Had a buddy that was a navigator on boomers in the 60s. He said they would tie a string tight from side to side of the sub and when they would do deep dives the string would sag in the middle.
It was a real treat going to the U.S. Naval Undersea Museum in Keyport, WA with him. They have a bridge form one of the 60s boomers set up there that you can walk thru. He showed me one of the control stations and said it was used to move water in ballast tanks in the front and back of the sub to help keep it level. He said whenever they would get a new guy on that station a bunch of them that were off duty would run back and worth from one end of the sub to the other making really hard for the new guy to keep the sub level. He passed away a couple years ago I really miss him he was a real character.
If your even in my neck of the woods its worth checking out.
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navalunderseamuseum.org]