Free Surface Effects tn Hull Propeller Interactton 
REPLY TO DISCUSSION 
Som D,. Sharma 
Hamburgische Sehtffbau Versuchsanstalt 
Hamburg, Federal Republte Germany 
To Professor Telfer 
We agree with Professor Telfer that our treatment of rela- 
tive rotative efficiency has been probably less than adequate. While 
we still regard it as an empirical catch-all for various unclarified ef- 
fects, we must admit in the light of our own results that these effects 
are not necessarily of insignificant magnitude ! We are well aware of 
the historical reason why R.E. Froude (1898) first introduced the 
concept of relative rotative efficiency and of the fundamental research 
in the thirties, notably by Horn (1932), which led to the modern inter- 
pretation of relative rotative efficiency as representing the effects of 
dissimilarity in flow conditions on the propeller behind the hull and in 
open water, while still retaining the original, somewhat misleading 
name. The multifarious nature of this dissimilarity can be illustrated 
by enumerating some of its aspects : 
1. Spatial nonuniformity of the incident flow, i.e. the variation of 
* with the three space coordinates (x,R,@), 
and the very presence of any non-zero radial and circumferential 
wake components w**, w ~ as well as their local variation with the 
coordinates (x,R,@). 
axial wake component w 
2. Temporal nonuniformity of the incident flow at any fixed point in 
space, e.g. due to unsteady boundary layer separation from the hull; 
including ambient turbulence in the hull wake. 
3. Guide vane effect of hull and rudder. 
4. Hub effect due to the opposite directions in which the propeller 
shaft generally extends in the behind-hull and open-water conditions. 
5. Possible differences in the Reynolds (and Froude) numbers bet- 
ween the behind-hull and the open-water tests. 
In the currently accepted method of propulsion factor analy- 
sis the relative rotative efficiency is indirectly determined as a deriv- 
ed quantity thereby lumping together all these effects into a single 
fudge factor. Despite some remarkable attempts in the thirties, such 
as by Lammeren (1938), to identify individual phenomena, it can 
1953 
