1568 
16.3 When an explosion takes place near the sea surface we will 
assume that: 
(a) Cavitation begins when the pressure in the water drops 
to a fixed value, Pe , the cavitation pressure. We will take R =0 
p. s-i- This assumption will be examined later. 
(b) The pressure at a point on the front of the reflected 
wave can be described as emanating from an image charge above the 
surface, having a variable apparent weight, Wi, so adjusted as to 
account for the pressure diminution due to cavitation. 
(ec) Wi is a continuous non-inereasing function of distance 
along a ray away from the image charge, being equal to the true 
charge weight W, before any cavitation takes place, decreasing 
monotonically in regions of cavitation, and remaining constant in 
regions of no cavitation. 
The assumptions (b) and (c) imply that cavitation propagates 
as long as the spherically expanding negative reflected wave is 
decaying in amplitude less rapidly than is the positive pressure 
region into which it is propagating in accordance with the criterion 
given by Kennard.°3 The assumptions, however, take no account of 
the effects of pressure gradients along the wave front. 
Referring to Figure 18c, where the origin of coordinates is 
taken at the image charge, the total pressure at a point (R),9) 
on the front of the reflected wave is the sum of the hydro-static 
head at this point, Py (B18), the pressure at distance Rp from 4 
charge of weight W at a reduced time @ (associated with propaga= 
tion through the distance x ) behind its front, pin? ,t), and the 
pressure at the front of a negative wave at a distance Rj froma 
3 
charge of apparent weight Wi, - pitt » 0). If cavitation is 
¢ 
