106 JOINT DISCUSSION ON TWO PRECEDING PAPERS. 
I have recently made a large number of comparisons between trials and calculated re- 
sults from Taylor’s 6 diagrams, and when the wake can be closely estimated the 
check has been very close indeed. The use of Taylor’s charts is much simpler than any 
other method, and when proper wakes, thrust deductions and hull efficiencies are used the 
results are, to my mind, even more accurate. I should like very much at some future time to 
present a summary of this investigation before the Society. 
Up to the present time our knowledge of wake has been from the investigations of Luke 
(presented before the Institution of Naval Architects in 1910, 1914, and 1917) and Mc- 
Entee’s investigations on self-propelled models. 
The results of Prof. Bragg, especially for single-screw ships, appear to be in close 
agreement with the investigations mentioned above. It is almost impossible to make a 
careful comparison, for as yet it is very uncertain what the controlling factors are. Luke 
plotted his results on block coefficient, but Fig. 4 in Prof. Bragg’s paper shows conclusively 
that block coefficient is not the most important controlling factor. Prof. Bragg’s sug- 
gestions of using a vertical prismatic coefficient is very interesting, and this may be one 
of the influencing factors. 1 feel, however, that the longitudinal coefficient (prismatic) of 
the after body is one of the most important factors. Luke’s and McEntee’s investigations bear 
this out, and Prof. Bragg’s model No. 1130 indicates the same thing. 
I would like to ask Prof. Bragg if in his study of the results he attempted to plot any 
of the data on this basis. 
The difference in the wake values between models No. 1130 and No. 1131 is very inter- 
esting and can hardly be charged up to the difference in beam-draught ratio. A comparison 
of these two models on the same block coefficients and percentage of run shows a variation 
in wake of about 20 per cent, which is hard to explain. 
Prof. Bragg has carried his investigations into new fields and has given a great deal of 
additional information to that already existing. 
Mr. Witiiam W. Smitru, Member:—The data given in Prof. Bragg’s paper are very 
valuable for estimating the wake fraction. However, for estimating propeller performance 
two other hull coefficients are required, viz., the hull efficiency and the rotative efficiency. It is 
hoped that in due time data for estimating these coefficients also will be added to the present 
valuable data. 
In making propeller estimates it is the regular practice of some engineers to take the hull 
and rotative efficiencies as unity. It is evident, however, that such assumptions may introduce 
considerable errors in certain cases. 
Referring to page 100, I wish to ask the following questions: (1) In passing from the 
model to the full size vessel, does the law of comparison with respect to size and speed hold 
exactly? (2) If not, what allowances should be made? (3) What, if any, data are avail- 
able on this subject? 
It also seems to me that the following elements require consideration in determining the 
hull coefficients of a vessel. These elements naturally divide themselves into three groups: 
1. Proportions and Form of After Body:—Length, draught, beam, prismatic coefficient, 
center of gravity forward of the after perpendicular and above the base line. 
2. Size and Location of Propeller :—Size ; longitudinal, transverse, and vertical positions ; 
and tip and axial clearances. 
