A playable Type XXIII. Replace the IIA with a XXIII, and make it only available from late 1944 (or perhaps mid 1944, would be interesting to go after the ships from Overlord).
Type XXIII's, while limited to two shots, did have some success in WWII. None were sunk by enemy action, and several sank ships, including the last two sunk in the war.
The Type XXI showed up too late to do anything.
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The U-Boat Commander of Love
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