402 



FULL SCALE OBSERVATIONS 



propeller an electrolysis grid was mounted, as 

 shown in Figure 5. The wires had a diameter of 

 0.2 ram and a current of 0.2A was used to generate 

 nuclei. The propeller shaft was at 0.4 meter below 

 the water level and the lowest wire at 0.5 meter. 

 Therefore the effect of electrolysis could only 

 be observed in the upper half of the propeller disk. 



-The propeller boundary layer. Two ways of 

 affecting the boundary layer were used. First, 

 sandroughness at the leading edge was used to trip 

 the boundary layer to turbulence. Second, the 



FIGURE 1. viscous effects on cavitation inception on 

 propeller A behing the model. 



cavitation pattern was present and identical at 

 every revolution. This "stabilizing" effect of 

 nuclei is important because it affects the induced 

 pressure fluctuations on the hull. 



Propeller C had a very distinct collapse of the 

 cavity when the blades left the wake peak, as can 

 be seen in Figure 3. This irregular collapse of 

 the cavity was thought to be caused by viscous 

 effects and it can also strongly influence the 

 pressure fluctuations on the hull. 



Propeller D was not tested in cavitating con- 

 ditions. It was used only for boundary layer 

 visualization. This propeller is an example of a 

 smaller propeller model used behind models with a 

 maximum length of 7 meters. This propeller was 

 made of a copper-nickel-aluminium alloy (CUNIflL) . 

 Propellers A, B and C were of aluminium. 



The most important geometrical characteristics 

 of the four propellers are given in Figure 4. The 

 complete description, necessary for the calculations, 

 is given in the Appendix. Most tests were done in 

 the NSMB Depressurized Towing Tank. To obtain 

 imiform inflow the propellers were mounted on a 

 right- angle drive unit, which was kept afloat by a 

 catamaran- type vessel, as shown in Figure 5. Only 

 a few comparative tests were done in a cavitation 

 tunnel. 



The following parameters were varied: 

 -The propeller loading. Two advance ratio's were 

 used, namely 70% and 40^ of the pitch ratio at 

 r/R=0.7. (Slip ratio's of 30^ and 60% respectively). 

 The slip ratio of 30% corresponds to a loading which 

 is about normal behind the ship, the slip ratio 

 of 60% corresponds to an overloaded condition, as 

 occurs when the blades are in a wake peak. Propeller 

 A was also investigated at an intermediate loading 

 with a slip of 40% . 



-The nuclei content. At 1 meter in front of the 



WITHOUT ELECTROLYSIS 



WITH ELECTROLYSIS 



FIGURE 2. Effect of electrolysis on propeller B 

 behind the model. 



