[Jay]

Thanks for all the suggestions.

What I'm doing now is using the snorkel almost all the time during daylight hours, with both periscopes raised. When sea conditions permit, I run the boat at a depth of 11m (rather than the default snorkel depth of 10m). I don't know if that extra meter actually makes a difference in terms of the game's AI, but it seems more realistic to do that.

I've noticed that when running at snorkel depth, approaching aircraft do show up on the Navigation Chart (F5), giving me time to react. Maybe this was happening before I kept the periscopes raised, and I just wasn't on that screen at the time.