100 



nVDRODVNAMK.S l.\ SHIP ni'SICN 



Src. 45.7 



bccaust" smooth surfaces are icpnxluiiblc and onlj' 

 in that way can consistent cxperimentul data be 

 assured. 



Despite the extensive studies of recent years 

 there is as yet no comprehensive, accurate formula 

 for bridging tlie gap between an experimental 

 friction plane or model and an actual ship, and 

 for taking account of the effects listeil in the seconil 

 paragraph preceding. In fact, workers in this field 

 are not even agreed on the form which such an 

 expression should take, or whetiier an attempt 

 should be made to embmly all the bridging 

 factors, as it were, into a single formulation. 



In the meantime the naval architect must 

 span the gap somehow, and with assurance that 

 his predictions are reasonably correct. The 

 American Towing Tank Conference decided in 

 1947 that for its work this operation is to be 

 performed by the use of: 



(1) The meanline developed by K. E. Schocnhcrr 

 for expressing the specific friction drag Cf of 

 turbulent flow on a flat, smooth surface a.s a 

 function of Reynolds number 7?„ 



(2) An additive allowance, in the form of a 

 specific friction resistance and called ACV , for 

 the effect of unavoidable roughness on a clean, 

 new ship. 



The Schoenherr meanline, as its generic name 

 implies, was based upon a careful analysis of 



available experimental result.s. While the.se experi- 

 ments antedate the year 1932, the line can alwaj's 

 be shifted to accommodate newer and better data. 

 It docs, however, conform to the physical laws 

 for the dependence of friction drag on Reynolds 

 number and it has given satisfactory ship pre- 

 dictions for many years. There is no specific 

 provision in the .\TTC 1947 procedure for edge 

 effects, transverse and longitudinal curvature 

 effects, variations in wetted surface and flow 

 patterns due to waves, or any other factors. 



The ATTC 1947 (Schoenherr) meanline, de- 

 ])icted graphically by the heavy, solid line in the 

 log-log plot of Fig. 45.E, expresses the relation 

 between CV and /?„ by either of the equations 



242 



"•g^ = log,o {R„Cf) 



(o.xiva) 



{C,)-"' = 4.132 log.o {RJCf) (S.xivb) 



Since this equation is not readily solved for 

 Cy , sufficient values of Cf and R„ , in the model 

 and ship ranges, have been calculated and tabu- 

 lated for practical use. The tables arc published 

 in SNAMIC Technical and Research Bulletin 1-2, 

 dated August 1948, for all who need them. Two 

 small portions of these tables, modified to express 

 all /?„ values in jtiillions, are reproduced in Tables 

 4.5. c and 45. d of Sec. 45.9, covering mniiol and ship 

 ranges, respectively. 



50 100 500 1000 



Reynolds Number ( I0"»j- ^ (lO"*) 



Fio. •I.I.FC Purr or Tiiiu;k TrrKs oi- ItocciiiNKHH Ai.uhvanik fou Siin-s, SimwiMi Vauiatiu.vs wrrii IlKV.Nouns 



Nll.MII>:lt 



