Until proper depth-charge fuses capable of exceeding 90m depth became available, the practice was to "soap" the fuses, ie to fill the external aperture on the fuse with a plug of soap. This delayed the operation of the fuse until it had exceeded the 90m depth, although it was not possible to "set" the fuse to a specific depth. All one knew was that it was probably/hopefully going off at 'more than 90m'.
This probably did not result in many, or indeed any, U-boat kills at 90m plus**, however, the relatively closer detonations would have done damage and kept the U-boat much deeper than otherwise might have been the case.
**it is hard to prove a negative or a positive in this case, unless the U-boat was forced to surface, survivors escaped and told their new captors at what depth they were hit. An exceedingly unlikely proposition!
Once fuses capable of 90m depth were developed, then one could draw a reasonable inference that if Asdic recorded the u-boat at 150m the fuses were so set, and the interval between release and detonation was correct, that it was a very near miss or hit.
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