418 



Rcm r 0.73 x10° 



SMOOTH 



60 [im CARBORUNDUM 



GO lim CARBORUNDUM 

 Rcm r 0.73 xlO^ 



^NT 



: 1.5 



-N 



- 2.5 



FIGURE 27. Effect of Reynolds number on spot cavita- 

 tion at roughness elements. Propeller A at 30% slip. 



FIGURE 28. Effect of roughness near inception. Pro- 

 peller A at 30% slip. 



cavitation inception pressure to a value greater 

 than the vapor pressure is possible , which would 

 create additional scale effects on cavitation 

 inception. To estimate the importance of a possible 

 increase in cavitation inception index, the ideal 

 inception radius is also given in Figure 18. This 

 is the radius where cavitation should start when 

 the calculations of the pressure distribution were 

 correct and when no scale effects would occur. As 



can be seen , no cavitation occurs inside the ideal 

 inception radius , indicating that the pressure at 

 inception with roughness is not far from the vapor 

 pressure. Assuming that full scale inception takes 

 place near the minimum pressure point at the vapor 

 pressure [oi=-Cp (minimum)], application of sand- 

 roughness can effectively simulate this situation 

 at much lower Reynolds numbers. Further experiments 

 are necessary to find out the precise effect of 



