How did the diving officer know when the boat was light or heavy?

While there was no gauge telling the diving officer whether the boat was out of trim, there was a clear indication available. The diving officer just watched where the bow and stern planes needed to be, up or down, to maintain depth. Then, through experience, he knew whether the boat was light or heavy, and whether the boat was in trim fore and aft. He would then move water in or out, or fore and aft to get the boat into neutral trim.

The best test for neutral buoyancy was to let the boat drift to a stop. At that point, the bow and stern planes had no effect. If the boat hovered at that depth, and didn’t rise or settle, then the boat was at neutral buoyancy. It was trimmed properly. This was often the first thing that a young officer on a submarine accomplished that was part of the art of submarining.