I too have had multiple torps which were set to mag detonate prematurely. I have had a few patrols (only on patrol #14 now) where I only came back with 4 ships sunk (mostly coastal) even though I fired all of my torps! Of course on those occasions, there were heavy seas (winds @15m/s) and the torp broke out of the water. I usually only fire my torps from less than 1200m and sometimes less than 800m... But what is a skipper to do, when you are trying to take out a ship by firing at less than 90 AOB and you are going for a keel shot?? I have been denied a few choice ships when the impact pistol fails to fire and the torp harmlessly bounces off the hull due to the ship taking evasive action to avoid the torp??
Happy hunting and fair seas!