~ 2S + = 
374 F. H. Todd 
when comparing the performance of surface and submarine vessels one has to be careful 
about the basis of comparison. Although for a given deadweight of say 25,000 tons the 
break-even point as regards power for certain designs may be as high as 25 or 30 knots, one 
has to bear in mind that a surface tanker of 25 knots would not be really economical com- 
pared with a commercial tanker which would have a speed of about 15 or 16 knots. As I 
said before, I am speaking here of economy on the basis of cost per ton-mile of cargo 
carried under present conditions. This was one of the findings arising from freighting cal- 
culations carried out by the British Shipbuilding Research Association in conjunction with 
ship-owners in connection with extensive nuclear energy studies, reference to which the 
author has made in the paper. This point is really covered by the second paragraph of the 
author’s conclusions with which I certainly agree. Incidentally it might be of interest to 
mention that I understand that freighting calculations have also shown that, if anything, the 
speed of some recently built conventional tankers is perhaps a little on the high side as far 
as economic running is concerned. From the military point of view, of course, the situation 
could be quite different as the author has clearly explained. 
What I am really saying here is that in my remarks on Dr. van Manen’s paper on Monday 
I anticipated these particular aspects of the author’s conclusions, although I was not aware 
of this at the time. 
I should also like to say that I agree with the author’s conclusions generally, in addi- 
tion to those to which I have referred already. There is just one point of detail I should 
like to raise, namely, the limit of power for a single screw surface ship which the author 
has taken as 40,000 SHP. Some years ago we looked into this at BSRA in connection with 
the nuclear studies for tankers referred to earlier and we were advised that for a number of 
reasons and in particular the possible incidence of excessive cavitation erosion, 20,000 to 
25,000 SHP was about the practical limit at the time. Perhaps the author may care to com- 
ment on this point. . 
F. H. Todd 
The British Shipbuilding Research Association has carried out a great deal of research 
into the subject of nuclear propulsion, and it is therefore gratifying to know that Mr. 
Lackenby is in general agreement with my conclusions as to the commercial merits of its 
application to submarine cargo ships and tankers. With reference to the use of an upper 
limit of 40,000 horsepower for a single-screw ship, such power absorption is, of course, 
fairly common on multi-screw ships. In single-screw surface ships the wake variation con- 
ditions are considerably more onerous, and as Mr. Lackenby has said this introduces the 
danger of cavitation erosion. In the submarine this is not nearly such a serious problem 
since the circumferential variation can be kept quite small by paying good attention to the 
shape and location of appendages. Moreover, we are here talking of the future, and with 
the advance in knowledge about propeller-excited vibration and the necessary stern shapes 
and clearances to reduce the forces to a minimum, we may look forward to a steady increase 
in the maximum power we can put through a single screw. 
J. M. Ferguson (John Brown and Co., Limited, Glasgow) 
I do not wish to discuss at all the technical side of Dr. Todd’s paper but I wish to 
express a thought which has caused a great amount of discussion between my colleagues 
and myself, mainly during lunch periods. We have been discussing the possibilities of 
