I have played this mission a few times. Usually approach Portsmouth during sunset with decks awash at high speeds. Before my sight range reveals the coast line, I submerge and continue with Große Fahrt on electric engines. The closer I get the slower I get, until just before the opening of the harbour I slow down to Langsame Fahrt. At this point it should have gotten pretty dark if you timed it correctly, around 10pm. So with slow speed you should be able to reach the inner part of the harbour at midnight when its the darkest. Run a long circle and head back out again so you are out at around 3-4am. Don't forget to replenish your oxygen every now and then, at best right before entering the inner harbour and right after exiting. And ffs don't raise your periscope!
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