Simon:

Yes, as Dan said, we used standard US Navy airplane altimeters as a visible reference for what sort of vacuum the engines were pulling in the boat.

The worst kind of snorkeling was in a choppy sea and a heavy seaway. The sea would wash over the head valve electrodes, and it would shut. The engines would continue to suck air out of the boat, so we'd "climb." It was a strange sensation as the air would suddenly chill in the boat. Then just as fast as it shut the sea would pass and head valve would open. "Down"in 'altitude" we would go as the atmospheric air rushed back in through the snorkel induction. And the cycle would repeat itself over and over, We called it a "chattering head valve."

Pretty soon everybody would be annoyed. People sleeping would be waked up by their popping ears and draining sinuses. People already awake, would just bitch about it and endure it. Usually, because submariners LOVE to agitate their shipmates, several of them would walk into the Control Room (one-at-a time)and ask the Diving Officer if he couldn't control the depth a little better.


Fritz