Nowaekt and Sharma 
R.E. Froude, when he first introduced the idea of rotative 
efficiency was then principally concerned with twin screw naval ves- 
sels, and there the erratic variation of rotative efficiency with oppo- 
site screws certainly puzzled him. Part of the trouble was due to the 
fact that the starboard and port propellers,being of opposite hands, 
were never made with identical accuracy ; and one frequently found 
different rotative efficiencies for the two screws, so offering some 
justification for the authors'own statement that rotative efficiency was 
earlier known as a ''catch-all'', Model manufacturing inaccuracy 
should no longer be allowed to occur and so cloud a fundamental issue. 
The effect of a positive rotative thus indicates a change in 
the wake distribution and therefore a bigger wake towards the root. 
If one makes the thrust wake integration as the authors have done ra- 
dially, it will be found to yield a correct rotative efficiency (in the 
real meaning of the word) which is actually less than unity. Thus if 
one does not presume the propeller to do the integration but if, as the 
authors did, one takes each radius separately, from such integration 
a lower rotative efficiency and a higher wake will be obtained. It is 
rather significant however that in the authors' tests the thrust wake 
is less than the torque wake. This is most unusual. Most single screw 
ships show the reverse, certainly with ordinary testing methods. 
When this is more deeply considered, as the authors have done, it 
becomes clear that the wake has actually increased, and one has then 
to recredit what was previously known as rotative efficiency to an in- 
crease in the wake due to the difference in the wake distribution. 
I am extremely glad that the authors have brought out this 
particular point and I hope my remarks are understandable in the 
light of their own work. One is tempted to believe that the original 
description of the words ''rotative efficiency'' was a justifiable one, 
but we now see that there is no justification for thinking that the pro- 
peller normally can work with a higher efficiency behind the hull than 
in the open, Yet certainly in the thirties, Teddington was reporting 
rotative efficiencies of about 1.2 and even higher, and Dr. Baker him- 
self expressed the view that these results were evidently ''phoney" and 
there was some other explanation to be found for the meaning of rota- 
tive efficiency. I suggest that the wake distribution effect is the real 
interpretation. 
The authors state that the wake is caused by the presence of 
the hull and the free surface. This is not quite correct. It is not caus- 
ed by the presence of the free surface, but provided there are waves 
on the free surface then admittedly they may have some effect on the 
1946 
