I always try to attack on the surface until late 1942 when it becomes too dangerous. I should qualify this by saying that I use the improved convoys mod which makes the whole nightime surface attack far more realistic.
Generally speaking the nightime surface attack allows you more options if its done right, you should rarely go in at full speed because you have more chance of being spotted. I find manual aiming far easier on the surface and its easier to get a much better idea of the disposition of the convoy and where the escorts are positioned. If I do it right I can sneak in fire off the torps and sneak out again and shadow the convoy, whilst reloading the torps, hopefully if there is still enough darkness left I can go in and attack again.
It is a better tactic than being submerged IMHO, but it takes practice to evade the escorts. Later it becomes more dangerous to do, but it still gives you a good simulation of the reality, the only downside is that Snowflake illumination flares are not modeled in the game and I find the escorts don't fire starshells as much as they should after your first torpedoes find their mark.
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