One torpedo can sink her, if you catch the bow or the stern. If you hit her amidships, she probably won't sink. I sink CVEs all the time (they seem to be extremely plentiful in GWX 3.0) and I usually give them two torps - one in the bow, and one amidships. If you don't sink her, you might slow her down and separate her from her escort screen. Then, though perhaps compromising historical realism, surface at about 5km and try to get a few deck gun rounds into her - dive, wait it out, maybe she sinks, if not, rinse/repeat.
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