A crash dive is a maneuver used by a submarine to dive urgently, typically in the event of an emergency. Used primarily in the diesel-electric submarines of World War II, the order to crash dive was given by the captain when contact with enemy surface or air vessels was detected. In a crash dive scenario, the forward ballast tanks are flooded in conjunction with using the diving planes to take the submarine down as quickly as possible. It is critical that all hatches be closed and sealed prior to the submarine becoming submerged in a crash dive.
A method used by German submariners to ensure the utmost speed in submerging during a crash dive is to have all available personnel move as far forward as possible. The weight transfer of having all personnel moving forward aids the submarine in nosing down into the water. This places the diving planes under the water's surface sooner as well, assisting in the expedited crash dive of the vessel. Periodically and in extreme situations, some crew members were occasionally left top side as the submarine went deep. This commonly resulted in death as well as the loss of the crew members' bodies.
The typical crash dive depth of a submarine is 90 meters or about 295 feet. Once the submarine has reached this level, it can then level out and begin to assume a cruising speed. Once submerged, the diesel-electric submarine was required to switch off the diesel engines and use the electric battery motors for power. Using the diesels while submerged will use all available air within the submarine in a matter of minutes. The electric motors are also much quieter than the diesels, which aids the submarine in becoming stealth while attempting to avoiding detection by an enemy.
Modern nuclear submarines remain submerged for most of the duration of the mission, making a crash dive an unneeded exercise; however, all submarine crews practice the crash dive in the event of a collision with another vessel or object while surfacing. Once the order for the dive has been given by the ship's captain, the chief engineer is commonly responsible for the orchestration and completion of the entire maneuver. When making the fast dive, it is critical that the rear ballast tanks be flooded in time to keep the rear of the vessel from coming out of the water. If this happens, the vessel becomes a sitting target while it struggles to once again regain the momentum to complete its dive.