JOINT DISCUSSION ON TWO PRECEDING PAPERS. 117 
(which were not very complete) the same propeller was used in the high place as in the low 
place. There are two methods of avoiding the loss of propeller efficiency pointed out above. 
Holding revolutions constant by increasing diameter and reducing nominal pitch, the slip 
per cent would be reduced and efficiency increased until we reach a balance between the rate 
of gain due to slip reduction and the rate of loss due to pitch reduction. This would show 
a substantial gain even in the extreme case above. But the fundamental trouble in the above 
case as regards propulsive efficiency is the high revolutions. The propellers, which as 
pointed out in the paper were smaller than would be used in practice, were all derived from 
assumed revolutions of 108 per minute for the full-sized propeller. While this is satisfactory 
for the purpose of investigating wake factor and thrust deduction, it is incompatible with a 
high efficiency of propulsion for the power to be absorbed and the speed of advance, par- 
ticularly the latter. 
When preparing a design, every factor present except propeller efficiency tempts the en- 
gineer to increase the revolutions of his propeller, and this has resulted in many unneces- 
sarily inefficient ships. 
I believe the copious experimental data published of late years have to some extent mis- 
led the designers because they have not covered the whole field. 
Take the case above. The maximum efficiency of the propeller is .65 at 17 per cent 
true slip. The nominal slip based upon the speed of the ship is 11.57 knots is about .09 per 
cent in the low position. Without reliable data as to wake, etc., the designer who, as a result 
of published model data, can estimate very closely the maximum efficiency and corresponding 
slip of his propeller would be tempted to say the nominal slip of .09 allowing something for 
wake is near enough the slip of maximum efficiency for practical purposes and let the revolu- 
tions remain high. 
If he had available, in the early stages of the design, however, reliable estimates of 
wake, it is not at all likely that he would pass an actual slip of 39 per cent corresponding to 
the high position, or anything approaching it ina lower position. Certainly before accepting 
such a high actual slip he would give a good deal of thought to the possibility of reducing his 
revolutions. 
THE PRESIDENT :—These are two extremely interesting and valuable papers, and the dis- 
cussions and the replies to the discussions have brought out additional information, and I 
am sure that 1 am only carrying out your wishes in extending the thanks of the Society both 
to Prof. Bragg and Rear Admiral Taylor for presenting these papers. 
That completes the program for the afternoon. The meeting now stands adjourned until 
ten o'clock tomorrow morning. ‘ 
