Totally realistic according to this thesis:
http://digitalcommons.georgiasouther...=1599&context=
However if you used magnetic pistols you should have set the torpedoes' depth to go just
under the keel of the ship. In the game a 1942 torpedo set to magnetic and hitting the ship would probably detonate, but a 1939 torpedo in the game is trickier. The idea in using magnetic pistols was that the torpedo senses the magnetic field of the ship and detonates a meter or so away from the ship's keel letting the full force of the explosion damage the ship. (often causing catastrophic damage compared to an actual contact and a following explosion right at the hull of the ship, apparently then some of the force of the hit directs away from the ship)
On the other hand, if you use magnetic pistols in 1939 the weather needs to be nice, otherwise the torpedoes will not keep their depth properly as H.sie has modded them to fail a lot more in bad weather. If you decide to follow the historic path of playing be prepared to have lots of failed torpedoes - up to the point of having all of your eels fail. That happened to some boats, even to Prien.