necessary to have the abscissa and ordinate values compatible. For abscissa, the speed- 



V 

 length ratio . — is preferred by many naval architects for its simplicity. In order to keep 



R J 

 the values of within a reasonable numerical range, it is usual to divide them by some 



A 

 function of (speed)^ since this makes the ordinates almost constant over the lower speed 



V 

 range. If it is desired to use y— as the speed parameter, then the ordinates should be 



if)' 



A . F2 



Rt ■ L -V 



If the comparison is to be made on a power basis, then the ordinate becomes ""■ — - 



A . F3 



EHP • L SHP • L 



A • f3 a • y3 



Dr. Telfer has made this point very clearly in discussing the Series 60 papers. 

 "... the designer's problem is usually to find the model having the lowest resistance per 

 ton displacement on a given length, length being usually approximately fixed by conditions 

 other than resistance" (discussion on Reference 44). And again, "Figure 16 gives us an 



SHP 



incompatible presentation of a power-displacement function presented in terms of 



A2/3y3 

 V . . 



a speed-length function ~^ . From this diagram a designer is led to infer that the finest 



ships are always the most economical. Such a conclusion from the basic data would be 

 completely erroneous. To review the data correctly they must be presented in a compatible 



V 

 form. As the speed-length ratio "pr is preferred by most practical ship designers it must 



by retained and the requisite change for compatibility made in the power-displacement 

 function. This must be converted to a power-length basis, still using, however, power per ton 



SHP • L 

 displacement. The conversion produces the function which correctly grades the 



A . V^ 

 power per ton of all vessels having the same length and speed" (discussion of Reference 61). 



The basic resistance and dhp data for the Series 60 parents are presented in this form 

 in Figures 93 and 94. To again quote Dr. Telfer: "A designer now sees that if his speed is 

 low the most economical ships have the fuller and not the finer forms. Certainly as the speed 

 is increased the finer form becomes the more economical, and by drawing a tentative envelope 

 to the individual curves a mean scale of optimum block coefficient and optimum power constant 

 for given speed-length ratio is at once available. "^^ 



Dr. Telfer has recently converted the results of the resistance experiments on the 

 Series 60 models to this method of presentation, and compared them with other available data. 

 ("The Design Presentation of Ship Model Resistance Data." E.V. Telfer, Trans NECI, Vol. 79 

 (1962-63).) 



XII-12 



