Hit here at about the 30ft mark at least on your first attack, hit her with all tubes at once to minimise the chance of her being battoned down and damage control teams being at the ready for later fish, try to get a good spread along her length, she seems to be a little more fragile in the stern and forward of her front batteries. As you see her start to list remember to adjust your torpedo depths accordingly.
Concentrate your fire on one side and try to capsize her.....
Good luck