Ideally what a submarine commander wanted to do was to lose the escorts, surface at night/in fog and escape at high speed.
Escaping underwater was very difficult considering a submerged speed of 1-2 knots per hour. Escaping on the surface was possible since the max. effective range of allied surface radar against a U-Boat, even in 43, was around 5 km.
obviously, whether it could be done depended on weather conditions/visibility, number/skill of the escorts, presence of aircraft.
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