Oxygen in U-boat - U-boat Officer's Reply

[Stiebler]

A discussion came up recently, in H.sie’s Realistic U-boat sticky thread, about:
i) how long the oxygen in a U-boat lasted before it became necessary to add compressed air;
ii) how long the compressed air supply lasted before it became all used up.
There appears to be no definitive answer to these questions available, either on the Internet, or in standard books on the U-boat war, or even in books about U-boat manufacture.

Since the answer is of considerable general interest, including to naval historians, I asked the same questions to an old U-boatman that I knew. The U-boatman was a real sailor in the U-boat arm in the Second World War, and is now about 90 years old. He carried out one war patrol as a midshipman on the ‘milk-cow’ U 461, and subsequently was an officer on a VIIC U-boat. I have given his reply below, in the original German:

Um Deine Fragen korrekt zu beantworten, habe ich meine Fachliteratur über Unterseeboote durchgesehen. Nirgends habe ich eine Antwort auf Deine Fragen gefunden. So kann ich Dir nur aus meiner eigenen Erfahrung berichten.
Wie lange ein Boot unter Wasser fahren konnte, hing sehr vom Sauerstoffverbrauch der Besatzung ab. Waren nur die Besatzungsmitgleider auf Position, die unbedingt benötigt wurden und die restlichen lagen in ihren Kojen um Sauerstoff zu sparen reichte der Sauerstoff im Boot länger, als wenn die gesamte Besatzung in Aktion war. Je nachdem wurde Sauerstoff schon nach wenigen Stunden, oder erst nach rund 12 Stunden zugesetzt.
Die Pressluftflaschen hatten 30 atü, so dass wir theoretisch aus 300 Meter Tiefe auftauchen können. Die Boote waren gebaut für 100m Tiefe mit dreifacher Sicherheit. Freiwillig ist jedoch kein Kommandant tiefer als 200 m.
Wie lange wir unter Wasser bleiben konnten, hing weniger von der Sauerstoffmenge, als von den Batterien ab. Selbst bei sparsamster Fahrt hielten diese meist nur 18 bis 20 Stunden Stand, dann waren dieselben leer und wir mussten auftauchen um die Batterien nachzuladen.
Wie lange der Pressluftvorrat reichte, hing davon ab, wie oft das Boot tauchte. Die Pressluft wurde zum Anblasen benötigt und jedes Anblasen verringerte den Bestand. Aus all diesen Gründen handelte es sich bei den Booten, des 2. Weltkrieges eigentlich nur um Tauchboote, nicht aber um Unterseeboote. Erst die Typen 21 und 23, die aber nur noch vereinzelt eingesetzt werden konnten, waren wirkliche Unterseeboote. Ich hoffe, ich konnte Dir mit diesen Angaben dienen.

The writer makes the excellent point that it is impossible to say how long the compressed air flasks would last to provide for the crew, since they were used also to blow tanks to surface the U-boat.
However, since my German is not fluent, and there are many users of SubSim who are fluent in both German and English, and since the naval officer’s response about air supply in a U-boat is probably the only personal experience that has ever been- / ever will be- reported, I should like to ask if any native German speaker can provide an exact translation into English of the German. Be very careful about it! Your reply may be used in reference sources, such as Wikipedia.

My translation:
“In order to answer your questions correctly, I have looked through my literature-at-hand about U-boats. Nowhere have I found an answer to your questions. Therefore, I can report to you only from my own experience.
“How long a U-boat could travel underwater depended very much on the consumption of oxygen. Only those crew-members were at their stations who were necessary, and the remainder lay in their bunks. The oxygen lasted longer, than if the whole crew was in action. Even so, oxygen was already added after a few hours, or only after about 12 hours. [I think this last sentence is an idiom, and I cannot translate it exactly.]
“The compressed-air flasks had 30 atmospheres, so that we could surface theoretically from 300 meters. The boats were built for 100 gravities with triple security. However, no commander would dive voluntarily deeper than 200 meters.
“How long we could remain submerged, depended less on the oxygen-supply than on the batteries. Even with the ‘thriftiest’ trip, these lasted usually only 18-20 hours, then they were empty and we had to surface to reload the batteries.
“Whether the compressed-air supply was sufficient, depended on how often the U-boat dived. The compressed air was required for blowing [water out of the ballast tanks], and every blowing reduced the compressed air supply. For all these reasons, the 2nd World War U-boat was actually only a submersible, not a submarine. Only the Type XXI and XXIII, which at first could be introduced only singly, were real submarines.
“I hope that I have helped you with these statements.”

Stiebler.