CHAPTER III 



CHOICE OF HULL FORM FOR THE PARENT MODELS 



In the past, the models of most methodical series have been derived from a single 

 parent form by proportioned geometrical changes. When carried to very different proportions 

 and to fullness coefficients suitable to very different values of speed-length ratio f _- J, 

 such changes must inevitably lead to unrealistic forms regardless of how good the parent 

 lines might be for the original design conditions. In planning Series 60, therefore, another 

 approach was tried. A review was made of tiie resistance results of the single-screw merchant 

 ship models available at the Model Basin, and some 20 were selected which appeared to give 

 good performance as judged by a comparison with Taylor's Standard Series. These models 

 covered a range of fullness, and plots were made of sectional area coefficients and waterline 

 half- breadth coefficients to a base of fore- and aft-body prismatic coefficients. Cross curves 

 were then drawn which, while being fair lines, followed the actual points as closely as 

 possible. In this way it was hoped to obtain, by interpolation at the correct values for the 

 parent forms, a series of models which would retain most of the good resistance qualities of 

 the models on which the coefficient curves were based, while also incorporating the changing 

 characteristics necessary to ensure good performance of each model at its appropriate speed- 

 length ratio. At the same time, these parent forms would be related to one another in accord- 

 ance with a definite graphical pattern. Once the series was complete and the resultant 

 resistance curves available, a form could be quickly obtained by interpolation of the cross 

 curves to fulfill any desired combination of Cg, L, B, H, A, and LCB position. Moreover, 

 this design could be immediately associated with a corresponding resistance and effective 

 horsepower. 



From these contours, five parent forms were drawn having block coefficients of 0.60, 



J D 



0.65, 0.70, 0.75 and 0.80, with — ratios, — ratios, and LCB positions as shown for the 



B H 



parent models in Figure 1. This group of models was designated Series 57 in succession to 



earlier TMB Series, and the details of their derivation and the results of the model resistance 



tests were given in a paper before the SNAME in 1951. 



The resistance results of Series 57 were compared with those for a number of recent 



successful modern designs of single-screw ships and found to be disappointing. In view of 



the apparently good qualities of the models on which the contours were based, this was at 



first sight surprising. Further investigation suggested that although the departures from 



the actual design lines made when fairing the contours were small, they may have been 



critical in certain cases, and also that possibly some of the results of the resistance tests 



on the chosen models were suffering from the effects of laminar flow. Apparently in ship 



III-l 



