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.

[Hitman]

Yes there are certain points in the translation that need fine tuning IMO

Here is how I understand some of the relevant sentences should be in english, with some additional comments in brackets to better ilustrate the meanings:

“How long a U-boat could travel underwater depended very much on the crew's oxygen consumption. If (Or "when") only the (minimum) necessary crew-members were at their stations (The minimum stations that had to be manned necessarily for the uboat to be under control), and the remainder lay in their bunks, then the oxygen lasted longer, than if the whole crew was in action. Depending on that, oxygen was already added after a few hours, or only after about 12 hours

It is my understanding that with full crew in normal activities, pure oxygen had to be added after only a few hours, and at barebones crew situation, only after 12 hours.

Sadly it is not clarified if normal compressed air was added first for breathing, before pure oxygen, but it seems to imply that it was not used for that purpose. Probably because normal air would add oxygen and dillute the proportion of CO2, but not restoring the correct and ordinary proportion of oxygen in the air, which can only be done both by adding Oxygen and by absorbing CO2 with chemicals.

We know however as you already reasoned. that UBoats could cross the Bay of Biscay submerged for long periods, and I see it pointless to asume that BDU would order them to do so if the Oxigen supply was exhausted in little time. So I guess that the key is probably in that the compressed oxygen reserve in the UBoat was larger than what we thought, maybe because of number of bottles carried or most probably because of higher compression than what we considered.

[Myxale]

Just curious;
The compressor in each boat, would replenish the lost or used compressed air. Right?

And it worked only along with the Diesel. So what was it's refill ratio?

[Wolfstriked]

Really interesting post by the real Uboat fella but it is off slightly.He is mixing up compressed air with compressed oxygen.Stiebler,I think what you guys have accomplished is perfect now that you can choose what you want the air supply to be like.

[Stiebler]

Thanks for this feedback, and especially to Hitman for improvement/clarification of my translation.

I've mentioned in H.sie's sticky thread, that my U-boat friend is an intelligent man, who would certainly have volunteered information about use of compressed oxygen, if he knew of it (or, perhaps, if he remembered it).

Stiebler.

[Stiebler]

I wrote again to my friendly and helpful U-boat officer (let us name him: Guenther Paas), asking specifically about the use of compressed oxygen in U-boats. Again, for reference purposes in future encylopaedias, I attach his reply in its original German, and underneath my very literal translation.

His reply:
"Es ist richtig, dass die Unterseeboote neben den Druckluftflaschen auch Flaschen mit reinen Sauerstoff an Bord hatten. Beide Flaschenarten hatten einen Druck von 20 atü = bar. Die Druckluftflaschen wurden zum Anblasen bei Auftauchen verwendet. Die Flaschen mit reinem Sauerstoff wurden benötigt, wenn das Boot längere Zeit unter Wasser war und der Sauerstoff im Boot zu Ende ging. Wie ich Dir bereits berichtet habe, wird die durch das Atmen der Besatzungsmitglieder verbrauchte Luft, durch Kali-Patronen, die sich an der inneren Bordwand befanden gesaugt. Die Kali-patronen halten das Kohlendioxyd zurück und die sauerstoffarme Luft kommt zurück ins Innere des Bootes. Wenn der Sauerstoff in der Luft durch längere Unterwasserfahrt immer weniger wurde und die Besatzungsmitlieder anfingen zu "japsen" und nach Luft zu schnappen, wurde aus den Sauerstoffflaschen Sauerstoff abgelassen und man konnte wieder normal atmen.

"Trotz Rückfragen bei Horst Bredow und alten U-Bootsfahrern konnte mir keiner sagen, wieviel Druckluftflaschen und wieviel Sauerstoffflaschen die Boote an Bord hatten. Daher weiss ich auch nicht, wie lange die Sauerstoffvorräte ausreichten. Aus meiner eigenen Erfahrung kann ich sagen, dass bei längerer Unterwasserfahrt die Batterien leer wurden und wir auftauchen mussten. Dass der Sauerstoff aus den Flaschen zu Ende ging, habe ich nicht erlebt."

My translation is this:

"It is correct that on board the submarines, besides the compressed air-bottles, had also bottles with pure oxygen. Both bottle-types had a pressure of 20 atmospheres = Bar. The compressed air-bottles were used for blowing when surfacing. The bottles with pure oxygen were required if the boat was an extended time under water and the oxygen in the boat came to an end. As I have already reported to you, the air spent through the breath of the crewmembers is absorbed through potash-cartridges, that were positioned on the inner hull. The potash-cartridges keep back the carbon-dioxide and the oxygen-poor air comes back into the inside of the boat. If the oxygen in the air became progressively less, through over-long underwater-patrol and the crewmembers started to "japsen" and to snatch after air, oxygen was released from the oxygen-bottles and they could breathe normally again.

"Despite queries, at Horst Bredow [owner of the well known U-boat Archive] and old U-boat sailors, no one could tell me how many compressed air-bottles and how many oxygen-bottles the boats had on board. Therefore I do not know also for how long the oxygen-supplies were enough. From my own experience I can say, that the batteries became empty with extended underwater-patrol and we had to surface. That the oxygen from the bottles came to an end, I have not experienced."

In summary, no one now knows, but exhaustion of the U-boat batteries was the determining factor in when to surface.

Stiebler.