You can easily destroy enemy sub with stealthy torpedo attack from periscope depth. However, you must use only electric torpedoes against them because steam torpedoes are easily detected, and enemy subs can evade them easily because of their small size.

I'm playing a Open Horizons II campaign as well. Sailing into the North Sea, past Norway, is when I noticed I was being followed by a sub marked in red on the map. It didn't attack me the whole time I was surfaced which made it seem as though it was another u-boat sent to help me patrol around Scapa Flow. Once I was visually close enough it looked like a British Triton Class. Though I'm no expert on British subs. Anyhow, I finally decide to sink it. I circled around to face it, still surfaced. He passed about quite close to me, less than 100m. The torpedo instantly killed it. In the log it noted that I sunk a U class sub. I have to admit I'm still not sure if it was a enemy or a friendly. I suspect it was enemy.