Size, Type, and Speed of Ships in the Future 4] 
E. V, Lewis (Davidson Laboratory, Stevens Institute of Technology) 
I wish first to compliment Mr. Oakley and the Bureau of Ships on their far-sighted view- 
point in considering such a wide range of possible vehicles for military use. The escort 
research ship in particular I am certain will be a significant milestone in naval ship design. 
In its own way the modern supertanker considered by Dr. van Manen has been a truly 
spectacular development. In this connection it is rather surprising to find in Fig. 2(c) that 
the author expects to maintain a constant overall propulsive coefficient of about 71 percent 
in ships up to 150,000 tons deadweight. Presumably he is assuming single screw vessels, 
and I would expect a reduction in efficiency as size continues to increase. My reason for 
expecting this trend is that for geometrical similarity the propeller diameter should go up 
and rpm down in order to maintain good propulsive efficiency. However, in actual ships 
practical considerations seem to have limited propeller diameter and prevented reduction in 
rpm. Dr. van Manen has apparently counted on the application of improved sterns and 
nozzles to maintain the efficiency of the heavily loaded propellers of larger and larger 
ships. Perhaps he will care to comment on this point. 
J. D. van Manen 
In reply to Prof. Lewis I will mention a total efficiency of about 71 percent in Fig. 
2(c) is only based on statistical data. 
H. Lackenby (The 3ritish Shipbuilding Research Association) 
I should like to raise one or two points on Dr. van Manen’s paper concerning the speeds 
and powers of displacement ships. 
Firstly, it is stated in his Introduction that up to 1940 the maximum shaft horsepower 
which could be installed per shaft in a merchant ship was about 10,000. I am sure the 
author is referring here to single screw ships and I think perhaps that this ought to be 
stated. For multiple screw ships much higher powers per shaft were, of course, installed. 
In this connection it is also stated that 16,000 shp is now normal and that 20,000 is 
frequently encountered. This is followed by the statement that at the same time tanker 
speeds have increased from 10 to 17 knots. This is quite true of course, but I think it 
might be mentioned that this speed increase was largely a natural consequence of increasing 
size and that in many instances it was a question of keeping the speed-length ratio about 
the same and increasing the speed according to the square root of the length. In other 
words there was a significant speed advantage in having a longer ship. 
The author’s references to the larger block coefficients now being associated with the 
high deadweight supertanker are of particular interest, and it is interesting to recall how 
views on this have changed, bearing in mind that between the wars one authority advised 
that no seagoing ship should have a block coefficient greater than 0.75. According to Fig. 
2(b) we find that the largest block coefficient for the Wageningen models plotted there is 
about 0.825. It might be of interest to mention here that the British Shipbuilding Research 
Association in conjunction with Ship Division, NPL, are now endeavoring to develop ocean- 
going forms with block coefficients up to 0.85. 
