In poor visibility you might be too close when you actually spot the ship. The range where torpedo gets armed is 300m. If the impact is prior to running 300m the unarmed torp will just hit and plummet into the ocean floor.
Somewhere in her there was a thread about shooting at ships only using data gathered from hydrophones. This is easier with lone targets. ( I have seen a tutorial movie about it too but cannot remember if it was posted here or at ubi forum.
I usually just shadow the convoy untill the weather permits the attack. Might be frustrating in areas where you are close to enemy harbours and will most likely never get the chance to attack as it soon enter waters too dangerous to operate on. I remember shadowing (listening) a convoy for 2 weeks untill the horrible storm and low visibility went away. This was when returning from Caribbean. I attacked the convoy with last 5 remaining torps and was able to bag 2 fat Tankers close to Bay of Biscay. All worth while and was one of those SHIII memorable moments.
Hope this helps
Cpt.Thorne
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