It has recently emerged that a submarine came close to a Stena Line ferry in the Irish Sea in November last year. The ferry, the Stena Superfast VII was on its usual route between Northern Ireland and Scotland when it sighted the periscope of the submarine.
A spokesman for the ferry company has reported:
“Stena Line can confirm that on Tuesday 6 November 2018, Stena Superfast VII and a submerged submarine came into close proximity during a scheduled crossing between Cairnryan and Belfast. At no stage were the vessel, passengers or crew in any danger. The incident is now under investigation by the Marine Accident Investigation Branch and we will, of course, co-operate fully in this.”
The Navy, apart from confirming that a sub sighted the other vessel, have made little further comment – which is normal for any such encounters between nuclear-powered submarines and civilian vessels where now actual collision occurred.
That said, the Marine Accident Investigation Branch (MAIB) has confirmed that:
“In November, we were notified of a close-quarters incident between the ro-ro ferry Stena Superfast VII and a submarine operating at periscope depth. We have carried out a preliminary assessment of the evidence in this case and the chief inspector of marine accidents has decided to open a safety investigation. The investigation is being conducted with the full co-operation of the Royal Navy. A report will be published when our investigation has concluded.”
Such incidents, though rare, are not unknown and it is possible that submarines operating at periscope depth have encountered many more civilian vessels in this way than are reported in the open press. Periscope Depth (PD) can be anything less than 100ft and it is common for a sub to cruise in busier waters at PD. It also means that the sub’s crew might be very aware of the situation around them whereas the ship’s crew might only see the periscope’s trail or “feather” at the last moment, causing a certain degree of alarm and the impression that they might have been in danger.