320 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 57.7 



Re'ynolds Number R^ 

 at which Wave- 

 mokinc^ Resist- 

 ance Beqins 

 on the Small 

 Model 



Line of Constant Froude Number Fn 

 for Models of All Sizes and for the Ship 

 Rbjnolds- Number Abscissa for each 

 Circle IS Determined by Usinij Same 

 Absolute Value of V as in the 

 Respective Froude Number and Same 

 L as for the Respecti\ 



Model orShip^~~^^ 



Loq of Reynolds Number, on Uniform Stale 



Fig. 57.C Schematic Representation of the Telfer Extrapolation Diagram to Illustrate Various 



Resistance Factors 



model has a greater effective volume, in proportion 

 to its length, because of the greater relative 

 displacement thickness 5(delta) of its boundary 

 layer. Because these three effects increase as the 

 model size diminishes they give the impression, 

 apparently not real, that the basic friction line 

 should be steeper in this region. 



If there are no effects other than those which 

 have been enumerated, and if the wavemaking 

 and other normal-pressure drags vary only as V' 

 in the normal model and ship ranges, it should be 

 possible to extend a hne such as D2D3 by drawing 

 it parallel to C2C4 . At a ship R„ corresponding 

 to the F„ value for this line, the point D4 should 

 give the total specific resistance coefficient for the 

 ship at the given F„ value. Similarly, other lines 

 such as E2E3 could be drawn, so that the complete 

 Cr curve HiGiDj for the ship would be predicted. 



Rarely is it found in practice that points such 



as D2 and D3 both lie on a line parallel to 

 H2H3H4C4. Almost certainly Di , Dj , and D^ do 

 not, as may be noted by consulting the many 

 diagrams in the references listed earlier in this 

 section. 



The accuracy and reliability of the Telfer 

 method therefore rest heavily upon several 

 factors, as yet not properly resolved: 



(a) Exactly the proper degree of turbulence 

 stimulation on small models to insure that the 

 transition from laminar to turbulent flow, and the 

 onset of separation, if any, occur at the same 

 relative positions along the length as on the large 

 model and on the ship 



(b) The correct allowance for roughness of the 

 full-scale ship surface. This is of course equally 

 important, whether or not the Telfer method is 

 employed. 



