Hi!
Assuming you are in relatively smooth seas, I suggest:
- 1. use a salvo of two torpedoes with the spread angle of one or two degrees.
2. Use using magnetic fuses with torpedo depth set to one or two meters below the draft of the ship as shown in the recognition manual.
That way you have a chancee of at least one hit causing heavy damage if the other torpedo explodes or duds out on you.
Even though the explosive power is the same, in real life the explosion of a magnetically-fused torpedo beneath a ship's hull could break the ship's keel, causing it to sink. The explosion basically created a big air bubble underneath the ship, and the ship's keel would break under the stress of trying to hold up a massive ship without the benefit of water supporting it from beneath.
Pablo