43 
explosion. In such trials with large charges, for example, 300 lb. T.NT., 
the pulse length is long canpared with the size of the gauge and the 
preceding theory for an infinite baffle is not applicable. A very 
similar approximate theory can, however, be used Thus if the pulse is 
very long compared with the gauge dimensions it can be argued that if the 
diaphragms be held fixed, diffraction round the sphere will rapidly 
equalise the pressure all over the gauge and this pressure will be 
approximately the same as that in the incident pulse. This carresponds 
simply to dropping the factor 2 in equation 42 and the remaining argument 
can then proceed as for ths infinite baffle. In particular, equation 49 
will then be modified by the insertion,of a factor 0.25 on the right-hand 
sids, that is the transferred energy is only 25% of the similar energy 
for the case of an infinite baffle. Even so, with a sufficiently long 
pulse giving a large value of c/na it iss#ill possible for the transferred 
energy to be many times the directly incident energy, although 
reflection from a baffle plays no part in the theory. In this case, the 
yielding of the diaphragm.reduces the pressure in its neighbourhood below 
the incident pressure, and energy is diffracted on to the diaphragm from 
the surrounding water which is effectively much more unyielding than the 
diaphragm It must therefore be realised that whilst the presence of a 
baffle can increase considerably the energy transferred to a plate, it is 
by no means essential to have a baffle in order that the transferred 
energy can be greater than the directly incident energy. 
112, Summing up, the main importance of the present theory involving 
diffraction effects is that it indicates a mechanism by which the energy 
transferred from the pressure pulse to a target can be greater than that 
estimated on the previous infinite plate theory. The present theory is most 
relevant to special types of single plate target used in small-scale 
experiments because for structural reasons they usually involve a baffle. 
For actual ships, it would appear that, whilst the infinite plate theory is 
in general more relevant, some increase of damage due to diffraction effects 
is possible. For example, the hull plating may be stiffened locally by 
seatings for machinery and sane of the energy incident on these stiffer areas 
will tend to be diffracted on to unstiffened panels in the vicinity. A 
similar effect can take place in the vicinity of panels forming one wall of 
a water or fuel oil storage compartment In general, such effects are 
unlikely to be large and if some allowance is to be made for them, the 
simplest correction is to replace the previous estimate (case 3, para. 99) 
that about two-thirds of the directly-incident energy is Me oie by 
the criterion that all this energy is transferred, that is t. 
Insofar as any simple criterion can be used for t sO oupee problem of 
damage by underwater explosions, this hypothesis =; is most in 
accord with experimental results. 
1135. Finally, for both the infinite plate and the finite plate theories, it 
may be noted that by expressing results in terms of the ene transferred 
to the target, the stiffness constant k only enters insofar as it has been 
assumed effectively small, Generally, provided the resisting force remains 
small in comparison with inertia forces whilst most of this energy is 
transferred, the exact nature of this resistance, for example, whether it 
is constant or varies linearly with displacement, is not important. To 
this extent, the mechanism by which the damaging effect of the explosion 
is transferred from the water to the target is independent of the precise 
nature of the deformation of the target This greatly simplifies the 
general problem and enables the transfer process to be discussed broadly 
without the added complication of a simultaneous consideration of the naturv 
of the deformation of the target. 
Bodily motion of targets due to pressure pulse 
114. Besides damage to the hull, a ship or target will tend to move as 
a whole under the action of the pressure pulse from an underwater explosivun 
Such bodily motion would be expected to decrease, if anything, the damage 
