465 



THE PRESSURES PRODUCED BY EXPLOSIONS UNDERWATER OF 

 SMALL CHARGES NEAR A RIGID BOTTOM 



G. Charlesworth 

 December 1943 



Summary . 



Tessure-t ime curves have been obtainefl> By means of piezo-clect ric gauges, at points 2. 3, « 

 and 5 'eet above 1 oz. charges of P.A.G. fired underwater at distances from to 3 feet from a rigid 

 tjottom. The ''esults have been compared with those observed previously with a gravel bottom. 



The shock wave tail was not reduced so much for the charge near the rigid bottom as when it 

 was fired near the gravel bottom. as befor?, it was found that lower intensity pulses of unknown 

 origin occurred some u to 7 milliseconds after both the shock waves and bubble waves, their occurrence, 

 but not their form, being independent of the presence of the bottom. 



The forms of the bubble waves with a rigid bottom were similar to those obtained with a gravel 

 bottom, but with the charge near the rigid bottom were more intense and of shorter duration. At a 

 given distance from the charge the pressures and impulses in both the 1st and 2nd bubble waves, when 

 plotted against the distance of the ctarge from the rigid bottom, had maxima for some value of this 

 distance between and 2 feet. with the gravel bottom this effect was only observed for the 1st 

 bubble wave impulses. whereas for the gravel bottom no second bubble-wave was detected when the 

 charge was 1 foot 6 inches or less from the botti^m the correspondino dist-incc from the rigid bottom 

 was 6 inches. 



The 1st bubble period incre?,sed as the charge approached the rigid bottom; with the gravel 

 Bottom the period decreased. 



Introductxon . 



The object of the tests was to termine the underwater pressures produced by exploding charges 

 near to a rigid bottom and to compare the results with those previously obtained near a gravel bottom. 



Expertmsntal . 



Site :- The tests were made in a concrete tank in a depth of 9 feet 6 inches of water. 

 The bottom of the tank was flat and the 'rigid bottom" used in the tests consisted of a steel plate 

 3 feet square and iJ inches thick placed flat on the bottom of the tank. 



Charg es:- AS before, 1 02. charges of P.A.G. were used, detonated by No. 8 A.S.A. detonators. 

 The charges were cylindrical in shape with height equal to diameter. They were placed with their 

 axes vertical ana detonated from the top. 



pressure measurement ;- Pressures were measured by means of piezo-electric gauges recording 

 photographicnl ly ^..ing cathode-ray oscillographs. 



Arrangement of tests ;- The gauges were suspended vertically above thr chsrge --.t distances 

 of 2, 3. « and 5 feet from it. Tests were made with the charge at distances from to 3 feet 

 from the bjttjm. 



