More from Thetis survivor's son on Mersey Ferry near-miss

The Cammell Laird-built submarine sank 70 years ago with the loss of 99 lives. Tragedy struck hours after it left Birkenhead for its sea trials with 103 people on board - twice the number she was designed to carry.

Only four managed to escape.

Derek Arnold, who lives in Bebington, followed his father to sea by joining the Merchant Navy. Now in his 70s, he is chaiman of the Liverpool Anchorage Club.

This is his take on the near-miss, including some comments that didn't make it into the stories in the Wirral News and Liverpool Echo:

"The recent near miss incident in the River Mersey between the ferry boat and the tanker clearly demonstrates that despite all the modern sophisticated navigational technology available to mariners, the one piece of visual equipment that remains the most reliable is the human eye especially when ships are manoeuvring in confined waterways.

"The rule of the road at sea is that for ships approaching on a converging course, as long as they keep green to green or red to red, perfect safety lies ahead i.e. Starboard sides passing each other or alternatively, port sides passing each other as the ships go by, all will be well.

"Sadly this rule does not hold good for a vessel passing across the bows of an oncoming ship. It will remain for the forthcoming inquiry into the near miss to establish if it may have been possible that each Captain assumed that the other was about to take evasive action until the moment that they both realised that they were wrong.

"Although the photographs taken from the deck of the ferry show that it must have been a heart stopping moment as the ships passed each other at what may be described as touching distance, it must be remembered that in fact distances over water can be very deceptive.

"No-one would appreciate this more than the person on the bridge of the ferry who has to use his navigational skills many times over in the course of a day whilst travelling backwards and forwards between the Pier Head, Seacombe and Woodside.

"Regular passengers must frequently feel that a collision with the quayside seems inevitable only to be pleasantly surprised when the ferry gently slides alongside with hardly a bump, particularly in the bad weather conditions that all too often occur in the Mersey.

"Passengers on the ferries should have full confidence in the abilities of the crews to use their considerable expertise to deliver a safe crossing."

An investigation into the incident is being carried out by Mersey Ferries, the Mersey Docks and Harbour Company and the Maritime and Coastguard Agency.