Depends on how dark the night is. On really dark nights you can get real close. I have approached to less than 500 meters without being spotted at 1/3 speed. If it is not too dark (you can see stars or the moon is up) you are probably ok at 1000 meters but I would not try much closer.
I usually approach with decks awash, launch my torpedoes, then dive to periscope depth to watch them hit. You are taking a pretty big risk if there is an escort and you stay on the surface. I don't care how dark it is outside, once those star shells go up you are pretty easy to spot. The spot lights will get you too.
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