Each one of the 14 US SSBN Strategic Missile Submarine Ohio-class nuclear submarines has 24 SLBM Surface Launched Ballistic Missiles of the type called Lockheed Trident II with stellar and inertial guidance (inertial navigation).
Each missile has eight MIRVs (multiple independently targetable reentry vehicles) that can be assigned unique targets with either 475 kilotons alt. 100 kilotons combat charges. Trident II D-5 has a range of up to 11,300 km (7,000 miles) according to a British Wikipedia page, and the corresponding US Wikipedia page rounds up the number to 12,000 km (7,450 miles) but claims it has a longer reach than that.
The exact range, mentioned on the US Wikipedia page, is a secret. Elsewhere it is mentioned that Trident II D-5 can hit targets at 7,400 km (4,600 miles) distance. There is a big difference between 11,300 km and 7,400 km, and the divergence is probably due to the fact that the two types of nuclear warheads weigh differently depending on their charge.
As it is almost 7,400 km from New York to Moscow for a Jet aircraft, and let us say that a ballistic missile fly 7,500 km from New York to Moscow, the shorter weapon range or 7,400 km, means that an Ohio-class submarine based in New York which is out for a month of service at sea will spend approximately 2.5 hours of these thirty days to sneak under the surface at ~46 km/h to reach the outer water where their nuclear weapons with the greatest total charge can reach target Moscow. And perhaps a more moderate speed will be used in the other direction towards the US coast, but I don’t mix that into the numbers here.
This means that maybe 1.5 percent of the second strike ability of the United States could be omitted already there. Approximately 25 percent of the US strategic submarine fleet at any given moment will be in port for service work and repair that cannot be completed onboard the submarine at sea and/or outside the dock.
In addition, the crews must be allowed to spend time with their families in port and the submarines must be buffeted with food regardless of whether their submarine is in need of service and maintenance in port or not. Although there are at least two sets of crews, and a submarine can often be out to sea many times longer if the submarine is buffeted from surface ships.
To all this must be added that the submarines’ nuclear weapons must be maintained at regular intervals, perhaps every five years, and this is not done on board the submarine.
Overall it reduces the second strike capacity by 35 percent or more in the worst case. Alternatively, the detonation blast from a multiple early nuclear missile launch from a given submarine is reduced, close to the home coast, with up to ~80 percent if half of the missiles have 475-kilo warheads with shorter range for the nuclear missile, but remain un-fired because of their shorter range, and the remaining half are only 100 kilos charges that can be fired from the waters directly from the New York coast.
If this ratio applies to as little as 1.5 percent + 25 percent of the United States second strike capability, at least 71,000 kilotons of TNT can be removed from the equation, at least in the immediate phase for a time period of up to just over five hours. For Russia the calculations might be similar or even more discouraging.
These Ohio-class Trident II nuclear submarines make up the US strategic second strike capability and a total of 50 percent of the US strategic nuclear capacity.
There are also 4 Ohio-class SSGN Cruise Missile nuclear submarines capable of carrying 154 Tomahawk Block III or Block IV cruise missiles. The Tomahawk Block IV missile is an all-weather surface- and submarine-launched precision strike stand-off weapon.
Designed for long range precision strike missions against high quality and heavily defended targets, the Tomahawk has a maximum range of 1,600 km (1,000 miles). It travels at subsonic speed – 890 km/h (550 miles per hour).
The US surface combat fleet phased out TLAM-A, the nuclear warhead version of Tomahawk, a number of years ago. So the US surface combat fleet is not any longer armed with nuclear weapons, just like the British surface combat fleet isn’t. Today, all of the Tomahawks the surface combat fleet have are conventional missiles, either in the form of a single combat part (TLAM-C, TLAM-E) or with submunition parts (TLAM-D).
It is simply not true that one strategic submarine can annihilate a not so small country as sometimes mentioned in the debate. But a few of them could. In this link you can see for yourself the effect of a nuclear weapon, which you can give various size charges, in an attack on any city or city in any country:
The numbers in this text is partly an approximation.
Roger M. Klang