I've been playing in the 1939 campaign in a type 2 sub.
I've gotten in real close to enemy merchants... about 400 meters close... shot off a torpedo... and it bounces off the hull when it hits! This has happened with 9 torpedoes in a row.
Is there some sort of minimum range you have to be in for a torpedo to detonate? To keep your sub safe or something?
I've also set them to MAX speed and trigger type "d" (detonate on contact I believe it is, instead of magnetic). No go. I've also hit the merchants from several angles.