114 JOINT DISCUSSION ON TWO PRECEDING PAPERS. 
hardly an indication of the wake produced by struts, since the surface of the plane is very 
much greater, relatively, than would be the surface of the struts. 
In reply to Captain Linnard I would say that I have made no experiments to determine 
how the wake given by a current meter wheel compares with the integrated wake of fila- 
ments as given by a Pitot tube. I should say, however, that it is a question of velocities 
rather than energy, as the meter wheel has ball bearings and can absorb very little of the 
energy of the stream. 
Rear ApMIRAL Davip W. TAytor:—There is very little I can add to what Prof. Bragg 
has said in the discussion, but there are one or two points I would like to mention. In the first 
place, as regards agreement between Prof. Bragg’s paper and mine, as far as I can make 
out there is no disagreement. The wake is liable to vary so much with comparatively small 
changes of the model that apparently it is difficult to make any exact comparison, but as 
far as I can judge there is certainly no material disagreement. It might seem at first sight 
that there is some conflict. I found comparatively small, almost negligible variation of the 
wake with diameter of the propeller. Prof. Bragg found material differences with variation 
of diameter of his current meters. But his diameter variations were very large, while mine 
were not large, being less than 25 per cent. 
There was another matter touched on, and that is the question of using a current meter 
instead of the actual propeller of the ship. 
As regards the determination of the wake, the results should be the same if diameters are 
the same. If you take the model of the actual propeller of the ship and drive it idle in its 
proper place, developing no thrust, I do not see but what you must get the same results as 
Prof. Bragg with his current meter, which has practically no thrust. Then when you speed 
up the actual propeller and drive the ship the additional disturbance should not materially 
affect the wake. With the use of the current meter, Prof. Bragg can accomplish what would 
take a long time, using the propeller alone, but his method is of no advantage in getting in- 
formation as to the thrust deduction, which is an important factor, and which is liable to 
alter conclusions drawn from the wake alone. 
I would be glad to add information as to the power used, referred to by Prof. Bragg, 
but, as indicated in the paper, with the possible exception of propeller 571, the propellers were 
smaller than would be used in practice. This resulted in getting much less variation in pro- 
pulsive efficiency than if the propellers themselves had been varied. That is also the reason 
why Mr. Smith’s cases showed no gain. His low propellers were so small they were already 
working at excessive slip and when raised the slip had to go up to an impossible figure. 
On the question of trim by the stern, we all have heard of it and discussed it for a 
good many years. Once I took it up in the Naval Institute and published an analysis of a 
number of trials of naval vessels, and that seemed to indicate, so far as the data went, that 
nothing would be gained by “trim” by the stern, as regards speed. 
Our model basin results have shown for years that, as to trim by the stern or by the 
head, in practice for ordinary models there is no material effect on the effective horse-power, 
so that any effect of change of trim on the actual ship must, be due to its effect upon the 
efficiency of propulsion—not the resistance of the ship. 
As regards the method of determination of wake factor and thrust deduction, the 
propeller was tested in the open and then tested driving the ship. In driving the ship it 
showed certain revolutions and certain torques and thrusts. They correspond, in open water 
