The American Enterprise Institute makes the case for Diesel Submarines.
The US Navy should procure a fleet of diesel-powered subs. Not only are diesels cheaper than nuclear-powered subs, but they have the advantage of being better platforms for many of the tasks the Navy faces today. The list of actual and potential submarine missions, including close-in intelligence, surveillance and reconnaissance, special operations, and blockade and mining, continues to grow.
These growing operational demands are coupled with the exigencies of new undersea requirements. In addition to the deep-sea dives and prolonged blue-water missions that became the staple of submarine operations during the Cold War, there are a number of scenarios today that are focused on the littoral areas, the green water within 100 miles of land, be they in the strait of Hormuz or Malacca, off the shores of Taiwan or in the South China Sea.
It is these missions that often favor diesel submarines. Diesel subs are smaller, stealthier and more maneuverable in tight spaces than nuclear submarines. For example, unlike a nuclear submarine's power plant, a diesel's primary engine can be turned off when submerged, reducing noise emission. Indeed, unlike a nuclear-powered submarine, a modern diesel can hide on the ocean's floor, deadly silent, while monitoring whatever passes over and around it.
And with the advent of Air Independent Propulsion (AIP) technology, today's diesel subs can remain submerged for weeks at a time. When deployed to bases in the Far East or Middle East, the range and reach of today's AIP-equipped diesels would put them well within striking distance of critical choke points.
Germany's Type 212 subs were sold for approximately $500 million versus the $2 billion for a Virginia-class nuclear attack submarine - the Navy would be able to ramp up submarine production without breaking the bank.
Wikipedia on Air Independent Propulsion
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.
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 for providing "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 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. A typical conventional power plant will provide 3 megawatts maximum, and an AIP source around 10% of that. A nuclear submarine's propulsion plant is usually much greater than 20 megawatt
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