The Manual says to take notice to fire the torpedoes over the minimal distance in order for the torpedo to arm itself, But It does not tell you what the minumum distance is!
According to the 'Time Life' book of "German Submarines" (In WWI & II) This distance is just 30 metres (!) for the savety screw to unwind itself and arm the Pistol.
Ingame I had several 'duds' when I fired under 300 meters Distance, from 350 they Ignite
But Those couple under 300 could have been vailures because I have the Vailing torpedo option active?