57 



455 



2.4 



2.0 



^ 1.2 



0.8 



0.4 



I 2 



16 



20 



24 



28 



32 



d , Charge Distonce in inches 



Figure 32 - The Central Deflection after the Diaphragm Has 

 Returned Elastlcally from the Maximum Deflection Zc^ 



separation into two groups is still in evidence in this graph. The effect of 

 the water-hammer blow is also evident if the graph is compared with that of 

 Zci, Figure 28. The deflection has increased to about 2.25 Inches at a charge 

 distance of 7 inches, an increase of about 1 inch relative to the U-branch of 

 Figure 28. At 32 inches the deflections are 0.29 inch in Figure 31 and 0.26 

 inch in Figure 28. The elastic return somewhat decreases these central de- 

 flections, as shown by the curves of Figure 32 in which the central deflec- 

 tion Zc , observed about 2 or 3 milliseconds after the initiation of the mo- 

 tion, is plotted against charge distance. 



The displacement of the diaphragm thus far has been described rela- 

 tive to the holding ring which is bolted to the mounting ring in the bottom 

 of the boat. The streak pictures show that the motion of the holding ring 

 is as follows: On the impact of the shock wave it is given an initial veloc- 

 ity upward, as discussed previously, on which there is superimposed a damped 

 vibratory motion of large amplitude having a frequency of about 6o cycles per 

 second. This frequency agrees roughly with that calculated for the free vi- 

 bration of a rectangular plate of the same dimensions as the bottom of the 

 boat. The holding ring and part, at least, of the bottom of the boat to 

 which it is attached Jump to a height of about 2.8 Inches when the charge is 

 fired 20 inches from the diaphragm. The holding ring is then accelerated 



