I fully take your point about "you-tube facts", although, like you, I've found this chap to be fairly reliable where I've independent material to compare. I think we can forgive duplicated use of footage!
Regarding depths of detonations, it became a widespread practice in the RN to "soap" DC charges when the u-boat'd depth was thought to deeper than the maximum settable depth of the DC. This was simply pressing soap into the aperture through which water-pressure was measured. It rapidly dissolved in sea-water, delaying the detonation. I do not agree that it was practice to set a range of depths in each pattern, as the depth setting used was commanded by the officer in charge of the Asdic. I cannot exclude that it "never happened", but it'd be unusual I think.
I fully agree that our DC's are rather too lethal, and are woefully short in terms of for how long searches and attacks are prosecuted for. I would regard the lethal range of a DC to be 5m incidentally) For my money what needs to happen is for the de-alert to commence once the u-boat is under (say) 30m or so, and for the asdic searches and attacks to continue for at least 20 minutes, during which period, the convoy alert ceases. (to allow other u-boats to attack without being inconvenienced by that alert, unless, they in turn become detected of course!)
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