I am not very experienced with setting my own solutions, but as a general rule i always fire 2 torpedoes at any target over 10.000 tons. I check the ship, identify it trough the ship identification-manual found at the Periscope and check the ships draft, i.e how deep down in the water the ship goes, and set my magnetic pistols to 0,5 m under that in calm waters, and 1m under it in rough seas. I try to mostly use electrical torps as they leave no trace behind, and that means a miss doesn't have to mean everyone chasing after you like madmen.
I never use torpedoes on targets under 1000 tons or targets that i am not 100% sure i can have a decent shot at. For example, i spot a ship in the horizon, heading away at 12 knots. I could race the ship, go around it and get a good line of fire in due time if i max out my engines, potentially burning lots of battery-energy or diesel for something that isn't 100% a clear shot.
Better live and let live sometimes, and not take stupid chances
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I have no experience at all with LUT or FAT-torps.
Numbers of torps fired at a ship is sometimes hard to tell. You have to see for your self but generally, a well placed shot with magnetic pistol right under the ship, detonating under the keel will tkae down most ships under 10k tons. Except for Passenger/Cargo. You will have to use nukes on them.
