When did the torpedo explode?

The torpedoes did not explode based on a timer. The Mark 6 exploder was used for most of the war. It could be configured in either of two ways. However, it wasn’t a quick, easy process to change the configuration. One setting was for a contact exploder, so that it exploded when it hit the target. The other way was to set it to explode when it sensed the magnetic field of the target as the torpedo passed under it. As noted above, the magnetic version of the exploder failed often during the war and eventually was no longer used. The contact configuration was then used all the time, even though there were problems with it that had to be fixed. The magnetic feature was improved after the war and is part of the mechanisms used today.