Hi simplyacrow,

First, I don’t know how your research came to the conclusion that the crew only had 14 hours of breathable air remaining.

Without knowing how many of their resources were depleted before they submerged it is impossible to answer your question. There are several variables that have to be taken into consideration. Here is a general overview:

When submerged, the only air the crew had to breathe was the air trapped inside the boat. There was a ventilation system with canisters of sodium hydrate a substance that would absorb the carbon dioxide exhaled by the men to keep the air clean located in both torpedo rooms and the control room. They were inserted into special manifolds attached to the ventilation exhaust line. The ventilation system continuously circulated the air in the boat. Measurement of CO2 concentration was made with use of air sampling tubes, a Drager measuring apparatus, or an Orsat testing setup.

However, the canisters eventually got saturated and had to be replaced. A total of 315 canisters (212 in the Type VIIC), each capable of absorbing 400 liters of CO2, were carried in the boat. Lastly there were 13 cylinders, each containing 50 liters of oxygen under 160 atmospheres (2,275 psi.) of pressure and a system to meter it into the ventilation system if the air quality got too bad. Additionally, oxygen could be bled directly into both torpedo rooms and the control room through reducing valves and a water trap, the latter enabling the amount of oxygen passing to be checked. The gauge on the low-pressure side of the reducer was marked “Mann” and presumably is so calibrated that the amount of oxygen supplied could be regulated according to the number of men in the compartment.

Normally if the air quality started to drop the commander would simply take the boat up and open the conning tower and control room hatches to let fresh air flood into the boat and the stale air out, assuming there was no reason not to.

However, if they couldn’t surface because they were damaged or under attack by the enemy, they just had to stay down. Each man not absolutely necessary to the operation of the boat would be immediately sent to their bunk or told to lay down someplace and be still. Any of the air purification canisters that need it would be replaced with fresh ones.

If the air quality couldn’t be maintained at a safe level, as a last resort the men were ordered to put on their personal air purification devices. They contained sodium hydrate just like the boat’s air purification canisters & each man had one. When these failed, they could switch to their escape gear. It was a rebreather that had its own air purification canister and its own cylinder of oxygen. It was intended to be used when escaping from a sunken U-boat and didn’t last long.

In theory if all the canisters were brand new and all the oxygen cylinders including those in the personal systems were completely full and everybody was asleep in their bunks a U-boat could stay down for 72 hours before everybody would suffocate. However, normally some of this capacity would be used up over the course of a patrol so a single submergence for 72 hours would not be possible.

A U-boat has to move while underwater or it will sink. A really clever Chief Engineer can keep it hovering at depth by manipulating the boats buoyancy for a short time but not for very long. Since a U-boat runs on its batteries while submerged, battery capacity has to be taken into consideration. At 4 knots a Type IXC will run out of battery power and have to surface or sink after about 15.75 hours assuming the batteries are fully charged at the beginning. At 2 knots the batteries might last a bit more than twice that or perhaps 36 hours.

However, there was the possibility of sitting on the bottom if the water was shallow enough and the boat hadn’t been detected by sonar and then if the air purification equipment was fully charged, perhaps 72 hours might be possible.

So, the amount of CO2 scrubbing capacity & available oxygen as well as the amount of battery charge all have to be taken into consideration when determining how long a U-boat can stay down in any given situation. For example, if the boat is in over 1,000 feet of water (or any depth past its crush depth), battery capacity becomes critical because if the boat sinks past that depth it will be crushed. On the other hand, if it is in shallow water and can just sit on the bottom but most of their CO2 scrubbing capacity has already been used, they will have to surface when the remaining capacity runs out.

36 hours isn’t out of the question and some boats have stayed down longer. After being attacked, U-219 located a salinity layer on which she drifted with motors turned off using the absolute minimum of battery power for 69 consecutive hours until the air became too toxic stay down any longer.

Regards,

Ken Dunn