The Karen was trailed backwards at seven knots in April 2015 after the sub snagged in its nets 15 miles from Ardglass – one of Northern Ireland’s main fishing ports in the south east.
Commanders were unaware of the collision until three hours later after they failed to realise they were passing beneath a fishing vessel with nets rather than a merchant cargo or passenger ship, the Marine Accident Investigation Branch (MAIB) report said.
Steve Clinch, chief inspector of marine accidents, said: “Eighteen months ago, the actions of the command team of a Royal Navy submarine placed the lives of the crew of the trawler Karen in danger.
“Regrettably, the reluctance of the Royal Navy to fully engage in the subsequent investigation resulted in this report taking significantly longer to deliver than would normally be the case.
“The accident happened because of insufficient passage planning by the submarine’s command team and their failure to follow guidance on fishing vessel avoidance.
“Had its trawl warps not parted, it is almost inevitable that Karen would have capsized and sunk; the collision also presented a very significant risk to the submarine.”
Part of the prawn trawler’s deck had to be lifted and another section was ripped off but the four crew escaped unharmed.
Mr Clinch said lessons learnt after the sinking of a trawler in 1990 had been lost.
“As a result, it is now important that the Royal Navy reviews its procedures and training for the safe conduct of dived submarine operations in the same vicinity as vessels engaged in fishing.
“By its actions, the Royal Navy also needs to rebuild trust with the fishing industry.”
The Karen’s crew managed to release net equipment but the accident caused it to heel heavily to port and its stern was submerged.
The report said: “Evidence of the collision on board the submarine was either not observed or misinterpreted.”
It said the accident occurred because the submarine’s command team detected no noise of trawling.
“The submarine was at a depth where it could, if necessary, pass safely beneath a merchant vessel, therefore the command team would not have perceived any risk of collision; as a result, no avoiding action was taken.”