29 
79.6 The spray dome is not of direct interest in connection with damage 
since it represent energy wasted from the attacker's point of view. 
However, the theory of its formation, using equations 27 and 28 or more 
complicated analogous formulae allowing for a finite tensile strength of 
water, can be of some use for subsidiary purposes in conjunction with 
experimental measurements from cine films of spray velocity, size and 
shape of the dome. In particular, the depth of explosion of charges 
dropped from aircraft can be estimated whilst on the more fundamental 
side such measurements can be used to obtain empirical data for the 
maximum pressure Py in the pulse. 
80. Occasionally, it is possible to distinguish a later secondary spray 
dome due to the first bubble pulse. The effect is generally indistinct 
because for deeper charges the bubble pulse and the resulting secondary 
dome are small, whilst for shallower charges this dome tends to be 
obscured by the plumes breaking through very soon afterwards. For 
charges near enough to the surface for the bubble to break surface on its 
first expansion, for example, 20 ft. or less for an amatol depth charge, 
there is of course no bubble pulse and no secondary dome. 
The plume 
81. The second phase of the visible surface effects is the eruption, 
through the dome of the water displaced bodily by the bubble motion. 
The resulting plume is somewhat variable in character and time of appearance 
but it has been possible in some instances to relate, at least approximately, 
the plume phenomena with the bubble motion. For a 500 1b. T.N.T. charge 
exploded at a depth of 50 feet, the first sign of plume appears 14 sec. 
after the first appearance of the spray dome. For shallower depths of 
explosion the interval becomes shorter until finally the plume and dame 
become indistinguishable. On the other hand, as the charge depth inoreases, 
the interval becomes longer and the effects decrease. At a depth for 
which the spray dome no longer occurs, all that appears of the plume is a 
churning of the surface some seconds after the explosion as the residual 
gases stream up, the gas bubble having degenerated into small bubbles with 
little internal pressure. 
82. A remarkable plume results from the detonation of a charge close to 
the surface when the gas bubble vents through the surface with a high 
efflux velocity. The plume structure from a n type of explosive of 
weight W lbs. detonated at a depth D feet, where W3/D is about 4 lbs.3 
feet1, i.e. a charge with its centre at about one charge diameter below 
the surface, is shown in the diagram. This illustrates the typical 
behaviour of such shallow underwater explosions. 
When the gas bubble vents, the gas rapidly overexpands, entraining 
water from the region of venting into the brush-like cloud zone which 
consists of finely divided water and gas, and so tends to disperse rapidly. 
Meanwhile the detonation impulse imparted below the surface generates an 
expanding cavity which is almost hemispherical. The water close to the 
free surface is, however, unconstrained and peels upwards to fom the stem 
of the plume, the base of the stem keeping in step with the expanding 
cavity. The hollow nature of the stem sheath has been confirmed by pulse 
X-ray photographs. The gas pressure within the stem rapidly falls to a 
sub-atmospheric value as the stem expands and the excess pressure of the 
outside atmosphere makes the fluid in the stem reconverge into a jet which 
moves upwards with a high velocity. A tall slender column ensues which 
reaches a relatively great height and is characteristic of such shallow 
explosions. 
