"Maximum Submerged WWII U-Boat Endurance?" Topic

I've found a few anecdotes on this, but no real definitive info.

Anyone know the maximum theoretical, and/or practical time underwater for a crew to stay submerged during WWII, disregarding battery endurance, e.g. just sitting on the bottom trying to avoid/evade enemy depth charge attacks?

Some numbers I've seen mentioned are 16 and 23 hours for attacks by Walker on U-Boats, but I've run across a couple of others (can't recall where now), mentioning 30 and 38 hours too. I've also seen mention of 48 hours, but that seems a bit long based upon the above (perhaps that is the maximum under ideal conditions).

I assume the larger Type IX boats might be able to stay down a little longer, but not sure about that either.

Thoughts?

A rough idea, or guest-imates based upon other anecdotes would also be useful.

Your question is difficult because it depends on the type of boat, but also on how well it was ventilated before being forced down. A U Boat that has just surfaced for air and battery charge but been forced to crash dive at once due to enemy presence will have a fairly short endurance.
12 to 16 hours is about average, with some lasting up to 24 hours.
There has been mention in some recent documentaries about extreme endurance's…."Hitlers last U Boat" for example. But the times mentioned were so long that I could only presume the film makers were taking some liberties with the truth or getting confused with modern boats. Three weeks would have been totally impossible without the crew all having died. Even the 72 hours kept below under continuous attack, mentioned at one point would be extremely unlikely.
They would have had to vent the boat at some stage, even if only for a short time.

If you are wanting to use this in a wargame, I would suggest you roll a d12, adding that to the base number of 18 for large boats. 12 for a type VIIC and 6 for a type II boat.

That would simulate the varying situations the could have applied on board at the time it submerged.

As Mal says, they COULD stay down for a really significant amount of time. I have read a couple of accounts that spent over 24 hours submerged, but I can't recall exactly how long, perhaps up to 48 hours or more. They were only able to accomplish this by very careful action – for example, all unnecessary personnel went to their racks to conserve oxygen, very careful conservation of battery power, etc. In one account the captain stated that upon surfacing and opening the hatches, they could actually see the foul air coming out of the hatches and the sailors were gasping for breath at that point. (This might have been in Kretschmer's book, can't remember right off.)

Thanks for the quick replies.

Sounds reasonable, and I like your suggestion for rules Mal.

I ran across an Allied intelligence report from June of 1944 that listed either the 30, or 36 hour time limit. Another one listed 48 hours, IIRC, but that is probably for optimal conditions.

So, now that we have that squared away, any ideas on how long would it take to ventilate the boat, upon surfacing?

So, now that we have that squared away, any ideas on how long would it take to ventilate the boat, upon surfacing?

Ten to fifteen minutes to do it adequately and get rid of all bad fumes etc. Again it depends on the boat and how efficient its fans are and if there are gasses accumulated.

If you wanted to also get rid of the bad smell of unwashed crew…a dockyard job with soap and water!!!

The best fan for ventilating a submarine is its diesel engines, which draw huge amounts of air. It was common on US boats to open the hatches, but not the engine inductions, and fire up the diesels. All compartments would feel a strong wind as the engines pulled fresh air through the hatches (and blew the old air out the exhaust). They'd revert to normal ventilation after a few minutes.