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


The noise from the propeller will depend on whether it is cavitating4 or not. Noise from a cavitating propeller dominates other propeller noise, other than singing (see 2.3), and all other hydro-acoustic noise from a ship [25]. In view of this, the IMO correspondence group concentrated its attention on various aspects of vessel propulsion, followed by hull design, on-board machinery, and operational measures [18].


Generally, it is possible to avoid cavitation at low speeds; however at high speeds this is not possible.


Surface


warships, particularly those used for Anti-Submarine Warfare, are designed to operate as fast as possible without cavitation occurring. However propellers will inevitably cavitate above a certain speed, no matter how well the ship and propellers are designed. Considerable research has gone into making some military vessels, which are already very quiet, even quieter. However these technologies are unlikely to be appropriate for reducing the noise generated by the noisier merchant ships.


The lowest speed at which cavitation occurs is known as the Cavitation Inception Speed (CIS).


The CIS for


warships will typically be below 15 knots. There are published examples of research vessels using advanced propeller technology to improve CIS where the CIS is about 10 knots [26, 27, and 28].


Warship designers try to ensure that cavitation does not occur at low operating speeds and hence other sources of noise


become important. The same applies for


specialised quiet vessels such as research vessels [29, 30]. However, this is not the case for normal merchant ships. Thus, the noisiest merchant ships, which have not been designed to reduce cavitation, will experience cavitation. above


components are largely irrelevant


If the noise from one component is 10 dB other components of noise, then [31].


certainly has the potential to generate noise that is greater than 10 dB above machinery and other noises [32]. Therefore, it is almost certain that cavitation noise will dominate the underwater


commercial vessels and noise reduction methods should be directed at reducing cavitation noise [18].


2.2


FACTORS AFFECTING CAVITATION PERFORMANCE


For a given propeller blade design a greater blade area can produce a given thrust with a smaller difference in pressure between the face (pressure side) and the back (suction side) of the blade. Thus, an increased blade area will result


in reduced cavitation. Unfortunately,


increasing blade area increases the torque required to rotate the propeller. Hence, for merchant ships, greater efficiency is possible with lower blade area, and so a


4 Cavitation occurs when the local pressure is lowered to the vapour pressure of the water.


©2012: The Royal Institution of Naval Architects 2.4 VESSEL LOAD CONDITION


Propellers are generally designed for the full load condition. However, few ships spend all their time in this state. For a range of practical reasons, when in ballast a ship is never loaded close to its full load


noise signature of large


the other Cavitation


Ships designed to reduce cavitation will have


performance. 2.3


well


designed after bodies with as uniform a flow into the propeller as possible. This cannot be overstressed as a major factor influencing


propeller cavitation PROPELLER SINGING


In some cases propellers can generate very high pitched notes, known as singing, caused by the shedding frequency of the trailing edge vortices coinciding with the structural natural frequency of the trailing edge of the propeller [33].


approximately 10 – 1,200 Hz, although it has been suggested that


Fortunately singing is usually very easy to cure.


normal procedure is to cut a very small section obliquely from the trailing edge of the propeller blade, leaving the trailing edge flat, with sharp corners on both the face (pressure side) and the back (suction side). The resulting shape is often referred to as an anti-singing trailing edge.


small amount of cavitation is optimum propeller design. Excessive


associated with the cavitation,


however, can reduce the thrust and also cause erosion, both on the propeller, and in some cases, on the rudder.


The other major contributor to the cavitation performance of a propeller is the flow into it. As the propeller rotates it will experience vastly varying inflow, known as wake, caused by the hull


ahead of it.


Typically, for a single screw propeller the axial velocity into the propeller at the top of the circle is much lower than the axial velocity at the bottom. In addition, there will be a tangential component of the flow into the propeller, which will be quite different at the top of the propeller disk compared to the bottom. This means that the angle of


attack of the propeller


constantly varying through the cycle and will not be at the optimum value. Although it is well known that non- uniform wake can have


operation of the propeller, and on propulsive efficiency, the effect on hydro-acoustic


cavitating propeller is not fully understood.


Variation in water flow into the propeller, combined with the lower static pressure (due to hydrostatic head) for a blade at the top of the cycle can often result in fluctuating cavitation, with cavitation occurring at the top, but not at the bottom of the cycle. In any case, the cavitation extent for each blade will vary throughout the cycle. This will affect the noise by providing a frequency component corresponding to the blade rate (and harmonics).


blade will be


a major influence on the noise generated by a


Audible singing can occur from it can be as high as 12 kHz [34].


The


A-81


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