395 



Charge Weight W in pounds 



Figure 6 - Curve Giving a Rough Estimate of 



the Minimum Depth D„ below the Surface 



at which a Charge W may be Detonated 



without Blowing through the Surface 



before Undergoing Recompression 



expected to execute a complete 

 oscillation and emit a second- 

 ary pressure pulse during a 

 first phase of recompression. 

 If the charge is too near the 

 surface, at least part of the 

 gas will blow through the sur- 

 face and no recompression of 

 the full globe can occur, as 

 Illustrated in Figure 3 on page 

 5. The proper criterion is un- 

 certain. It may be assumed 

 tentatively as plausible, and 

 as supported somewhat by exper- 

 iment, that recompression will occur only when the depth exceeds both the 

 maximum radius as calculated for a spherical gas globe and the calculated 

 gravitational rise to the first peak compression.* The minimum depth D„, de- 

 termined in this way Is plotted on a basis of charge weight W in Figure 6. 

 If the depth of detonation exceeds Z)„, recompression of the gas globe should 

 occur, although the emitted pulse of pressure may not be very effective if, 

 because of the gravitational rise, the recompression occurs very near to the 

 surface of the water. 



GENERAL EFFECT OF SURFACES 



A nearby surface limiting the body of water attracts or repels the 

 gas globe. In a rough way this effect is superposed upon that of gravity, as 

 is evident from the linear combination of the two terms in B,^, By, B^, as in 

 Equations [7a, b, c], [17]i [20], [21]. Some interaction of the two effects 

 arises, however, from the fact that the integral [/In Equation [6a, b, c], or 

 the quantity B defined by Equation [l6], depends upon both effects. 



Comparisons for charges of different weights are most easily made 

 at distances from the limiting surface in proportion to the maximum radius iZg- 

 Then at corresponding distances the factors N^, Ny, N^ in Equations [Ja, b, 

 c], [T7]i [20], [21] actually vary as 1//22. so that th'fe corresponding surface 

 terms In B^, By, B^ do not vary at all, whereas under fixed hydrostatic pres- 

 sure the gravity terms vary as iJj o^ ^s W^, where W is the weight of the 

 charge. For this reason it turns out that, at distances of the order of 2R2 



For small charges, where the rise is decreased or even made negative by 'an effect of the free surface, 

 O,,, is determined by the maximum radius. 



