USS Minneapolis-St Paul was 'close to grounding'
A US nuclear submarine came close to grounding during an incident in which two crewmen were swept overboard off Plymouth, a Navy report said.
The USS Minneapolis-St Paul was leaving Plymouth in rough seas when the tragedy happened on 29 December 2006.
The report said "human failure" was to blame and said the outcome could have been "much more catastrophic".
The 2007 report has been made public following a Freedom of Information request.
The incident happened as the US submarine navigated through Plymouth Sound following a week spent docked at Devonport naval dockyard for Christmas.
While weather conditions inside the Sound were relatively calm, beyond Plymouth breakwater they were becoming increasingly rough.
Three crewmen who were preparing to help the British harbour pilot on board disembark from the submarine to a waiting pilot boat were swept overboard by high waves when the submarine left the shelter of the breakwater.
According to the report, two of the men, who were wearing safety harnesses, were repeatedly thrown against the casing of the submarine "like rag dolls".
Senior Chief Thomas Higgins, 45 and Petty Officer 2nd Class Michael Holtz, 30, were rescued a short while later but pronounced dead in hospital.
The third crewman managed to climb back into the submarine and two others who were swept overboard without harnesses were picked up by nearby boats.
The report said that while the men were being rescued the submarine "came within less than her own length of the Panther Shoal [breakwater] which, given the height of the tide and sea state, was close to grounding the submarine."
It concluded that the incident happened because there were men on the casing [outside] of the submarine as it entered "very rough seas".
It said: "This was a severe incident with multiple loss of life. There was a very real possibility of the boat grounding in very rough seas.
'Lessons not learned'
"In addition the crew's mess hatch remained open in these conditions allowing a considerable amount of water into the submarine.
"Tragic as the loss of lives of Holtz and Higgins was the outcome could have been so much more catastrophic."
Following the incident the commanding officer of the submarine and the British pilot both lost their jobs.
They had been responsible for assessing whether it was safe for the men to help the pilot disembark beyond the safety of the breakwater.
The report said that lessons had not been learned from a similar incident involving the British submarine, HMS Sovereign, in February 2006.
It also said there had been a failure to identify potential hazards and risks on the day and that people who had concerns about the adverse whether conditions had failed to speak up.
It concluded that: "A more effective safety culture is required across all authorities that operate in the Dockyard Port of Plymouth, if further such incidents, perhaps with even more severe outcomes are to be avoided."
In repose the Royal Navy said: "Naval Base Commander Devonport (NBCD) commissioned a review of the findings from the reports into the investigation to identify lessons learned.
"As a result, there have been improvements in the dissemination of lessons learned more widely, not only those resulting from the Minneapolis St Paul accident."
An investigation by the US Fleet Forces Command found the Plymouth harbour pilot had not shared concerns about "potentially dangerous sea states" in the area with the American crew.