U-Boat trainee crewmen instructed on submarine buoyancy with the use of a transparent model

An interesting demonstration of the basic function of submarines in this 1943 Kriegsmarine footage. 0:16 The submarine is a state of positive buoyancy and the ballast tanks are filled with air. The tanks are open at the bottom, so when the top valves are opened, water pressure forces the air out and water fills the tanks, causing the submarine to lose buoyancy and submerge. 0:27 The submarine is down by the bow, however by moving liquid in the internal trim tank from the bow to the stern, balance is restored. Now neutrally buoyant, the submarine can maintain depth 0:58 With the submarine negatively buoyant and resting at the bottom, compressed air - in this case from a volunteer's lungs - forces the water out of the ballast tanks, restoring positive buoyancy and allowing the submarine to rise to the surface. I do this for my own amusement but if you would like to support the channel consider