MCM are a type of naval ship designed for the location of and destruction of naval mines which combines the role of a minesweeper and minehunter in one hull. The term MCMV is also applied collectively to minehunters and minesweepers. A "minesweeper" is a small naval warship designed to engage in minesweeping. Using various mechanisms intended to counter the threat posed by naval mines, waterways are maintained clear for safe shipping. A "minehunter" is a naval vessel that actively detects and destroys individual naval mines. (11 of 14 in active service, 2 decommissioned, 1 lost to accident) Patrol ships Patrol ships PC are relatively small naval vessels generally designed for coastal defense duties. There have been many designs for patrol boats. They may be operated by a nation's navy, coast guard, police force or customs and may be intended for marine (blue water) and/or estuarine or river ("brown water") environments. They are commonly found engaged in various border protection roles, including anti-smuggling, anti-piracy, fisheries patrols, and immigration law enforcement. They are also often called upon to participate in rescue operations. Vessels of this type include the original yacht (from Dutch/Low German jacht meaning hunting or hunt), a light, fast-sailing vessel used by the Dutch navy to pursue pirates and other transgressors around and into shallow waters. (14 built, 13 in active service, 1 transferred to the Philippine Navy) Submarines Submarines SSN, SSBN, SSGN are watercraft capable of independent operation underwater. There are currently two types; attack and ballistic. "Attack submarines" (SSN) have tactical missions, including controlling naval and shipping activity, serving as cruise missile-launching platforms, and intelligence-gathering. "Ballistic submarines" (SSBN) primarily have the single strategic mission of nuclear deterrence by being hidden launching-platforms for nuclear SLBMs. However, some of these boats have been converted to (SSGN) and launch standard cruise missiles. (attack submarines: 28 in active service, 34 retired of 62 planned) (attack submarines: 3 in active service of 3 planned) (attack submarines: 21 in active service of 48 planned) (ballistic missile submarines: 14 in active service of 18 planned) (Ohio class converted to guided missile submarines: 4 in active service) Future requirements In a 2012 study called the "Force Structure Assessment", the Navy determined a post-2020 battle-force requirement of 306 ships. 12 fleet ballistic missile submarines 11 nuclear-powered aircraft carriers 48 nuclear-powered attack submarines 0-4 nuclear-powered cruise missile submarines 88 large, multi-mission, surface combatants 52 small, multi-role, surface combatants