77 
APPENDIX ry. * 
Airblast trom underwater explosions 
Not a great deal is known about the airblast caused by charges 
exploding below the surface in water. The few measurements which have 
been made have shown that if the charge is just above the surface of the 
water, the blast wave is the same as that from a similar charge resting 
on the ground. If the centre of the charge is a depth D expressed in 
units of the "charge radius", the blast wave in the air is believed to 
be approximately hemi-spherical in shape and intensity, centred at a point 
in the water surface above the charge, and to be roughly the same as that 
caused by a charge of weight SW exploded on the ground, where the 
reduction factor S is shown in the following graph as a function of D. 
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Hin DD, exrRessca vt Canny roti 
If part of the charge is above the water surface, and part below, 
then it seems necessary to integrate over the charge, allowing for the 
depth of each element, in order to find the equivalent charge. For 
example, in the case where a cylindrical charge, long compared with its 
diameter, is detonated with the length vertical, part under water and 
part above, the part below water is very appreciably muffled, as is olear 
from an inspection of the curve S above. 
An explosion which cases a strong airblast wave over the surface 
of water throws up spray. A layer of water is tripped off the surface 
and breaks up into droplets which move in the turbulent boundary layer. 
It can be shown that the height to which the spray reaches is approximately 
1/80 ft., where I is the positive impulse in the blast wave, measured in 
pounds weight milliseconda/in?. This formula will begin to fail for small 
charges (1 1b. or less), but should be roughly valid for larger charges. 
The failure is due to the finite time taken for instabilities to develop 
in the water surface (of the order 1 millisecond), a time which is 
independent of the scale of the explosion. 
