J^2% SURFACE AND OTHER EFFECTS 



and to a large extent are drawn from, analyses of idealized cases such as 

 the ones treated in the preceding sections. It is to be remembered that 

 any simple conclusions must lose in precision with their extension to 

 other than the simple cases for which they strictly apply. Even so, 

 these qualitative conclusions can at least serve to suggest the relative 

 importance of the various phenomena in the water for specific cases of 

 explosion damage. 



A. Relative importance of shock wave and later pressures. A first 

 question is of course whether the shock wave, bubble pulse, or pressures 

 at other times are of major importance. The more important high ex- 

 plosives are remarkably similar in that roughly half of the available 

 chemical energy is radiated in the primary shock wave. Although a 

 third or more of this energy may be lost as heat within distances of sig- 

 nificance for damage, the fact remains that the shock wave constitutes 

 the largest single source of available energy throughout the surround- 

 ing water. Under many conditions, the shock wave is of decisive im- 

 portance and in no case should its effect be dismissed as negligible with- 

 out good reason. 



As compared with the shock wave, secondary pressure pulses from 

 gas sphere contractions are of much greater duration, but the total 

 energy radiated is much smaller, being certainly of the order of one- 

 third or less of the radiated shock wave energy. At the same distances 

 from their effective source, the maximum pressures in the secondary 

 pulses are a small fraction of that in the shock wave. The bubble pulse 

 may well be responsible for supplementary damage, however, particu- 

 larly if migration of the bubble places it close to the target when the 

 pulse is emitted. Examples of greatly increased damage as a result of 

 this migration are well established by both direct and indirect evidence. 

 Because of the long duration of these pulses, if they are not seriously 

 attenuated by possible loss of energy to the free surface of the water, 

 their effect in many cases is more nearly that of static pressure and can 

 be gauged by the peak pressure. The magnitude of this pressure and 

 the possible importance of the bubble pulse are strongly affected by the 

 position and state of motion of the gas sphere, and both the explosive 

 and its position relative to the target must be considered. It is reason- 

 able to conclude, however, that the bubble pulse is of major or decisive 

 importance only if the geometry of the charge, target, and nearby sur- 

 faces is particularly favorable. 



A frequently recurring question in underwater explosion damage is 

 that of the importance of the essentially noncompressive outward flow 

 of water following emission of the shock wave. Most of the remaining 

 energy appears as kinetic energy of outward flowing water at a later 

 stage of expansion of the explosion products. This fact has presumably 

 been the reason leading a number of writers to ascribe major significance 



