The pressure inside a diesal boat is always at atmospheric. The only thing that changes the pressure is when you are snorting, and the float valve in the snort induction mast shuts when a wave goes over it. The diesals suck all the air creating a vacuum. When this happens, it feels like your brain is being sucked out through your nose.
It is worse when you are sleeping, and have been snorting for awhile. Your ears dont clear automatically, and when you wake up you feel like you are in another dimension from the pressure build up. Then when you do val salva to clear your ears it feels like your brain explodes. Then you have one hell of a headache for awhile.
Over time you do get a slight build up of pressure in the boat, and if you have been running shut down, when the hatch is opened when you come alongside, you can feel the pressure in the boat equalize. I have never seen it strong enough to loose your hat, but I guess it would be possible.
We used an oxygen candle which was activated by a 410 guage shotgun shell, and gave off oxygen as it burned to enhance the air quality when dived for a long time, and we had a CO2 absorbtion unit in the fore ends, and one in the after ends to get rid of the CO2 buildup. I don't know if U-boats had this, but the O-Boat I served on was not that much further advanced, so it may have been possible in some form.
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Former O-Boat diesel Submariner
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