84 A THEORY OF FLUID FRICTION. 
Proressor H. C. SapLer, Member of Council:—I think this is a very valuable 
paper, especially in connection with the results of the propeller experiments. We have all 
had experience with somewhat similar designs of propellers giving very different results, 
and if we examine the curves given here for a rough surface versus a smooth surface, prob- 
ably that will explain some of the difficulties that some of us have had in showing why, 
two propellers of apparently similar design have given very different results. If there is 
a difference of from 36 to 72 per cent in the efficiency by roughening the surfaces of the 
propeller, I think that will explain in a good many cases the differences that are obtained 
in practice with similar propellers, even though the limits are not as high in the actual 
propeller as in the model. 
In Fig. 7, Plate 60, the effect of even using a smooth surface as against a moderately 
smooth surface is also brought out in a very good way. There is quite a considerable differ- 
ence in efficiency, thrust and torque in the smooth surface propeller as against the mod- 
erately rough surface propeller. 
With regard to the question of wind resistances, the installation of the wind channel 
at Washington, I am sure, will prove a very valuable adjunct to the model tank. In ves- 
sels particularly of this collier type, it is very difficult to estimate what is the effect of wind 
resistance. There are so many uprights, braces, stays and riggings that we cannot tell, 
without actual experiment, what the horse-power would be in order to overcome the wind 
resistance. I can only hope that in the future further experiments will be made on this 
subject of wind resistance of models. 
