. Most propellers are designed for full load condition, in calm seas, whereas many ships operate at lighter draughts in a seaway.


Many shipping companies are now adopting ‘slow steaming’ philosophies, to reduce fuel consumption (see 3.9). This will also mean that the propeller has not been designed for the actual operating conditions.


3.4 SPECIAL MERCHANT SHIP PROPELLERS


As discussed above, cavitation from the propeller is without doubt the most serious source of hydro-acoustic noise from large merchant ships. Therefore, the best way to reduce noise is to make use of a propeller specially designed to minimise cavitation.


Propellers designed to avoid cavitation altogether below a given speed are less efficient than a conventional merchant ship propeller, and hence are probably not likely to be applied routinely on commercial vessels. There are, however, some basic principles that can be applied to


reducing the propeller noise without decreasing efficiency [25].


There are also a number of proprietary propeller design concepts that claim increased efficiency and a reduction in cavitation/vibration. These include propellers [25,


42]; Contracted and proponents independently verified.


of these concepts Also,


have yet to although claims


High Skew Loaded Tip


propellers [43, 44], Kappel propellers [45] and New Blade Section (NBS) propellers [46]. Claims reported by the


be of


reducing cavitation are made, it is not clear exactly how much these will reduce the hydro-acoustic noise. Most of the emphasis of the concepts has been to increase efficiency, and to reduce noise and vibration propagating into the ship.


It is important to recognise that there are many other concepts which claim to increase efficiency and to reduce noise and vibration, and that different approaches may suit different vessels. For all these designs of propeller, noise measurements are required to verify whether claimed improvements in efficiency are matched by reductions in noise.


3.5 PROPELLER HUB CAPS


A propeller generates vortices from its hub, which reduce its efficiency, and are prone to cavitate. The magnitude of these vortices will depend on the blade radial loading distribution, and on the size and design of the hub. Vortices from the hub tend to be steadier than those generated from the propeller tips, and consequently have


an influence at the higher frequency range, rather than direct harmonics of the blade rate frequency.


Properly designed hub caps can reduce the hub vortex cavitation, and consequently the hydro-acoustic noise, as well as improving propeller efficiency, particularly for controllable pitch propellers [47]. Two concepts which can be used to reduce hub vortex cavitation are Propeller Boss Cap Fins [48, 49], and Propeller Cap Turbines.


3.6 WAKE INFLOW DEVICES


Improving the wake into the propeller will reduce cavitation, and probably also increase efficiency. If the wake is already good flow modification devices are unlikely to improve the situation, however such ships are not likely to be amongst the noisiest, and hence not a priority for noise reduction.


There are a number of devices that can be fitted to the hull of a ship to improve the flow into the propeller including Schneekluth duct, Mewis duct, Simplified compensative nozzle and Grothues spoilers [42, 48, 49, 50, and 51].


3.7 PROPELLER/RUDDER INTERACTION


The interaction between the propeller and the rudder has a significant impact on propulsive efficiency.


Various


concepts such as a twisted rudder (better designed to account for the swirling flow from the propeller) and rudder fins (designed to recover some of the rotational energy) have been developed to increase efficiency [52].


In addition, the Costa Propulsion Bulb (CPB) is a concept where the propeller is integrated hydrodynamically with the rudder by fitting a bulb to the rudder in line with the propeller shaft. It is claimed that this can reduce the hydro-acoustic noise levels by 5 dB [25].


3.8 CHANGES TO THE HULL FORM


The hull form will have a considerable influence on the power required to propel the vessel, but also on the hydro-acoustic noise from its propeller. A well designed hull form will require less power for a given speed, which is likely to result in less noise. In addition, a well- designed hull form will provide a more uniform inflow to the propeller, thereby increasing the propeller’s efficiency, and reducing noise and vibration caused by the uneven wake flow. underwater noise.


This will further reduce the


One special technique for improving the flow into the propeller of a single screw merchant ship is to adopt an asymmetrical afterbody. The flow around single screw ships is not symmetrical about the centreline, since the propeller tip is moving one way at the top of the propeller disk, and the other way at the bottom. The


©2012: The Royal Institution of Naval Architects


A-83


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