308 THE INFLUENCE OF SHAPE OF 



DISCUSSION. 



The Chairman: — This paper, No. 11, entitled "The Influence of Shape of Transverse 

 Sections upon Resistance," is now open for discussion. 



Professor L. B. Chapman^ Member: — The Society is very fortunate to have this very 

 valuable paper presented by Professor Sadler and Professor Bragg. An entirely new field 

 has been covered, and the results will be very useful in merchant ship design. The profes- 

 sion is certainly indebted to them for this piece of work which, we all know, must have taken 

 a long time to perform. 



I should like to speak briefly about the method of presenting the results. Last year we 

 had considerable discussion along this line in connection with Mr. Robertson's paper. The 

 method used by Admiral Taylor and, until this year, by Dr. Sadler is to divide the resistance 

 up into frictional and residuary. The residuary is plotted in pounds per ton of displacement 

 on a base oiV^/L. This is a very convenient method, for the frictional resistance can be 

 carefully and accurately determined and the residuary estimated from published data. The 

 frictional resistance is generally about two-thirds of the total, and this can be definitely com- 

 puted if the wetted surface is known. For ships without parallel body we have Taylor's 

 Standard Series as a basis for comparison of residuary resistance, these curves being very 

 useful as a standard for comparing the residuary of a new ship. Also, for this type of ship 

 the wetted coefficient C in the formula W. S. = C V D.L. is well known. 



Now when we come to ships with parallel bodies practically all of the published data 



fcl V^ /?^'^ 

 is given in terms of [C], where E. H. P. = *— ' the standard ship being 400 feet 



long. 



The experimenters in this field — namely, Baker, Semple and Robertson, all use a differ- 

 ent base, either V VL or some variation of this ratio. McEntee gives all his data as E.H.P. 

 for a 400-foot ship ; so \C\ can be easily obtained. For ships of this type there are little 

 published data on the value of the wetted surface coefficient C; and as all the data have been 

 given in the form mentioned, it is a very useful and convenient method. 



Now Doctor Sadler comes forward with a new method of presenting results — namely, 

 E.H.P. per ton, on a basis oi V V L. As the curves of pounds of resistance per ton are 

 known to cross each other when changes are made in the draught he probably used this method 

 in place of the \C\ method. In order to keep his curves within reasonable limits he has 



P TIT p 



divided the values of t^- " ' by {V V L)^ This is perfectly clear on the curves and can 



be easily corrected for. However, I believe that the correction necessary to apply before using 



EH P 

 these results — namely, that ^. ' ' has to be multiplied by the square root of the length ratio 



( VX ) — is going to be overlooked bu the practicing naval architect. Thus, if the ship 

 in question has some other length than 425 feet, is not the man picking up the paper going 

 to overlook the necessary correction factor. It seems to me that if Doctor Sadler had pre- 

 sented his results as E.H.P. -i- £>'/" the possibility of this error would have 'been avoided. 



