The snorkel was no panacea, but it was anything but easy to detect, according to this
report
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The sweep width is then about 1/10 of a mile, only about I per cent of the value on a surfaced U-boat. In operations, therefore, Schnorchel has been very successful in countering radar detection. In the daytime, when visual sightings are also possible, its immunity from contact is not quite so great. The sweep width has been estimated at 0.6 mile
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From what I recall the plan in case the XXIs were to be encountered in numbers was to essentially give up most patrol areas and massing the aircrafts to carry out overlapping searches in the path ahead of convoys. Surface escorts armed with forward throwing A/S mortars on the other hand retained most of their value against snorkel boats.