[Hartmann]

In real life some boats stay 30 or more days without surface.

but there are a lot of problems with health and machinery breakdowns.

"On the latest health inspection by the doctor if was detected that seven men still had crabs, another one is full of rash. There is fever, several headaches and angina. This constant marching submerged was a health hazard. Relative humidity in the boat was always 97 to 100 percent. The sweat water on the walls went down in buckets, and all leather and other clothes were covered with a thick layer of mold, everybody is constantly sweating. The only dress we wore were our swim trunks.
" Finally the boat had passed the line, the Equator. Still running submerged 37 days out from Bordeaux, she is now exactly on the dangerous track of constant planes between Bahia and Freetown.".....
When the boat was 25 degrees south on May 5, she finally ran day and night surfaced and I jumped with all hands on cleaning our 3.7cm and the two 2cm twin barrel anti-aircraft guns, which had suffered badly, having been all the time under water. There was jubilation in the boat when the first automatic shots were fired without jamming. The men could finally see the sun again after some seven weeks of life under electric lights. Many had bad skin rashes and several had jaundice, which could not be detected in the boat. Our doctor was down with malaria."...