I assume he is an enemy always, so I prepare the interception course and I start to execute it.
Then I carefully get close enought to see its flag. It means, I try to show the less uboot profile as I can, and if it's, lets say 13:00, I will go almost sure underwater at < 6000m, at night I can go closer without going underwater.
Once I identify him as enemy, and (in 40-41) know if he has deck cannon:
1. Has he cannon and he is very large? Usually three of torpedos in early 1940, because one of them may fail. 1941, just two.
1.a) Does him survive but at zero or reduce speed? Wait and if he doesn't sink, fire another torpedo, take a better position before if you need. (if you fire under the keel, take in mind its keel is deeper now!).
1.b) Do you fail the shot? if you cannot do another fast... better go away, or do a more careful plan against an aware enemy in zig-zag.
2. Has he cannon and and he is medium size? Same than above, but just one torpedo (2 if you are in early 1940 and you do not want to risk).
3. Has NO cannon and he is very large? Same than 1, but in this case, just one torpedo and finish it with your deck gun.
4. No cannon and he is medium or small? Abuse your deckgun!
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