MCM,
It was a Flying Submersible Dutchman...

;-)

(sorry, couldn't resist)

I think it was Class U British or Polish sub.

The brit sub is pretty easy to ID. I fired three fish at one my crew managed to spot at ~ 1000m during night time. The first two were not set up correctly, but realizing my mistake at the last minute I set the last fish for a zero gyro and shot from the hip. It hit right unter their conning tower. AOB was ~70.

I, as well as my crew, I suppose, never felt a worse victory, watching her take her final breath within seconds. She sank before we could man the deck gun.

Fascinating ... never guessed you could encounter enemy subs.

The only Subs I so far encountered were german ones ... one in the Kiel Kanal, another one in the baltic sea, near Danzig. Travelled some time alongside the second one, before I veered away to follow my own mission

Just started a new Open Horizons II campaign with real navigation and I'm in the Baltic off Danzig and there's a sub nearby. I'm trying to track a Polish minelayer and a destroyer which are patrolling nearby (weather is clear, no wind, no waves - dangerous to surface at daytime). Keep getting a sonar contact but my hydrophone cant ID it. Sound is a faint twin screw turbine though. I haven't properly located it yet though because I don't want to waste my effort doing a full four bearing track of it and then lose the more dangerous contacts...

In the past I've usually found engaging them pointless because as soon as you open up with the deck gun they'll submerge (unless you get them with a lucky shot early on) and once they're under you've got no chance with the torpedoes.

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.