Collision Risk in Underestimating Vessel Speed & Range


MAIB reports the investigation of a near miss accident between the ro-ro ferry Stena Superfast VII and the Royal Navy submarine.

Summary of the incident 

At 1256 on 6 November 2018, Stena Superfast VII’s officer of the watch took action to avoid collision with a submerged submarine that had been spotted at close range ahead of the ferry.

Stena Superfast VII was on a scheduled North Channel crossing from Belfast to Cairnryan; a Royal Navy submarine was at periscope depth conducting pre-deployment safety training
in the same vicinity.

The submarine’s command team detected and tracked the ferry using visual, sonar and automatic information system data. As the ferry’s range reduced, the submarine’s officer of the watch altered course to avoid it.

However, this turn was towards the ferry and reduced the time available for the submarine to keep out of the ferry’s way.


Safety issues directly contributing to the incident:

  • Until avoiding action was taken by Stena Superfast VII’s OOW, there was a serious risk of collision between a laden ferry and a submerged Royal Navy submarine.
  • Stena Superfast VII passed inside the submarine’s go-deep range, therefore, it was  unsafe for the submarine to remain at periscope depth.
  • It was extremely fortunate that Stena Superfast VII’s bridge AB spotted the submarine’s periscope, though there was no reasonable expectation he would do
  • Safety-critical decisions on board the submarine, specifically to turn towards the ferry and remaining at periscope depth, were taken based on inaccurate information.
  • Overestimation of the ferry’s range and under-estimation of its speed resulted in the submarine’s command system presenting an inaccurate surface picture. However, this situation meant that the unsafe decisions might have seemed rational at the time.
  • The submarine’s command team and the command qualified FOST sea rider demonstrated a bias towards the safer SMCS track that was based upon visual over estimations of the ferry’s range. This bias created a situation where other clues to the close proximity of the ferry could be ignored.
  • Perceived pressure to remain at periscope depth for training purposes might also have influenced the decision not to go deep.
  • Although the submarine’s passage plan had identifed the North Channel ferry hazard and the commanding officer had directed the OOW to remain south of the ferry lanes, the submarine was actually operating in the hazardous area.

Actions taken

Post this incident the RN reported that the following actions had been taken:

  • FOST shore-based simulator training was updated to enhance the management of close quarters situations with merchant or fishing vessels.
  • Submarine command teams were briefed on the critical importance of operating safely at periscope depth in coastal waters. This included a brief on the facts of this case to raise awareness of the potential risks posed to submarines and other vessels nearby.
  • Comprehensive learning from experience (LfE) events were delivered to submarine command teams prior to proceeding to sea.
  • Training and documentation for the operational use of AIS was reviewed.
  • FOST training was amended to ensure that, if a close quarters procedure was commenced, this was run to conclusion and not interrupted.
  • Incident reporting procedures have been reviewed and the amended policy reiterated to the submarine flotilla; commanding officers are also briefed on reporting requirements prior to taking command.
  • The decision to conduct safety training in areas of known high density shipping was reviewed and found to be justified. However, direction was given that a formal risk assessment should be conducted by FOST prior to safety training commencing.
  • All submarines operating near known shipping lanes and when operational circumstances permit, were recommended to use radar to provide increased accuracy of ranging.


The Royal Navy has taken a series of actions in response to this and similar previous accidents.

As a result, a safety recommendation (2020/124) has been made to the Royal Navy to undertake an independent review to ensure that the actions taken have been effective in reducing the risk of further collision.

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Source: Maritime Accident Investigation Branch


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