The USS Boise is an unusual visitor to Portsmouth Naval Base at the moment.
A US Navy Los Angeles Class Nuclear Attack Submarine, USS Boise has a displacement of 6,000 tons, and a crew of 129 men. She can fire Tomahawk Land Attack Missiles, Harpoon anti-ship missiles and conventional torpedoes. She was commissioned in 1992, and saw action in the second Gulf War in 2003.She is normally based in Norfolk, Virginia.
The security around US vessels is always tight when they visit foreign ports, and especially so with nuclear submarines. There is a 100 metre exclusion zone around the vessel, maintained by MOD police boats. When the boat I was on passed the Boise the US sailors on the deck tracked us through their binoculars!