Free Surface Effects tn Hull Propeller Interactton 
DISCUSSION 
Edmund V. Telfer 
ets Nets 
Ewell, Surrey, U.K. 
Anybody who has attempted to read this paper, as I have 
during the past two or three days and this morning at 2 o'clock after 
coming back from our delightful banquet, will appreciate that it con- 
tains a lot of matter for real thought. The first point I would like to 
deal with is the authors' attack on the subject of relative rotative ef- 
ficiency and their quite innocent reference to relative rotative effi- 
ciency being an empirical ''catch-all' for various unclarified effects 
of relatively insignificant magnitude. I am not sure whether that is 
fair. Undoubtedly during the thirties, a lot of attention was given to 
the subject. In 1951 I published a North East Coast Institution paper 
on various aspects of the propeller/hull interaction problem and I 
made a suggestion that the real meaning of relative rotative efficiency 
could be very simply understood by plotting Kp toa Kop base so 
getting the well-known propeller polar. Suppose we have such a plot 
of all propeller polars over a range of pitch ratio, we will then have 
the higher pitch ratios on the right and the lower ones to the left. 
What then happens in self-propeller tests is that the measured Kp 
value does not locate on the correct pitch polar with the correspond- 
ing Ko value. In most cases it will be found that the Kp lies to the 
low pitch ratio side of the actual open polar and it is this that produ- 
ces the phenomenon of relative rotative efficiency. Values of relative 
rotative efficiency exceeding unity are then obtained, which to the lo- 
gical mind appears to be impossible, but if it is realised that what 
has really happened is that the actual correlation of the behind thrust 
and torque has left the open line and come to a smaller Kp value or 
in other words the centroid of the thrust of the behind propeller has 
been moved radially inboard, as a consequence of the normal wake 
distribution in a single-screw ship, having the heavier wake towards 
the shaft centre. Thus when we see this taking place we realise that 
all that positive relative rotative efficiency is showing is that there 
has been a change in the wake distribution compared with the uniform 
distribution of the open condition, Therefore, really, relative rotative 
efficiency is merely a wake distribution factor and if one thinks of it 
in that way one can get a much clearer understanding of the problem. 
1945 
