151 



HVDROUVNAMIC.S IN 6H11' DESIGN 



Sec. 47.9 



Examples of photoRraphs of this kind arc 

 puMi.shctl by J. \V. Fisher ("Photonraphy at Sea 

 of Ship Propeller Cavitation," XKCI, 1951-1952, 

 \o\. (^18, pp. 1!) m ami D-1 through D-S). Other 

 examples are emhotlietl in a paper by A. F. Weeks, 

 entitled "Ship Propeller Cavitation Patterns," 

 presented at the XPL Sympasium on Cavitation 

 in September 1955 [SBSR, 3 Nov 1955, pp. 

 5G9-570]. 



It is to be expeetetl that the instrumentation 

 and procetlures for making shipboard cavitation 

 photographs on screw propellers will improve 

 rapidly. This will provide the naval architect with 

 extremely valuable data which may be studied 

 and analyzed at leisure. 



47.9 Propeller Cavitation Criteria. An analj'- 

 sis of the loss of thrust due to cavitation on ship 

 propellers, when it first occurred on fast naval 

 \essels some sixty or more years ago, led to the 

 establishment of a criterion proposed by S. W. 

 Barnaby, involving a maximum average unit 

 loading on the propeller-blade area. The original 

 figure emploj-ed was 11.25 lb per sq in, corre- 

 sponding to a pressure coefficient (based upon 

 atmospheric pressure) of 11.25 divided by 14.7, or 

 approximately 0.765. This unit loading was 

 subsequently raised by Barnaby to 13.0 lb per 

 sq in, corresponding to a pressure coefficient 

 based on atmospheric pressure of 0.884. In 

 neither case was there any allowance made for 

 the depth of submergence of the propellers. 

 Sub.sequently, different criteria were proposed by 

 D. W. Taylor, J. M. Irish, and others, ba.sed upon 

 a limiting tip speed for any propeller, independent 

 of- or related to the propeller loading but, as 



before, not a function of the depth of submergence 

 or the associated speetl of advance [Schoenherr, 

 K. E., PXA, 1939, Vol. II, pp. 175-17G). 



In 1932 E. F. Eggert developed a set of rela- 

 tionships which enabled the propeller user or 

 designer to predict, with reasonable accuracj', 

 the rate of rotation or the blade-section velocity 

 at which the propeller thrust would begin to fall 

 o(T from cavitation efTects ["Propeller Cavitation," 

 SXAME, 1932, pp. .58-74]. Discussions of this 

 relationship, embodj'ing somewhat different com- 

 binations of variables, arc found in: 



(1) Schoenherr, K. E., P.\.\, 19;», Vol. II, p. 178 



(2) Taylor, D. \V., S and P, 1043, pp. 110-117 



(3) Van Lammeren, W. P. ,\., UPSS, 1948, pp. 180-182. 



Still later W. P. A. van Lammeren worked out 

 a relationship between the two 0-diml ratios 

 [.//(virtual P/D)] and [(t,- (.4 c/.4o) (virtual P/D)] 

 which is an excellent indicator of the point where 

 the thrust breaks down on a screw propeller 

 [De Groot, D., NSP Rep. 89; Schip en Werf; Gth 

 ICSTS, 1951, published by SXAME, 19.53, Fig. 

 30 and pp. 93-95]. ^\Tlen plotted in graph form 

 this appears as the simple curve of Fig. 47. d. The 

 graph is based on the tests of many model pro- 

 pellers, all of which give results remarkably close 

 to the line, independent of the tj'pe of blade 

 section. It is thus possible to predict, before 

 carrying out cavitation tests on model propellers, 

 the rate of rotation at which thrust breakdown 

 will take place. 



The indications given by the graph of Fig. 47. G 

 arc still sufficiently precise for engineering 

 purposes if the actual or average P/D ratio is 



04 



oe 



08 1.0 12 



<*vCae/a<;)(pv/d) 



i.e 



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