How often were German U-boat pens attacked, and how much did that affect operations?
Short answerThey were attacked frequently, but their construction sharply limited damage until very heavy bombs and advancing ground forces arrived late in the war.
NotesThe pens were attacked more often than simplified accounts suggest, yet their thick concrete and steel made them resistant to all but the heaviest late-war bombs. By the time suitable weapons became available, Allied ground forces were also closing on several fortified ports. Canadian armoured formations are cited as winning part of that race on the ground, while other German-held Atlantic fortresses remained isolated until the war ended. The result was a race between specialized bombing and the capture or containment of the bases rather than a simple story of bombing stopping U-boat operations.