CHAPTER XI 



EFFECT OF VARIATION IN AFTERBODY SHAPE UPON 

 WAKE DISTRIBUTION AND POWER 



The original conception of Series 60 was a'set of related basic hull forms which, in 

 terms of fullness and proportions, would cover the general field of single-screw merchant 

 ships. The principal purpose was to indicate the trends which might be expected in resistance 

 and power by changes in these basic parameters, but it was also realized that the results 

 were likely to be used for making power estimates for new designs. It was therefore necessary 

 that the resistance and power characteristics of the series models should be of reasonably 

 good standard, and an effort was made to ensure this by the preliminary work with Series 57 

 and the later comparisons with models of existing ships of accepted good performance.^^ •^•^ 



On the other hand, any effort to explore all the possible changes of shape in waterlines 

 and sections before embarking on the series proper would have been prohibitive both in time 

 and money. One of the objectives in setting up the Series 60 design contours has always 

 been the hope that they would be used as a point of departure in future research so that there 

 would always be a link with new work, and this hope has been in a large measure fulfilled. 



The variation of hull shape as exemplified by changes in waterline and section shape 

 is one such research which could well begin from Series 60 as a basis, and a start on this 

 phase has been made at Taylor Model Basin. Since a great deal of interest has been gener- 

 ated in recent years 'in the effects of afterbody shape upon the wake distribution, propeller- 

 excited forces on the hull, and horsepower, the first experiments covered the measurement of 

 wake pattern behind the five parent models, together with the effect upon wake pattern and 

 power of two additional models of 0.70 block coefficient having, respectively., more U- and 

 more V-shaped afterbody sections than the parent. This work was sponsored by the Bureau 

 of Ships, the Maritime Administration and SNAME, and carried out at the Taylor Model Basin. 

 The results have been given in detail in Reference 64. 



The parent 0.70 block coefficient model was No. 4280, made in wood, and identical as 

 regards lines with the parent wax model No. 4221. Two additional wooden models, No. 4281 

 and No. 4282, were made with the same forebody, identical with that of No. 4280, but with 

 more U- and more V-Type stern sections, respectively. The section area curve, load water- 

 line (WL No. 1.00), deck waterline (WL No. 1.50), and stern profile remained unchanged. As 

 a result, all coefficients of form and dimensions except those related to wetted surface and 

 section shape are the same for all three models (Table 42). A comparison between the after 

 end sections is shown in Figure 60. Table 42 also includes values of a coefficient r to 

 describe the slope of Station 18 at the level of the propeller shaft. This coefficient was 

 first proposed by Harvald and is measured as shown in Figure 61. Average values of t 



XI-1 



