108 JOINT DISCUSSION ON TWO PRECEDING PAPERS. 
Captain JosepH H. Lrynarp (C.C.), U. S. Navy, Member of Council:—Prof. Bragg 
in his paper describes the various methods for obtaining the various wake figures, and then 
he goes on to state how he gets the average value by the use of the current meter, which, in 
effect, is a propeller, and I will ask Prof. Bragg if he will state the manner in which the cur- 
rent meter averages the wake. 
I also ask whether he has made experiments in comparison with the Pitot tube, to deter- 
mine that average with equal areas, as would be determined in the case of the propeller aver- 
ages; whether he averages the amount simply or whether there is any relation to energy or 
speed which would make the higher speeds count more heavily owing to their relatively 
greater energy in their effect on the current meter propeller than on the speeds that are lower, 
so that they would not average as the mere numerical sum, but more or less to the square of 
the wake speeds? 
ProFessor Hersert C. SADLER, Member of Council:—I would like to relate a little ex- 
perience that occurred some years ago when we first started work in the Experiment Tank 
at the University of Michigan. I made some models of some of the lake freighters, and 
worked out the effective horse-power and had an opportunity to check them up with the 
actual vessels. 
I was surprised to find that the propulsive coefficient had to be something like 65 per 
cent or more in order to get the tank resultsto agree with the practical results. At that 
time it seemed to me that there must be something wrong somewhere, because we hardly 
expected large capacity freighters to have a coefficient of 65 per cent, such as you get in 
battleships. 
Admiral Taylor’s paper gives the answer. You will notice the hull efficiency is consid- 
erably over 100 per cent, and Commander McEntee showed the same thing a year or so ago. 
In the case of lake vessels, vessels with 19 feet draught and propellers 14 or 15 feet in 
diameter, we are reaching out into the top regions of the wake, much more so than in the 
case of the ocean liner where the draught is much greater, and the propeller no greater in di- 
ameter. I would like to thank Admiral Taylor personally for clearing up a matter which 
originally gave me considerable trouble. 
Admiral Taylor suggests that the stern of the models tested should be made finer in 
future tests. I would also suggest that it be made fuller also so as to cover vessels of .75 
coefficient. 
Mr. Wituram W. Smitru, Member:—On page 103 Admiral Taylor states that the hull 
efficiency must be affected by speed. If Admiral Taylor could furnish data showing what 
this effect is, it would be of value. 
I have sought reliable data for estimating the effect of speed on the performance of pro- 
pellers but have found none which seem reliable except the charts given by Admiral Dyson. 
These data, however, are only applicable to his method of estimating propellers. It would 
also be very desirable to have similar data for estimating full size propellers from model 
experiments. 
Referring to Plate 43, it is noted that the author does not give the rotative efficiency. 
It would be of value if Admiral Taylor would define the method of referring the “behind” 
to the “open” tests. 
Referring to Plates 40 to 42, it is my understanding that the C+ value from the “behind” 
