Concept

Crash dive

Summary
A crash dive is a maneuver by a submarine in which the vessel submerges as quickly as possible to avoid attack. Crash diving from the surface to avoid attack has been largely rendered obsolete with the advent of nuclear-powered submarines, as they normally operate submerged. However, the crash dive is also a standard maneuver to avoid a collision. A crash dive in a diesel-powered submarine requires careful orchestration of the crew. On German U-boats of World War II, a crash dive began with the Captain or senior lookouts giving the order "Alarm!" which led to a bridge officer activating the alarm bell. All crew members then immediately stopped what they were doing and proceeded to their diving stations. Once the lookouts were below deck and the upper deck hatch was secured, the Captain or Chief Engineer shouted the order, "Fluten" ("flood the tanks"). With the bow planes at a maximum down angle, the crew then flooded the forward ballast tanks. Often, all available crew members moved as far forward in the boat as practical (a "trim party"). This extra weight forward gave the boat a bow-down angle so its momentum helped pull it below the surface. A few seconds later, the crew would flood the rear ballast tanks to prevent the bow-down angle from lifting the boat's stern out of the water. The entire crash dive was generally coordinated by the Chief Engineer. Before hatches and air induction vents fall below the surface they must be closed. Before that, the diesel engines must be stopped or they will suck the air out of the boat in a matter of seconds. On submarines with direct drive, the crew disengages the diesel engines from the propeller shafts and switches to electric motor propulsion. The motors run at high speed to maintain the forward momentum. Once all hatches and induction vents are closed, the diving planes (like the control surfaces of an airplane) pull the boat below the surface and level it out at the desired depth—typically between . In a World War II-era boat, the whole operation could take as little as 30 seconds with a well-trained crew.
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