Sec. 41.5 



SHIP AND PROPELLER CAVITATION 



TABLE 47.b— (Continued) 



149 



is repeated here. This derived cavitation index 

 2.48 is a measure of the pressure which must be 

 available at the log position to create a transverse 

 pressure gradient which will cause the water to 

 close in about the nose and follow the shape of 

 the log, with no cavities. It must exceed numeric- 

 ally the negative-pressure coefficient —Ap/q at 

 any point along the sides of the log, to insure that 

 there is more than enough pressure gradient to 

 turn the water inward abaft the nose of the log 

 and keep it there. As long as {p — p„) at any 

 point is smaller numerically than p^ — e, or 

 that {h — Ka — hn) is smaller numerically than 

 (h^ + hH — hv), cavitation will not occur at 

 that depth and speed. 



There are now (1955) in existence a consider- 

 able amount of data, and in time there will be 

 more, which give by inspection the critical cavi- 

 tation numbers acR for the onset of cavitation on 

 a variety of body forms, as well as the shapes and 

 positions of the cavities to be expected at different 

 cavitation numbers. A set of the first, applying 



to symmetrical hydrofoils, is given in Fig. 47. D 

 of Sec. 47.5; a few of the second are shown in 

 Fig. 47.E of Sec. 47.6. 



47.5 The Prediction of Cavitation on Hydro- 

 foils and Blades. Using the procedure described 

 in the preceding section, it is possible to determine, 

 from a knowledge of the cavitation number for 

 a certain set of operating conditions and an 

 inspection of the pressure-coefficient curves for 

 any body, such as those illustrated and referenced 

 in Sec. 44.8, whether or not cavitation is liable 

 to occur, and where it will be found along the 

 body surface. One has only to look for locations 

 on the body where the pressure coefficient 

 — Ap/q is greater numerically than the cavitation 

 number a = (p^ — e)/q. For example, if the 

 R.A.F. 30 section of Fig. 44.G, running at an 

 angle of attack of 7 deg, is placed in a region 

 where the cavitation number is only 2.0, it may 

 be expected to cavitate on the upper or — Ap 

 surface directly abaft the nose. 



For certain symmetrical or asymmetrical hydro- 



