The explanation offered for German U-boats entering the Mediterranean more easily than leaving is the inward current through Gibraltar. A boat could ride that current submerged on entry but would have to overcome it on the return passage. Figures cited here put the current at roughly seven to ten knots and a German boat's submerged maximum near seven. British U-class and Italian boats are credited with enough underwater speed to offer a better chance of leaving. Even at the higher speed cited, an outward transit is described as risky.
Notable Point · 7:56:19-7:57:32
Gibraltar's inward current and the problem of leaving submerged
A strong inward current let submarines enter the Mediterranean submerged but made escape nearly impossible for German boats whose underwater speed could not overcome it.