ECONOMICAL CARGO SHIPS—SOME MODEL EXPERIMENTS. 57 
In the Taylor method and also in the method of Dr. Sadler you can get the frictional 
horse-power out of the way and begin to make a study of the residuary per ton for the 
various coefficients, and finally, at the proper coefficient, get the desired residuary per 
ton and thus do not have to work out the effective horse-power for each model. 
It seems to me that the Taylor and Sadler methods of working with pounds per ton 
are more satisfactory, can be extended either to a large boat or a small boat, and are theo- 
retically correct. The method of using the circular P works nicely for a boat of a certain 
size and within certain changes of lengths. 
Tue CHAIRMAN -—Is there any further discussion? If not, we will ask Mr. Robertson 
to close the discussion on the paper. 
Mr. Ropertson :—Professor Sadler drew attention to the diagram, also referred 
to by Mr. Rigg, on Fig. 11, Plate 23, which I think is perhaps the most important diagram 
presented here. It is an attempt to explain the difference existing in the tanks between 
this country and abroad. Mr. Rigg tells us that these differences have been known for 
some time. It has been extremely difficult to investigate that point thoroughly because 
the quantities with which we are dealing really come to the frictional resistance of the 
model, and that model resistance may total about one-tenth of a pound. The discovery 
of a 5 per cent difference in the friction of that one-tenth of a pound needed a good deal 
of careful comparison and investigation, and I just wish to say that the curves, though 
theoretically correct, are not actually correct. The models tested at Washington and 
at Ann Arbor did not test just as shown on these curves. In the case of the Washington 
models of light draught, small cross-sectional area, the difference is approximately what 
is shown here; but in the load draught portion, where the cross-section of the Washington 
model is larger in proportion to the cross-section of the tank than at Ann Arbor, the Wash- 
ington model seems to test high, and, of course, that is a thing which will have to be 
investigated further—we are just feeling our way to eliminate some of the causes of the 
distrust of tank tests. 
Mr. Rigg referred to the elimination of the P value in presenting the results by the 
Froude method. Of course the P value is not a Froude method. The P value is a Baker- 
Kent method, and I think that the very best proof of the value of that P can be obtained 
by the analysis of the results given in this paper. I mean to say that you can substantiate 
the value of the P, or discover it is not of as high value as at first we thought, by a careful 
study of these tests, where the parallel middle body was varied through so many stages 
as it was in this series of experiments. My own opinion is that in these tests, where the 
entrance and run proportions are not the same, the P value does not hold good. 
I am very much obliged to Professor Peabody for throwing a little additional light 
on the Tideman’s coefficient. Most of us who have been using them for a time were 
aware that Tideman had not actually carried out an intensive investigation, such as 
Froude had done, but he did give the world coefficients at an early date, which were 
adopted as if he had, and I think Mr. Froude deserves more credit than he gets for the 
frictional coefficients, even if in this country Tideman’s slightly higher coefficients have 
been adopted, both at Washington and at Ann Arbor, for the full-sized ships. 
Professor Chapman complains about the form in which the results have been pre- 
sented, the circular C values. Of course the only difference between my form and those 
of Dr. Sadler and Admiral Taylor is simply in the C, because we use the same abscissa 
