How long could these submarines stay submerged?

It depends. Submarines could stay submerged for more than the 16 hours that the fresh air would last, as indicated above, but it takes some extra steps. A short answer is that a practical limit was about 30 hours. However, the record in WW2 was just short of 38 hours by the USS Puffer (SS-268). At that point both the battery and the crew were pretty messed up. (Technical term!) The boat was being depth charged and tracked, and they had no choice. Fortunately, they were eventually able to get away and return to base.

There were two limitations at play here, battery power and air quality. The limits on both depend on how fast the boat used them.

As discussed above, battery duration depends very much on the speed. At two knots (about 2.3 miles per hour), the battery is rated at 48 hours. Note that you probably walk faster than that. If you double the speed to four knots, you need to divide the time the battery would last by about eight. (The ratio of speed to battery power is cubic, two times two times two.) If you double the speed again to eight knots, you divide the time by eight again and the battery would last less than an hour, and this is called the “one-hour rate”. At maximum speed submerged, about 9 ½ knots, the battery may only last about 30 to 40 minutes. As a result, captains tended to use the battery very conservatively.

Air quality also depended somewhat on how fast you use it, although the differences aren’t as drastic. Again, the air in the boat was intended to last 16 to 18 hours. However, since most of the crew in WW2 smoked, the air would likely last 16 hours. How can you use it more slowly? One way is to put everyone not actively operating the boat in their bunks. Expending less energy uses less oxygen. Another way would have been to put the smoking lamp out. That didn’t usually turn out to be practical due to nicotine addiction. Still, it could have been done in a real emergency.

Fortunately, there were some other options to extend the air limitation. There were six oxygen bottles on the Pampanito, and they could have been bled into the boat to drive up the O2 percentage. There were also many cannisters in the overhead marked “Do Not Paint.” These contain lithium hydroxide or soda lime which is a CO2 absorbent. The cannisters would be opened and spread on bunks or decks even though the chemicals are caustic. These steps added a couple hours of habitable air to the boat.

If more time were needed after the above steps, it was possible to bleed air into the boat from the high-pressure air banks. It seems unlikely that any boat had to go to this extreme, but it was possible to safely double the air pressure in the boat. The oxygen and CO2 percentages were about halfway back to normal and the pressure was not extreme. The pressure would be about the same as swimming at about 25 feet. This wouldn’t have been terribly uncomfortable and wouldn’t require decompression.

However, now another challenge arises due to the increased pressure. When the boat surfaces, we want eyes on the bridge as soon as possible. We need to know for sure that there isn’t anything up there that we missed on sonar and periscope sweeps. Although there isn’t a great overpressure in the boat, that increased pressure was in all of the pressure hull that was 16 feet in diameter and about 275 feet long. It all wants to go out the Bridge hatch at once. If you put a man in that small hatch, he could have been pushed out much the way a bullet is pushed down the barrel of a gun, albeit more slowly. To keep him from being launched, they would have a big sailor hang on to his ankles as he opens the hatch. Really! The procedure was not uncommon. The overpressure could also have been an issue after just launching multiple torpedoes which brings more air and water into the boat.

There were some other steps that could have been taken to reduce the pressure in the boat such as running the low-pressure blower or the air compressors. However, that would usually take too much time and you don’t want to introduce a vacuum.