Pepsican is giving sound advice.
I find the center lead destroyer usually cannot detect your sub if you are stopped or going 1/3 silent running especially below a thermal layer.
It's somewhat debatable as to how many torpedos per merchantman as they seem to absorb an unpredictable amount of damage from convoy to convoy.
It's also a huge dissapointment to put 2 torpedos into a ship and later discover it manages to limp away at 2 knots keeping up with the convoy who all slow down to 2 knots ??? Some of the larger tankers will stay afloat after 3 torpedos and proceed at 3-5 knots well down in the water with sometimes heavy lists .... amazing stuff.
Depends on the time of day, weather, visibility, number of escorts, depth of water, how many torpedos you have.
If it's getting near dark I try putting 1 each into as many ships as possible hoping they will come to a standstill or limp away at slow speed, surface after the escorts have gone, finish the immobiles & stragglers with deck gun, reload torpedos then chase down the remaining convoy doing an end round and repeating the procedure of submerging in front of anticipated path of centre of convoy.
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