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Air-independent propulsion (AIP) is a term that encompasses technologies which allow a submarine to...

Air-independent propulsion (AIP) is a term that encompasses technologies which allow a submarine to operate without the need to surface or use a snorkel to access atmospheric oxygen. The term usually excludes the use of nuclear power, and describes augmenting or replacing the diesel-electric propulsion system of non-nuclear vessels. The United States Navy uses the hull classification symbol "SSP" to designate boats powered by AIP, while retaining "SS" for classic diesel-electric attack submarines. AIP is usually implemented as an auxiliary source. Most such systems generate electricity which in turn drives an electric motor for propulsion or recharging the boat's batteries. The submarine's electrical system is also used to provide "hotel services"—ventilation, lighting, heating etc—although this consumes a small amount of power compared to that required for propulsion. A benefit of this approach is that it can be retrofitted into existing submarine hulls by inserting an additional hull section. AIP does not normally provide the endurance or power to replace the atmospheric dependent propulsion, but allows it to remain submerged longer than a more conventionally propelled submarine.