A submerged submarine must displace exactly as much water as it weighs. Ships on the surface must do the same, but they have no choice. If they fail to obey this law of physics, they sink. We spent much time with paper and pencil to ensure that we weighed exactly the right amount. Before returning to port, we would submerge one last time for a "trim dive", during which we measured exactly the contents of every tank on the boat. The next time at sea, we calculated our mass, and our center of mass, as best we could. When we dove, we always found ourselves within 1,000 pounds of neutral buoyancy. I found this accuracy to be impressive, since there were so many different variables that entered into our mass, such as the number of people on board, where their bunks were, how much food we had bought, where we had stowed it, how much fuel, lube oil, hydraulic oil, even fresh water we had on board. Even the compressed air in our main air flasks could vary in weight by as much as 5,000 pounds.

Upon submerging, we cranked up full speed ahead, to maximize the water flow across the diving planes, which are horizontal rudders mounted fore and aft. The diving officer analyzed the behavior of the boat, and if all seemed in order, he slowed to 2/3 speed. After some fine-tuning, the diving officer slowed to 1/3 speed, which indicated to the Officer of the Deck (Submerged) that the boat was in trim, and the OOD could take charge of the boat for military exercises. Quite often we cruised for hours at 1-1/2 to 3 knots.

The minimum standard of competency as diving officer was to have your buoyancy correct within 500 pounds overall, with no more than 200 pounds of error fore and aft. With experience, it was not difficult.

A submarine is more compressible than sea water. As a result, if a submarine descends, it must pump sea water from the ballast tanks to sea. If a submarine ascends, it must flood sea water into the ballast tanks. A submarine's mass is almost always in a condition of "negative stability".

Note that I said "almost" always. Upon occasion, the density of sea water does not constantly increase with depth. This occurs as a result of thermal layers in the ocean. When submariners find such a layer, they frequently just stop. It is called "resting on the layer". It is a wonderful way to hide from sonar.

Have I mentioned that submariners are human? When a new officer is standing watch as diving officer, and the boat is resting on the layer, an unusual practical joke is possible. Since the buoyancy of the boat, fore and aft, is critical within 200 pounds, it is possible for, say, forty sailors to gather in the crew's mess, walk to the after torpedo room, and return. Needless to say, this cannot be detected from the control room, and it throws the boat into apparently desperate straits. The stern sinks alarmingly. The bow threatens to break the surface of the ocean, giving away the location of your hiding place. Pumping water from aft forward can't keep up with the change. The diving officer must admit that he has lost control, and ask the OOD for propulsion, just to save the boat from catastrophe. One-third speed ahead was sufficient to regain control. Of course, the instructors don't teach about "trim parties" in submarine school. The new guy is baffled as to what went wrong.

So, if forty guys at a time thought that this was a form of entertainment, just how bored do you think they were to begin with?