characteristics, can be obtained with a crude wake simulation. However, if the vessel is to be fitted with angled shafts it is important that these are represented correctly in the experiment.


Cavitation does not always result in erosion, and to date it is not possible to confidently predict whether cavitation observed in the tunnel will, or won’t cause problems. Currently, a lot of work is going into predicting when erosion will occur, and when cavitation will not result in erosion.


Cavitation erosion can also occur on the rudder. As


containerships are getting larger and faster this is becoming a well known problem for them, even although they are not usually considered high speed craft. Again, this is difficult to predict, and work is being conducted to understand this phenomena, and hence to be able to predict whether it will occur, or not.


3.2 SUPERCAVITATING PROPELLERS


Supercavitating propellers are designed to operate with the back fully cavitated. Although this produces a lower thrust, there is also a reduction in torque, so the resulting efficiency is not substantially lower at high speed. Erosion is eliminated as this is actually caused by the cavitation bubbles imploding when they enter regions of higher pressure – this doesn’t occur on a well designed supercavitating propeller blade.


The blade shape


required for supercavitation, however, is not particularly efficient at


lower speeds. Numerical prediction


techniques are not sufficiently reliable to predict the performance of supercavitating propellers, however the issues are generally well understood by the specialists in the field, and full scale predictions can be made from model experiments in a high speed cavitation tunnel.


3.3 SURFACE PIERCING PROPELLERS


Surface piercing propellers operate with the shaft close to the waterline, and


hence half of the completely out of the water.


Numerical techniques, such as panel methods or RANS, are not currently suitable for predicting the performance of surface piercing propellers, so the designer has to resort


to empiricism, and model experiments.


propeller operates with much of it out of the water it is necessary to use a tunnel with a free surface.


The need to both lower the pressure, to get the correct cavitation number, and operate with a free surface makes the experiment somewhat complex, and there are only a limited number of tunnels worldwide capable of being used to conduct such tests. The photograph in figure 3.2 shows the test set up in the Circulating Water Channel at Haslar.


propeller is


Figure 3.2 Test set up for surface piercing tests (Courtesy Peter Dyson)


3.4 WATERJETS


Waterjets are often used for the propulsion of high speed craft.


As noted in section 2 there is a complex


interaction between the waterjet and the hull which is not generally as well understood as the interaction between a propeller and the hull, and which needs to be taken into account.


As the


There are two critical elements to the waterjet which both affect its performance: the inlet, and the pump unit. The performance of both, in the absence of cavitation or separation, can be predicted reasonably well using RANS CFD techniques.


In some cases, however, it is necessary to resort to model experiments, and if cavitation is suspected these need to be conducted at the reduced pressure to achieve the correct cavitation number. The procedure for doing this for the pump unit is fairly straightforward, although care needs to be taken to ensure that the wake into the unit is modelled correctly.


On the other hand, in order to study the performance of the inlet it is necessary to suck water at the correct flow rate out of the working section and through the inlet. To do this it is not necessary to correctly simulate the pump


© 2007: Royal Institution of Naval Architects B-15


Figure 3.1 Schematic of surface piercing propeller test arrangement (courtesy Peter Dyson)


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