How did the superyacht Bayesian sink? Experts say weather was just one factor

A complicated search effort is underway deep beneath the surface of the Mediterranean Sea, off the coast of Italy, where a superyacht sank early Monday during a fierce storm.

Dive crews are attempting to enter the wreck of the Bayesian, a 56-metre long British-flagged luxury vessel, which is sitting in some 50 metres of water near the Sicilian fishing village of Porticello.

Fifteen of the 22 passengers and crew members on board were rescued. Diverssearching for six people considered missing, including British businessman Mike Lynch, located fivebodies inside the yacht's hull on Wednesday, and retrievedfourof them from the water.

Searchers previously recovered the body of Recaldo Thomas, the ship's Canadian-Antiguan cook,in the water not far from where the Bayesiansank.

Questions have emerged about why a boat designed to handle severe weather sank so rapidly and whether or not someof its features could've been a factor in its demise. Maritime experts say investigations may, in time, reveal what led to the disaster.

"There needs to be an investigation as to why this happened, what went wrong and, you know, how to prevent it in the future cases,"said Simon Boxall, an oceanographer and senior lecturer at the University of Southampton in England.

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How did the Bayesian sink so quickly?

Grainy footage from closed-circuit cameras on the shorebroadcast on the website of the Giornale di Sicilianewspaper showedthe Bayesian's majesticmastjust before it disappeared.

Karsten Borner,captain of the Sir Robert Baden Powell, which rescued the survivors who managed to get into a lifeboat, told The Associated Press he was close enough to be able to see the Bayesian as the storm came in.

"A moment later, she was gone," he said.

The survivors told the rescuersthey went flat on the water "and were sunk in two minutes."

Boxall said vessels rely on being able to steer and navigate stormy seas, but the Bayesian was anchored and stationary, which likely made it more vulnerable to the stormand, potentially,a waterspout, or mini tornado that creates a whirlwind over the surface of the sea.

He said it was also dark, meaning "you wouldn't see this sort of very unique event coming towards you."

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The mast lights of the yacht Bayesian, anchored at Porticello, Italy, can be seen in this CCTV footage as the storm intensifies and they become obscured. It is unclear at which point the yacht capsized.

"It's a freak of nature," Boxall said. "I think the fact that so many have survived, so far, is probably a miracle."

Tom Sharpe, a retired Royal Navy commander and defence commentator, told CBC News the weather was likely not the only issue.

He saysit's rare that a weather event like this takes down a boat.

"There's nearly always a sequence [of events]," he said in an interview from Guildford, England, explaining that everything from safety protocols to the culture on board the vessel needs to be taken into account.

Was the boat's design part of the problem?

The Bayesian was built in 2008 by Italian luxury yacht maker Perini Navi.

Andrea Ratti, a nautical design professor at Milan Polytechnic, told Reuters that a boat the size of the Bayesian could only sink so rapidly by taking on a huge amount of water.

He suggested that one or more portholes, windows or other openings may have been broken or smashed open by the waterspout, letting in water.There has also been media speculation that a major hatch might have been inadvertently left open.

Reports have also highlighted that the Bayesian featured a 72-metre mast one of the tallest in the world.

Ratti said an unusually tall mast is not by itself an element of vulnerability in a storm.

A second expert, structural engineer Filippo Mattioni, was also skeptical about the suggestion the boat may have sunk due to a broken mast, which likely would have caused major damage smashing against the hull.

Fire department diver Marco Tilotta told the newspaper Il Messaggero that the wreck was "apparently intact," with "no gashes, no signs of impact." However, only half of the hull is visible to divers.

The Bayesian also had a retractable keel the fin-like structure under the hull that helps stabilize boats and acts as a counterweight to the mast.

Both Ratti and Mattioni wondered if the yacht had been anchored with the keel up, reducing the vessel's depth under water and making it less stable.Ratti saidstrong winds might have caused the boat to start oscillating wildly, "like a pendulum," putting exceptional strain on the mast.

Sharpe pointed out that a mast the size of the Bayesian's is designed fora massive sail, and without that sail raised and catching the wind, the gusts likely would'vehad a negligibleimpact onthe aluminum pole.

He instead suggestedthe anchor may haveplayed a pivotal role.

"My kind of working assumption is that she was probably a bit further in at anchor, and it's very likely, in these sort of conditions, that her anchor dragged," he said.

In such a situation, he said, a crew is better off steering toward the anchor to stabilize the vessel or raising the anchor and heading out to sea to ride out the storm.

"They might have got caught in that middle ground where they're not on a particularly good anchorage, but the anchor is now controlling the bow of the ship."

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What caused the extreme weather?

Although Sharpe says the weather is unlikely the sole cause of the sinking, he notesthe Mediterranean isn't the calm sea often pictured in travel brochures.

"It can get pretty nasty," he said.

The type of storm that struck Monday is fuelled by warm water and the Mediterranean is warmer than ever, said Boxall, noting there's been about a three and a half degree increase in the 20-year average temperature.

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Karsten Borner describes seeing the British-flagged Bayesian capsize, and his subsequent rescue of some passengers, after the luxury yacht was struck by an unexpectedly violent storm and sank off the Sicilian capital Palermo.

Climatologists say global warming is making such violent and unexpected tempests more frequent.

Luca Mercalli, president of Italy's meteorological society, said the sea surface temperature around Sicily in the days leading up to the shipwreck was about 30 C.

"This creates an enormous source of energy that contributes to these storms," he told Reuters.