On board HMS Northumberland with ship's chaplain
HMS Northumberland monitors undeclared submarine activity, and protects the UK's nuclear deterrent, among other roles.
The ship's chaplain, Reverend Dr Louisa Pittman, told the BBC that the four-month deployment has been tough and unusual.
The BBC joined her and the crew on board.
Watch The Submarine Hunters on BBC iPlayer now
Reporter: Claire Jones
Video Journalist: Alex Dackevych