632 



Figure 1 made up from data taken at Woods Hole shows the 

 time of appearance of the plumes as a function of the charge 

 depth, taking the zero of time as the first appearance of the 

 spray dome which is the same as the detonation time, save for 

 the negligible travel time of the shock to the surface. The 

 charges used were the equivalent of 400 lbs. of TNT. The blacked- 

 in points represent the more pronounced velocity change in the 

 case of two charges. The period, T^, of the first oscillation I 

 is calculated taking account of the influence of the surface, ' 

 and the period of the second oscillation, ^2) is taken, for 

 these depths, as equal to that of the first. In this depth 

 range the decrease in period due to loss of energy is roughly 

 compensated by the increase in period due to the rise of the 

 gas globe. At the end of the first oscillation, at time t = T^, 

 the gas globe is small, compressed, and moving rapidly upward. 

 It is probable therefore, that for charges originally at depths 

 of 25 to 30 feet the columnar upheaval through the spray dome 

 at about one second is caused by the gas globe reaching the 

 water surface in a state of high pressure and high velocity. 

 The known facts concerning upward migration are wholly consist- 

 ent with this view. The spray dome at this time may be 30 or 

 4o feet high and the columnar plume will require at least a 

 tenth of a second to emerge from it traveling at a typical 

 velocity of 250 feet per second. The initial velocity for the 

 spray dome due to the shock wave reflection will be about 120 

 feet per second for a 400 lb. charge at 25 feet. This indicates 

 that the velocity of the plume is a mass motion of water driven 

 by the gas globe and is not analogous to a spray dome of tiny 

 droplets. Hence, the potentiality of this plume for damaging 

 a ship is probably high . 



For charges at greater depths the plume emerges at later 

 times with respect to the period of oscillation. There are two 

 reasons for this: First the gas globe has further to travel 

 and second, its velocity is less in the less compressed phases. 

 The blacked-ln points In Figure 1 for the depth region from 35 

 to 45 feet show that the gas globe reaches the surface in the 

 early stages of expansion after the first collapse. The addi- 

 tional points to the right along the T^ + T2 cvirve correspond 

 to the expansion after the second collapse of the gas globe. 

 This second collapse is necessary if the globe Is at all large 

 when it nears the surface. According to this vievj, there will 

 be no surface events at the time the gas globe nears the surface 

 if this happens at its maximum size. In this case there will 

 be a delay of about one half period until the collapse and con- 

 sequent rapid upward velocity occur which will result again In 

 a columnar display as indicated by the heavy points on the 

 Ti + T2 curve. For charges of this size at depths less than 25 

 feet the dome and plume phenomena merge together. Hov;ever, 

 radial plumes associated with radial expansion of the gas globe 

 are observed at about 20 feet whereas for very shallow explosions, 

 in say 5 to 10 feet of water again a columnar formation is 

 observed . 



