November 30, 191 1] 



NATURE 



145 



THE INTERACTION BETWEEN PASSING 

 SHIPS. 



ONE of the prominent questions of the day in naval 

 architecture circles is that of the influence of 

 passing ships upon each other. It has been known 

 for many years that such an influence exists between 

 vessels in confined waters, canals, for example, and 

 for such canals there are usually stringent regulations 

 as to speed and manner of passing of ships. Such 

 conditions are, of course, extreme, but the narrowness 

 and shallowness of the canal, merely intensify a 

 phenomenon which is present in deeper or broader 

 waters, but not always apparent. 



tendency for it to cant in towards two; in the second 

 position the forces are all tending to draw the two 

 ovals together, there being 'throughout the body of 

 water between them less pressure than exists on 

 their outer sides. In the third case, oval one is sub- 

 jected to forces tending to cant it towards or away 

 from two, as the stern is in a field of pressure below 

 normal on its nearer side, and the bow in a field of 

 increased pressure on the inner side. 



The extent and importance of these forces will de- 

 pend on the lateral distance apart, on the bounding 

 conditions of the fluid, and on the speeds, both abso- 

 lute and relative, of the ovals. In order to give an 

 indication of how greatly this influence is increased 



Fig, I. — Variation of pressure around an oval slong the line X X. 



A ship's form unfortunately cannot be expressed by 

 any formula, and its stream lines cannot be easily 

 calculated, but by considering the case of two ovals 

 formed by " sources and sinks," we can get a rational 

 idea of the character and variation of the forces which 

 come into play. The equation for the pressure at any 

 point in the fluid surrounding an oval is given by : — 



where r^, ^, and r„, <i>2 are distance and angle from 



source and sink respectively, and / varies with the 



distance between source and sink 



and length of major axis, and v^ 



is the relative velocity of fluid and 



oval. 



The line b & in Fig. i shows the 

 variations of the pressure from the 

 normal, along a line distant one- 

 third the length from the centre 

 line, and it can be seen from this 

 that, speaking generally, there exist 

 at each end of such an oval fields of increased pressure, 

 and that the space between them is a region of 

 diminished pressure. 



If two such ovals are moving in the fluid, it can 

 be readily seen that the variations of pressure due to 

 one will modify the pressures due to the other, and 

 that the pressure conditions on the sides of the ovals 

 nearer to each other are different from those on the 

 outer sides. 



In Fig. 2 the ovals are shown in three positions 

 relative to each other, and the arrows at each end of 

 oval one show the motion which the forces due to 

 interference tend to set up. If one is overtaking two, 

 in the first position shown, there will be a strong 



NO. 2196, VOL. 88] 



when a body moves from deep to shallow water, the 

 curve a a has been drawn in Fig. i. This shows the 

 variation of pressure for a plane oval {i.e. with two- 

 dimensional flow), having the same axes as the oval 

 for which curve h h has been drawn, and it will be 

 noticed that the scale of a a is one-tenth that of h b. 



Such general reasoning, however, although showing 

 the nature of " interference," does not give a measure 

 of the forces involved with actual ship forms under 

 similar conditions, and to obtain this experiments must 

 be made with models in water of different depths. 



Experiments conducted in 1898 in a German canal 



cr^^^-:^ 



Fig. 2. — Action between passing ovals. 



with barges having a sectional area approximately one- 

 third the sectional area of the canal, showed a region 

 of considerable excess pressure in front and at the 

 rear of the barge, and on the bank for the whole 

 length of the barge a remarkably strong negative cur- 

 rent or diminution of pressure. 



The experiments made by Naval Constructor D. W. 

 Taylor in the Washington Tank, throw a considerable 

 light on the problem. The ship-shaped models used 

 were of the same length and were run at an 

 average speed corresponding to r3"5 knots for 500-feet 

 ships. 



The results of the experiments show that in very 

 deep \\.i(i r ^u^l^ ^liip"; do not begin to influence each 



