A French trawler that sank suddenly off the Cornish coast with the loss of all five crewmen most likely became pinned to the sea bed when her nets caught in sediment, an inquest has heard.

The Bugaled Breizh went down on January 15, 2004 off the Lizard Peninsula in favourable conditions, with no apparent defects to the vessel.

The possibility a submarine may have become entangled in the trawling gear and dragged the vessel under has been proposed since it sank.

But an inquest at the High Court heard on Thursday that the system of wires, chains, weights and nets that made up the trawling rig was found relatively intact on the seabed.

It did not have the level of damage consistent with being entangled with a fast-moving, powerful military submarine.

Captain Yusuf Soomro, an independent maritime investigator, conducted an analysis of the evidence gathered by France’s marine accidents investigation body Bureau d’Enquetes sur les Evenements de Mer (BeaMer).

It found that one of the vessel’s trawl doors – two sheets of metal used to keep the mouth of the net open – had become buried in sediment and mud close to a depression in the seabed.

Captain Soomro said that it was likely that the weight on the port trawl door had caused the net to close, putting a huge amount of pressure on one of the cables, known as warps, connecting the net to the boat.

“Whenever you have some object pulled by two identical forces and one gets destabilised, the other one automatically gets destabilised as well,” he said.

“This is exactly what has happened – the geometry of the trawl rig has been disturbed.”

Investigators believe the Bugaled Breizh’s trawling gear became entangled on the seabed Picture: Field Fisher/PA

Investigators believe the Bugaled Breizh’s trawling gear became entangled on the seabed Picture: Field Fisher/PA

According to modelling, the vessel was travelling to the north east and the prevailing conditions would have caused it to be rolling from side to side.

The pressure on the port warp would have caused it to lean heavily, allowing huge volumes of water to be shipped on to the main deck.

Because the vessel’s deck would now have been very close to the waterline, the water would not be able to escape via the “freeing ports” that are cut at the top of the hull.

“The vessel would have come to stop within about five seconds, there would be an increased tension on the port warp and it would have a sideways component, the vessel would turn to port and take on a sideways list (lean),” Captain Soomro said.

“The combined effect of the wind and sea swell would reduce the vessel’s stability and prevent it from righting itself.”

BeaMer’s analysis found that the water would very quickly have found its way into the crew accommodation in the vessel.

“As more and more water gets deposited on deck and more and more water finds its way down into the crew accommodation you reach a point where there is no return,” Captain Soomro said.

He said once the vessel had reached an angle of 30 degrees in the water, capsize “would have been very rapid”.

There was evidence the crew of the Bugaled Breizh had tried to right the vessel by releasing the port warp of the trawl net, which was found to be 140 metres longer than the starboard warp when the vessel was recovered.

“I think the issue here is that snagging is quite a routine event that happens on a fishing vessel, but sometimes if you don’t react correctly to the situation then things can turn nasty very quickly.”

Captain Soomro said boats that become snagged usually release both warps and cut their engines.

They can sometimes try to “wriggle” out of the snag by steering rapidly to port and starboard – but only if conditions are calm.

When asked if the soft snag theory put forward by BeaMer was the most plausible, he replied: “In my opinion, yes.”

BeaMer were unable to ascertain whether the vessel was being steered manually or on autopilot when it ran into difficulty, the court heard.

The inquest is only considering the deaths of skipper Yves Marie Gloaguen, 45, and Pascal Lucien Le Floch, 49.

Their bodies were recovered in the hours after the ship sank and taken to the Royal Cornwall Hospital.

It means that under English law an inquest must be held here.

The body of a third man, Patrick Gloaguen, 35, was recovered during a salvage but was taken to France, while the bodies of Georges Lemetayer, 60, and Eric Guillamet, 42, were never found.

As a result, their deaths are not the subject of the inquest, although their families are taking part in the process.

Judge Nigel Lickley QC is expected to deliver his conclusion into the deaths next Friday, October 22.

The inquest continues.