Seven crab fishermen were taken to hospital after their boat triggered a bomb from the Second World War off the Norfolk coast.
Five crew members on board the Galwad-Y-Mor suffered significant injuries in the explosion on December 15, the Marine Accident Investigation Branch (MAIB) said.
“Although the physical injuries were significant to five of the seven crew, they were fortunate not to be killed,” it said.
The fishermen were hauling in a string of 100 crab pots from the seabed when they noticed “a lot of tension” on the main line, an MAIB report said on Thursday.
The skipper revved the engine in a bid to break the line free, but at 11.22am, three loud bangs resounded on the main deck.
The move had disturbed an unexploded 250-kilogram bomb that had been dropped in the sea during the war, the investigation found.
The MAIB report said it was a German-made high explosive — probably an SC250, a bomb widely used and feared during the Blitz.
The blast triggered shock waves that sent the boat rocking, the MAIB said.
The boat's power immediately cut out and water began flooding the deck as the skipper roused the nightwatchman and sent a distress call to the coastguard.
The wheelhouse was wrecked and the hull plating ruptured, while the engine, gearbox and switchboard were sheared from their mountings.
At 11.48am, a search and rescue helicopter was sent to the accident site, about 35 kilometres away from Cromer in north Norfolk.
The captain of another nearby vessel, the Esvagt Njord, heard the distress call from around six kilometres away and sent a rescue boat to save the crew.
The fishermen were hoisted on board the Esvagt Njord and given first aid.
Two were flown out and taken to hospital at about 1.50pm, while the others were taken to hospital later in the afternoon.
A shard of metal later found in a crab pot was sent to specialists for forensic examination.
Analysis suggested to “a high degree of certainty” that it came from an SC250.
The report said unexploded bombs remain highly volatile, even after many years underwater.
It ruled the crew, whose training, experience and emergency preparedness helped them survive, could not have anticipated what happened.
“The aim of this report is to highlight the dangers that still exist with unexploded ordnance in the seas around the UK and the actions to take should fisherman encounter any,” the MAIB said.
“In this case, the skipper and crew could not have foreseen the explosion and their level of preparedness to deal with such an emergency saved lives.”