[UnderseaLcpl]

Impact pistol FTW. All merchants worth bothering about are 3m. All destroyers are 1.5
Very, very rarely will this ever fail in GWX 2.1
Sure it may take 2 torpedoes to sink a ship you could have sunk with one mag, but I have found an overall aggregate savings of torpedoes when using impact fuses.
This is primarily due to 1) premature detonations and 2) the fact that it is harder to set up a really effective magnetic torpedo solution that will traverse the beam of the ship. This is doubly true when the target zigzags or weather is poor.
The only time I use mags is against battleships because they are as wide as some ships are long and I don't have to worry much about the torpedo exploding anywhere other than directly under the ship.