Reality and popular belief often differ.
Uboats, did not dive at or below 300 meters, at least, not on purpose, and definatly not with any regularity. I doubt you'll find more then one or two accounts of a uboat having gone to 300 meters deep or more and lived to tell about it. Furthermore, the stress undergone by the pressurehull at that depth by any submarine of the day, would have caused fatigue on the hull to such a degree, that retirement of the boat might be a consideration. At the least, the strength of the hull in future dives would have been drastically compromised, making the crush depth of the boat progressively shallower as the pressure hull became more and more fatigued.
The type 21 Uboat, wasn't all that and a bag of chips. Firstly, many of the internal lines and piping (such as hydrolic systems) were external to the pressure hull, making repairs by the crew virtually impossible. Secondly, you have to look at the hulls themselves. Made in sections at various yards, under the hammer of allied bombing. There are accounts of the various sections not fitting together correctly at the time of assembly. In otherwords, the hulls lacked quality due to the frantic/desperate situation in nazi germany's late war construction.
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