CHAPTER 47 



47.1 

 47.2 



47.3 

 47.4 



47.5 



47.6 



The Inception and Effect of Cavitation on 

 Ships and Propellers 



Scope of This Chapter 145 



General Rules for the Occurrence of Cavita- 

 tion on Ships and Appendages 145 



Vapor-Pressure Data for Water 146 



Tables of and Nomogram for Cavitation 



Numbers 147 



The Prediction of Cavitation on Hydrofoils 



and Blades 149 



Cavitation Data for Bodies of Revolution 

 and Other Bodies 151 



47.7 



47.9 

 47.10 



47.11 

 47.12 

 47.13 



The Effect of Cavitation on Screw-Propeller 



Performance 152 



Photographing the Cavitation on Model and 



FuU-Scale Propellers 1.53 



Propeller Cavitation Criteria 154 



Predicting Hub Cavitation and Hub Vor- 

 texes or Swirl Cores 155 



Prediction of Cavitation Erosion 156 



Propeller Performance Under Superoavitation 156 



Selected Cavitation Bibliography 157 



47.1 Scope of This Chapter. The phenomena head of 33 ft, gives an effective head of less than 



and mechanism of cavitation in its various forms, 

 and the factors associated with it, are described 

 in Chap. 7, Sees. 7.12 through 7.19, of Volume I. 

 The occurrence of cavitation on ships and pro- 

 pellers, as well as many of its practical features, 

 are discussed in Chap. 23, Sees. 23.9 through 

 23.16, of that volume. Practical examples illus- 

 trating the calculation of the cavitation number 

 are found in Sees. 41.3, 47.4, and 47.5. 



Treatment of cavitation in the present chapter, 

 as associated with ships, propellers, and append- 

 ages, is limited to data of a quantitative nature, 

 of general interest to the naval architect and 

 marine engineer. A selected cavitation bibliog- 

 raphy, containing many recent papers on the 

 subject, concludes the chapter as Sec. 47.13. 



47.2 General Rules for the Occurrence of 

 Cavitation on Ships and Appendages. Cavitation 

 occurs when, for any one of several reasons, the 

 pressure in a particular region in the water drops 

 to that of the water vapor, or to that of a com- 

 bination of water vapor and dissolved air or gas. 

 As the purpose of practically all moving ship 

 appendages, as well as propulsion devices, is to 

 create useful forces by the development of positive 

 and negative differential pressures, it is natural 

 that the +Ap's should be made as high, and the 

 — Ap's as low as can be accomplished without 

 setting up harmful cavitation. Most such parts 

 of a .surface ship operate within a depth of about 

 37 ft below the free surface. This hydrostatic 

 head, when added to the atmospheric-pressure 



70 ft before cavitating conditions are reached. 

 To demonstrate this, set down the pressure 

 equation for steady irrotational flow of an ideal 

 liquid, Eq. (7.i) of Sec. 7.12. Transposing p„ , 



P = P- 



+ liu- 



u'-) 



(47.i) 



For the condition described the critical mini- 

 mum pressure p is equal to the absolute vapor 

 pressure e, say 0.4 psia. The ambient pressure p„ 

 is that corresponding to the full absolute head of 

 70 ft, or 31.13 psi for standard salt water. The 

 value of the term 0.5p(rho) for standard salt 

 water is 0.99525 slugs per ft^ Introducing the 

 factor 144 to transform psi to psf , then substitut- 

 ing and solving, 



0.4(144) = 31.13(144) + (0. 9952.5) (f7^ - C/') 



whence 



(C/ = 



U') = 



(31.13 - 0.4)(144) 



0.99525 

 = -4,446.2 ft' per sec' 



If F = C/„ is 30 kt, equivalent to 50.67 ft per 

 sec, so that Ul is 2,567.4 ft" per sec", then 



U^ = Ul - (-4,446.2) = 2,567.4 + 4,446.2 = 

 7,013.6 ft' per sec', and 



U = 83.75 ft per sec. 



This means that to keep clear of cavitating 

 conditions the relative velocity of the water 



145 



