How do the escape trunks work?

The after escape trunk is one-time use only. The boat must truly be lost since this process makes it almost certain that the submarine would not surface again. In order to use the after escape trunk, the crew must:

1. First, gather as many crewmen as possible together in the space and close the watertight door to maneuvering.
2. Then charge Momsen Hoods with air and distribute them.
3. Flood the compartment with water up to the lower lip of the escape trunk.
4. Pressurize the compartment to exceed the outside sea pressure if it doesn’t already. At 200 feet, this was about 90 PSI. The higher pressure in the compartment was needed to be able to open the hatch against the sea.
5. Launch the marker buoy.
6. Someone climbs up into the trunk to open the hatch (Everyone would be treading water already, so no ladder is needed.)
7. Each man then would duck and go out and follow the line up to the buoy.
8. Each man needed to exhale most of the way up to get the excess air (up to 90 PSI) out of his lungs. Yell “ho, ho, ho” most of the way up.
9. Stop at each knot in the buoy line for a minute to decompress
10. Do not ascend faster than your air bubbles.
11. At the surface, stay together, preferably near the buoy.

Again, this escape trunk has never been used successfully. The two times that sailors were able to escape from the forward trunk on a boat like this, the after part of the ship was flooded from the initial incident and everyone aft had already died.

The forward escape trunk works on the same principles but holds far fewer people and can have been used multiple times. The boat doesn’t have to have been sunk. The different steps are:

1. After the trunk is full with just a few men, it is flooded to just above the side door. It is pressurized, as above. That door would have been used to leave the boat rather than the hatch at the top.
2. Exit out this side door and follow the buoy line up.
3. The last person out closed the door behind him. If he failed to do this, there is a long lever in the torpedo room that could be used to close the door.
4. The escape trunk would then be drained into the torpedo room and depressurized so the next group can begin their escape.

Part of the reason that this escape trunk was designed differently was so that it could be used for both exit and reentry. In WW2 divers and UDTs (underwater demolition teams) and, later, Navy SEALs could exit the boat, accomplish their missions and have been recovered all without the submarine needing to surface. The boat would have been much less likely to have been discovered. The boat and the divers were therefore safer.