CHAPTER II 



SELECTION OF THE RANGE OF PROPORTIONS FOR THE SERIES 



At the time of the inception of the program, a survey was made of the current practice 

 in shipbuilding to ensure, as far as possible, that the series would cover the normal 

 range of proportions of modern ships. In the course of this, some 40 individuals and organi- 

 zations were consulted, and after analyzing these comments, the SNAME Panel agreed upon a 

 series of parent forms and variations which would cover the general field of design for single- 

 screw merchant ships. This was in 1949, and already it is obvious that the Series is no 

 longer adequate for modem single-screw ships, which, on the one hand are being made finer 

 and driven to higher and higher speeds in order to obtain the increased efficiency possible 

 with single-screw as compared with twin-screw propulsion, and on the other hand are being 

 made larger and fuller to achieve the resultant economy in bulk carriers of ore, oil and 

 similar cargoes. At the time of the inception of the program, it appeared that lower and 

 upper limits in block coefficient of 0.60 and 0.80 would be satisfactory, but the intervening 

 years have shown that 0.55 and 0.85 would have been better forecasts. The future extension 

 of the series to such forms would be a very worthwhile project. 



The basic parameter chosen for defining the series was block coefficient (Cg). 

 This was used in preference to the prismatic coefficient (C p) because in the preliminary 

 design stages for merchant ships it is a direct measure of the displacement carried on given 

 dimensions, usually a basic consideration. This approach in no way prevents the use of 

 prismatic coefficient in the subsequent presentation of the results if so desired. 



The decision to use Cg in preference to Cp has been a point of comment by numer- 

 ous contributors to the discussions on the Series 60 papers. In general, the ship designer 

 and operator seem to favor block coefficient. Sir Amos Ayre said that "for the type of ship 

 dealt with, I am pleased to observe that the block coefficient has been chosen as the basic 

 parameter in preference to the prismatic coefficient" (discussion on Reference 44). Mr. 

 Ericson, commenting on the same paper, stated that he "should like ... to put in a few 

 words which will present the viewpoint of the ship operator himself. First, 1 should like to 

 endorse the use of the block coefficient as a basic parameter. It is fairly useful in making 

 a study, particularly an economic study, where displacement is considered, which is reflected 

 immediately in the carrying capacity of the vessel." 



On the other hand, naval architects and hydrodynamicists have emphasized the merits 

 of the prismatic coefficient as being a more meaningful parameter for interpreting resistance 

 results, although even here some doubts have been expressed by Dr. Weinblum: "Other 

 calculations show the now well-known extreme sensibility of the wave resistance to varia- 

 tions of pure form for a given prismatic coefficient. The wave-resistance values correspond- 

 ing to two such forms can easily reach a ratio of 3:1, so that sometimes one even is inclined 



II-l 



