Although no claim can possibly bo made that Series 60-or indeed any other Series- 

 represents optimum resistance qualities, the above remarks and other evidence do suggest 

 that the contours will be of help in preliminary design work and can be used with some confi- 

 dence in the estimation of ship performance, both in the absolute sense and also in the 

 investigation of various alternative choices which may face the naval architect. 



In using the Series 60 results, it is worth recalling that the hull forms are all related 

 to one another in a clear and unambiguous way by means of graphical methods. As has been 

 pointed out before, this has the advantage over geometrical variation of one parent form in 

 that the characteristics can be varied with fullness to suit the corresponding changes in 

 speed-length ratio, '^he alternative use of a single parent form to cover such a wide range 

 of variables as used in Series 60 would have led inevitably to unrealistic designs towards 

 the limits of the area covered. Another point to remember is that all models were of the 

 same length and run in the same tank with the same instrumentation, thus eliminating other 

 possible sources of difference. 



In the course of the discussions on the many Series 60 papers, much has been said 

 about various methods of presenting the data. The two most commonly used are to give 



values of residuary resistance per ton of displacement ^ in terms of speed-length ratio 



V ^ 



^ , almost universally used in the United States, or values of ©400 ft i" terms of (^ 



'W L 



as used in Great Britain. Both systems have merits and demerits, as one might expect, but 

 they are well-entrenohed in their respective homes. The Series 60 results have therefore 



been given in both ways as contours of and of (0 to their respective bases. There is 



A 

 a vast amount of model data expressed in one or other of these forms, with which the Series 



60 results can be compared directly. The SNAME Model Resistance Data sheets also give 



the information in both these forms. 



Rr V 



The presentation of in terms of , has the advantage of simplicity but suffers 



A yJLr 



'W L 

 from two drawbacks. In the first place, a true merit comparison has to be made on the basis 



^R 

 of total resistance per ton of displacement, and comparisons on the basis of can be 



quite misleading. Skin friction resistance is the major component of total resistance in most 

 if not all single-screw merchant ships, and this depends on wetted surface, not directly on 

 displacement. To make a merit comparison from data presented in this way, it is therefore 

 necessary to estimate the frictional resistance in each case and so obtain total resistance 

 or ehp. 



The true merit comparison of interest to the naval architect and ship owner is the 



R -r 



total resistance per ton of displacement ■■ . To present this properly in curve form it is 



XII- 11 



