Poreh and Dtmant 
wealth which has been wasted in all the experiment tanks in the world 
by the intrusion of laminar flow affecting the accuracy of their results, 
I wonder whether the question of doping the tank water with something 
which will have entirely the opposite effect from that of the drag re- 
ducing polymers should be considered. In other words, is there a 
dope which could go into the tank water which would make the occu- 
rence of drag reduction or laminar flow quite impossible ? If that 
could be found, a lot of the quite arbitrary and, to many people, unac- 
ceptable devices currently in use to eliminate turbulent flow would be 
avoided. I offer this thought sincerely to the author of this paper, and 
would like to congratulate him on the way the paper was delivered and 
on its content. 
REPLY TO DISCUSSION 
Michael Poreh 
Technion Israel Institute of Technology 
Hatfa, Israel 
The concern of Pr. Telfer is well understood. The presence 
of drag reducing agents in the towing tanks is favorable from the mo- 
deling point of view. Dr. J. Hoyt has published an excellent review 
paper in the Journal of Basic Engineering (June 1972) in which he dis- 
cussed this problem. The uncertainty of the data which has been ac- 
cumulated in the past is due to the fact that the drag reduction was 
not controlled or recorded. Pr. Telfer suggests, if I have understood 
correctly, to eliminate the effect altogether. I believe that this can be 
done by adding chemicals which will inhibit the growth of algae and 
drag reducing bacteria as well as creating unfavorable conditions for 
the stretching of the molecules. However, the more difficult task of 
maintaining a controled standard level of drag reduction in towing 
tanks seems more attractive as it enables one to improve the simi- 
larity in modeling ship motion. 
The interest of Pr. Voitkounsky in the effect of roughness is 
natural.I have published a paper in the Journal of Hydronautics in 
which I proposed an approximate model for describing this effect. The 
work in the Soviet Union in this area is not known to me and I'll appre- 
ciate if Pr. Voitkounsky will help me in receiving the papers he has 
mentioned. 
* (Jour. of Hydronautics, 4,4,Oct. 1970) 
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