The term "silent running" is sometimes used to describe the standard operating procedure used by the crew on a submarine when they wish to become as undetectable as possible. It involves the observation of "noise discipline," minimizing noises which could alert other ship traffic to the presence of the submarine. Many people have romanticized the concept of silent running, thanks to war films featuring tense-jawed submarine captains and nervous crew.
In anti-submarine warfare, the biggest challenge is finding a submarine to attack. Fortunately for people trying to find subs, submarines present very large acoustic targets, showing up on sonar with some very distinctive signature sounds. The engines on board the ship generate noise, as do the propellers used to move the ship, and the air pockets created by propulsion also create a unique signature. Simply lowering a microphone into the water can identify all of these sounds, and very sensitive acoustic equipment can pick up the sounds of walking and talking crew members and other ambient noise created on board ship. This is known as passive acoustic observation, and it is very useful, because a passive ship can look for other ships while remaining silent, whereas in active mode, it would have to emit soundwaves to search for subs, making itself a potential target.
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During the Second World War, when a submarine captain ordered "silent running," the engines of the ship would be shut off, allowing the ship to sit motionless in the water or on the seafloor, depending on where the sub was. Crew members would be expected to remain silent while they listened for signs of the enemy. At times, submarines entered into stalemates with each other, with one ship ultimately having to turn its engines back on to surface for fresh air.
Many modern submarines have propulsion systems which are designed to address the noise issue, using things like nuclear power to generate energy as noiselessly as possible. In addition, their hulls are heavily insulated to provide sound dampening, and crew members may wear special garments and shoes to make themselves as noiseless as possible. When silent running is ordered, people are simply expected to be as quiet as possible, making minimal conversation and being very aware of what they are doing to reduce the risk of dropped tools and other sounds.
Silent running and noise discipline are drilled into crew members at an early stage of their training, ensuring that the habit of noise discipline comes naturally to them. Submarines may go into silent running while out on patrol in hostile areas, or to practice their skills; in some instances, submarines have also been known to lie in wait on the continental shelf in total silence, waiting for an enemy ship to cross into territorial waters and then springing into action.