3 to 5 metres under the keel? no way.... i'm sure you mean feet
shoot the torps at 50cms (that's 0.5 metres) under the keel. but i dont know if you are suposed to shoot under the keel of a battleship (never met one)
3 metres below the waterline means that torpedoes will surely strike the anti-torpedo bulge, and again, i'm not sure if that tipe of "armor" actually works on the game.
so, i'm not actually sure why i'm replying
oh, yeah, 3 to 5 metres is too much for magnetic detonators, the torp will pass right by, the closer the better. try 0.5 to 1 metre