<P> The class's design allows the boat to operate for about fifteen years between major overhauls . These submarines are reported to be as quiet at their cruising speed of 20 knots (37 km / h; 23 mph) or more than the previous Lafayette - class submarines at 6 knots (11 km / h; 6.9 mph), although exact information remains classified . Fire control for their Mark 48 torpedoes is carried out by Mark 118 Mod 2 system, while the Missile Fire Control (MFC) system is a Mark 98 . </P> <P> The Ohio - class submarines were constructed from sections of hull, with each four - deck section being 42 ft (13 m) in diameter . The sections were produced at the General Dynamics Electric Boat facility, Quonset Point, Rhode Island, and then assembled at its shipyard at Groton, Connecticut . </P> <P> The US Navy has a total of 18 Ohio - class submarines which consist of 14 ballistic missile submarines (SSBNs), and four cruise missile submarines (SSGNs). The SSBN submarines are also known as "Trident" submarines, and provide the sea - based leg of the U.S. nuclear triad . Each SSBN submarine is armed with up to 24 Trident II submarine - launched ballistic missiles (SLBM). Each SSGN is capable of carrying 154 Tomahawk cruise missiles, plus a complement of Harpoon missiles to be fired through their torpedo tubes . </P> <P> The first eight Ohio - class submarines were armed at first with 24 Trident I C4 SLBMs . Beginning with the ninth Trident submarine, Tennessee, the remaining boats were equipped with the larger, three - stage Trident II D5 missile . The Trident I missile carries eight multiple independently targetable reentry vehicles (MIRV), while the Trident II missile carries twelve, in total delivering more destructive power than the Trident I missile and with greater accuracy . Starting with Alaska in 2000, the Navy began converting its remaining ballistic missile submarines armed with C4 missiles to carry D5 missiles . This task was completed in mid-2008 . </P>

How many nuclear armed submarines does the us have