Sec. 47.4 SHIP AND PROPELLER CAVITATION 



TABLE 47.a — Vapor Pressure of FRESH WATER fob Various Temperatures 



147 



47. a with corresponding values of deg F, deg C, 

 psia, and absolute head in ft of water. 



The vapor pressure of the salt water of the 

 seas of the world is, as mentioned in Sec. 47.2, 

 a function of many variables other than tempera- 

 ture. The effect of these variables is not yet 

 isolated and only somewhat random data are 

 available for calculation purposes. The best of 

 these data are given by S. F. Crump in TMB 

 Report 575, issued in October 1949. Some data 

 are abstracted by P. Eisenberg in TMB Report 

 712, dated July 1950, pages 18 through 20. A 

 general statement about the range of vapor 

 pressure for salt water is added at the top of Fig. 

 47. A. Supplementary data are given in Table 

 X3.1 of Sec. X3.6 of Appendix 3 at the end of 

 this volume. 



47.4 Tables of and Nomogram for Cavitation 

 Numbers. For convenience in making rapid 

 predictions there have been calculated the values 

 of the cavitation index or number o-(sigma) for 

 standard salt water, covering a rather wide range 

 of velocity and a reasonable range of net head 

 [Qia + /iff) — hv], where hy is the vapor-pressure 

 head. These data, computed for standard con- 

 ditions and in accordance with the formula 

 o = Qia + flu — /ly)/ (Velocity head hu), are set 

 down in Table 47.b. 



Fig. 47. B is a nomogram by which the speed 

 of incipient cavitation may be determined by 

 inspection from a knowledge of the depth of the 

 object below the water surface and the critical 

 cavitation index acR for the body shape [Macov- 

 sky, M. S., Stracke, W. L., and Wehausen, J. V., 

 TMB Rep. 879, Jan 1948, Fig. 4; Mandel, P., 

 SNAME, 1953, Fig. 8, p. 472]. For example, 

 assume that a speed log of some kind is projected 

 4 ft below the bottom of the ABC ship of Part 4, 

 and that it is desired to have the log free of 

 cavitation at the clean-bottom trial speed of 

 20.5 kt. With a ship draft of 26 ft, the nominal 

 submergence of the log is 30 ft, neglecting the 

 effect of sinkage, trim, and ship-wave profiles. 

 To have a suitable margin for an increased vapor 

 pressure due to aeration and similar factors, the 

 speed of incipient cavitation should be at least 

 24 kt. Then applying a straight edge across the 

 nomogram from h = 30 ft to F = 24 kt it is 

 found that the critical cavitation index acR for 

 the portion of the log in question is 2.48. This 

 means that the pressure coefficient E„ at the 

 point of minimum absolute pressure on the 

 exposed portion of the log cannot exceed —2.48. 



To make the situation perfectly clear to the 

 prospective designer, a portion of the discussion 

 of Sec. 41.3 on the Euler and cavitation numbers 



