152 



m DROOYNAMias I\ SHIP DESIGN 



Sec. 47.7 



variety of londiiiR ends of ivlimlrical bodies, 

 tested in n variable-pressvirc water funnel at 

 zero yaw angle. The princi|)al source of these data, 

 "Cavitation and Pressure Distribution: Head 

 Forms at Zero Angle of Yaw," by H. Rouse and 

 J. S. McNown [State Univ. Iowa Studies in 

 Eng'g., Bull. 32, 1948], gives the magnitude and 

 the meridional-distance distribution of pressure 

 coefficient about a series of fourteen head shapes, 

 covering most practical applications. In addition 

 there are recorded the shapes, axial positions, and 

 dimensions of the cavities or of the envelopes 

 ("pockets") of tiie regions in which vapor bubbles 

 were formed around the bodies carrying these 

 heads. 



Diagrams 1, 2, and 3 of Fig. 17. E give the 

 cavitation "pocket" data for a hemispherical 

 head, a 1-caliber ogival head, and an ellipsoidal 

 head having a major-axis to minor-axis ratio of 2, 

 adapted from the referenced report. The several 

 curved lines represent the boundaries of axial 



, Leng th ond Fbsition o f Covitotio n 'Fbck e t" or Covit^ 

 I for Covitotion Number tf • 0.20\ H 



-\ 037D 



T Covity Boundaries,. 

 I ore Indefinite 



Bod\( of Revolution with Hemispherical Head 



No Covitotion for rf >0.50 



Covitotion Number tf • a g 



Apprommote Shape of Rjcket' or Covity 



u- ' 



Bodif of Revolution with l-Collber Oqivol Heod 



No Covitotion for tf >040 



All Doto ore for Zero Yow Arv^le 



Indefinite Covilii6oundor<j 



(ith 2-to-l Ellipsoidal Head 



No Covitotion for <J >035 



Fio. -IT.E Cavitation Indexes roB Thrbb Ttpe.'j 

 or AxutniMETOic Ucaub 



sections tlirough the cavitation regions on one 

 side of the head and bfnly, for eacji of a series 

 of cavitation munbers a. The meridional positions 

 of these "pockets" and the cavitation numbers 

 belonging to them correspond to the horizontal 

 lower flat portions or "bottoms" of the pressure- 

 distribution curves drawn on pages 49 through 

 64 of the reference, for the given cavitation 

 numbers. A regular pressure-distribution curve or, 

 more exactly, a curve of pressure coefficient on 

 meridional distance with no flat bottom, indicates 

 no visible cavitation for that condition. Broken 

 lines at the trailing ends of the cavity regions of 

 Fig. 47. E indicate indefinite "pocket" outlines. 



The calculated and ob.served speeds for the 

 onset of cavitation around a rather wide variety 

 of 2-diniI and 3-diml bodies when submerged in 

 water are given by H. B. Freeman in TMB 

 Report 495, November 1942. A few of these are 

 pure bodies of revolution, others are 2-diml 

 cylinders having a wide variety of section shape, 

 while still others are combinations of the two, 

 with the axisymmetric body attached to the lower 

 end of a streamlined stmt. 



47.7 The Effect of Cavitation on Screw- 

 Propeller Performance. The typical character- 

 istic curves of a model screw propeller, derived 

 from tests under cavitating conditions in a 

 variable-pressure water tunnel, exhibit a pro- 

 nounced falling off of thrust coefficient Kt and 

 torque coefficient Kq as the values of the advance 

 coefficient J decrease and as the cavitation 

 becomes more severe. This is equivalent to .saying 

 that the thrust and torque of a cavitating pro- 

 peller drop with increase in real-.slip ratio, 

 slowly at first and then more and more rapidly. 

 This situation applies to the range of advance 

 coefficient in which a normal .ship or motorboat 

 propeller operates. It docs not necessarily apply 

 to operation in the supercavitating range, men- 

 tioned in Sec. 47.12. The decrease in Kt is greater 

 than in Kq , so the efficiency T;(eta) likewise 

 fliniini.shes. 



As the cavitation number is reduced, there is a 

 progressive retluction in the numerical values of 

 all three 0-diinl characteristics, exhibited in the 

 typical cavitation characteristic curves of Fig. 

 47.F. The manner in which cavitation enlarges in 

 extent and spreads over a screw-propeller blade as 

 the advance coefficient ./ is reduced, or as the 

 real-slip ratio «« is increa.seil, is shown by L. P. 

 Smith in his i)aper "Cavitation on .Marine Pro- 

 pellers" [AS.Mls, .hil I'j:!7, Vol. 51), PI). 409 l.'Jl; 



