When would the signal tube in the after torpedo room have been used?

If it were needed for signaling, we would launch flares from that tube. This would have been used mostly when doing training exercises with other U. S. ships and planes. The color of the flare contains the message. For example:

1. A green flare is fired by the submarine when the boat would have launched torpedoes if this weren’t an exercise. That would mean that the boat had sunk the carrier or tanker, etc.
2. A red flare is fired by the submarine when it is in trouble and needs to surface quickly. All surface ships need to get clear of the area as soon as possible.
3. Recognition signals would very and the prescribed colors and sequence would be followed.

After the war, the signal tube can also be used to launch a decoy which was fairly basic. It consisted of a bubble generator, much like the German pillenwerfer, made of calcium hydride. When it contacts water, it generates a large volume of bubbles. Air bubbles reflect sound (sonar) just as the submarine’s hull would. It is the change in density that reflects the sound. If you are familiar with Alka Seltzer, you know that it reacts with water and creates fizz or bubbles. The countermeasures after WW2 were similar. (However, Alka Seltzer uses different chemicals, bicarbonates.) A fictional example can be seen in the movie “The Hunt for Red October.” When the Russian aircraft drops a torpedo to try to sink “Red October,” the boat releases a pair of decoys that tumble and generate bubbles in an attempt to confuse the torpedo’s sonar.