How does a submarine adjust its depth?

This answer addresses the modern military submarines. There is a bit of variation on the theme, and a bit more when considering other types of boats. When submarines are submerged and underway, they are neutrally buoyant. The boat is set up this way so that minimal effort is required to control depth. (If the boat isn't sinking or trying to float to the surface, the operators can tend to other things.) With a boat submerged and properly trimmed, the planes are added to the rudder as the hydrodynamic surfaces. There are a couple of set-ups for planes. Boats have stern planes back aft. From there, some have bow planes up front, and some have planes on the superstructure, or sail (conning tower). A boat will have stern planes and bow planes, or it will have stern planes and sail planes. And the planes are used to control the depth. When the boat needs to surface or dive steeply, the bow and stern planes are used to "point" the boat in the desired direction. The angle that the vessel is moving through the water, the "bubble" of the boat, is changed. By tipping the bow planes up and the stern planes down, the boat can be pointed to the surface. Tipped oppositely, the boat heads down. For small changes in depth, both planes are eased in the same direction. Tip both up a bit, and the boat is pulled up through the water. And the boat's bubble isn't changed. Tip both planes down a bit, and the opposite effect results. The boat remains level while slowly changing depth. On the boats with the stern planes and the sail (or fairwater) planes, the depth is changed to dive or surface by using the stern planes in the same manner as in the boats that have both bow planes and stern planes. The fairwater or sail planes are not used. But for small changes of depth, the fairwater planes can be used by themselves. The fairwater planes are approximately amidships, and by tipping them up, the "down" force they impart "pushes the boat down" without changing the bubble of the boat. Make sense? The sail planes take advantage of the boat's neutral buoyancy. The force generated by the planes when they are used will act in a way that causes the boat to sink a bit more or rise a bit more without changing the bubble of the boat. It remains level while slowly changing depth. Use the links to look at pictures of boats and check out their plane configurations.