Sharma 



050 



Fig. 11 - Measured total resistance coeffi- 

 cients of a cargo ship model with and with- 

 out a bulb, at two drafts 



a good result. In the present context it is more important to observe the follow- 

 ing features in Fig. 11. The wave resistance coefficient C^ as computed from 

 the spectra displayed in Figs. 9 and 10 is also plotted in Fig. 11 as a deduction 

 from the measured total resistance coefficient c^. Now, if the standard corre- 

 lation line recommended by the ITTC 1957 is used as an estimate of frictional 

 resistance Cf , it is obvious that the residuary resistance C^ - Cj is not ac- 

 counted for by wave -making alone. Thus there is indirect evidence of consid- 

 erable pressure drag of viscous origin. Considering differences, it is worth 

 noting that the resistance reduction due to the bulb at the load draft can be fully 

 explained by the observed attenuation of the wave system, whereas at the ballast 

 draft only about two thirds of the total resistance reduction can be attributed to 

 the attenuation of the visible surface deformation. This indicates that in the 

 latter condition either the bulb is simultaneously acting to reduce viscous re- 

 sistance or that the present method tends to underestimate the "actual" wave 

 resistance. That this second possibility cannot be ruled out is suggested by the 



760 



