U-331 could make 17.7 knots on the surface but only 7.6 knots submerged. Currents and the geography of the Mediterranean approaches made the return passage extremely difficult for German submarines. Boats attempting to leave would have to spend considerable time on the surface, exposing them to patrol aircraft. Sending a U-boat into the Mediterranean therefore reduced the submarine force available in the Atlantic, even as it increased the threat inside the Mediterranean.
Notable Point · 2:27:19-2:29:30
Why Mediterranean U-boat deployment was nearly one-way
The Type VIIC's low submerged speed made entry into the Mediterranean easier than escape, turning geography and Allied air patrols into strategic constraints.