try to hit her in the front part (near the ancorage) just 1 or 2 meters swallower than her depth with impact and see what happens....It usually takes 2 eels to make her slow down enough for you to catch up with her once the escorts leave the place....
sunked her twice in the present career....
playing at 72% realism with NO manual targeting
generally i find that auxiliary cruisers, large merchants, liners and generally big ships go down easier once hit near the ancorage.....