I sometime get questions on the emergency escapes from U boats.
In general I can can only say the survival statistic is not very impressive and I only know about a handful successful attempts of the German boats. In the last accident in the RN I remember in my time the RN T class submarine HMS Truculent which collided with the Swedish tanker M/S Devina at the entrance of the Thames January 1950. There was quite a number of people on board as she had some yardpeople in addition to the normal crew. I guess some 57 people managed to surface but were taken by the strong tidal current and perished in the cold water some 20 were picked up. The escape is one thing, but survival on the surface depends on what kind of support is available. The WW2 standard of escape equipment was by no means comparable to to days standard.
In general the VIIC`s pressure hull is divided into 4 separate pressureproof compartments, the control room separated by pressureproof bulkhead from aft and fwd compartments and conningtower separated by the pressurehull including by lower tower hatch. In addition you have the aft torpedo- and E- room, main engine room, the compartment consisting of galley and PO messroom, the compartment consisting of the sonar,radio, wardroom and CPO mess, and forward torpedoroom separated by watertight bulkheads. The watertight bulkheads have doors a bit low from the upper part of the pressurehull and the pressure hullhatches are equipped with a coaming.
In the event of an emergency escape, the lower doors and coamings all keeps the residue air in the compartments stable when flooding. The whole boat has to be flooded and the pressure tight compartment shut. You have now three separate escapehatches of which all if possible should be used as you have limited escape time, both with regards to pressure (bends) and chlorine gas emission from the flooded batterycompartments. When the uboat is flooded you pull down the escape hatch coamings lengthenings trunks which now ends submerged in the floodwater as the pressure equalizes with the ambient seawater pressure, at the same time the watertight bulkheads keep the trapped pressurized compartmentair up towards the pressurehull.
The crew is ordered to put on the Draeger breathing- and escape vest. This equipment is used both as and breathing apparatus or as an ascend/lifevest having an oxygen canister and a hydrosodium cartridge for CO2 absorption. In case of excessive chlorine gases the vest is used as a breathing apparatus otherwise the crew is waiting above the waterlevel breathing the trapped compressed compartmentair. A man dives under the hatchcoaming up to the escapehatch which now can be open as the pressure inside the u boat is the same as the ambient seawaterpressure. He has to be aware of the danger as the trapped air in the duct escapes as the hatch might slam shut when the airbubble collected at the top escapes. Now the waiting crew can take a deep breath of the compressed compartment air, dive under the coaming and have a continuous column of water up to the surface. While ascending the crew has to exhale all the time to let the compressed air out otherwise you might experienced a lungburst. Usually you are told to start whistling when escaping and due to the risk of "bend" you should adjust the ascendspeed ( by adjusting the air in the Draeger vest) to that of a small following airbubble. Normally a successful escape depends on the support you get on the surface almost everybody needs a decompression chamber treatment. The effect of having a human body exposed to high airpressure might be compared to a sodapop bottle, as long as it is kept under pressure it stays calm, but when you open the cork it start bubbling. The same happens to the human blood and can cause a fatal embolism hence you have to release the ambient airpressure slow ( in a pressurecamber).
