How stable are cruise ships like the Costa Concordia?

One of Europe's largest cruise ships, the Costa Concordia, carrying 4200 passengers and crew, suffered a fatal and spectacular accident on Saturday. The vessel was holed on rocks off the Italian island of Giglio ? and then quickly keeled over, preventing lifeboats on its port side from being lowered and trapping some passengers and crew in the bowels of the ship. At 12:00?GMT today, six people had been confirmed dead and a further 15 were missing. So how stable and safe are these vertiginous floating multistorey hotels?

Why was this massive ship so close to shallow rock outcrops?
Mark Staunton-Lambert, technical director of the London-based Royal Institution of Naval Architects, says this is the main question investigators will want answering. GPS and sonar instruments should have warned of the danger, he says.

Why might the Costa Concordia's depth-sounding sonar have been ignored?
Like aviation, seafaring is in the midst of major computerisation, with bridges in modern ships like Costa Concordia becoming "glass cockpits". The transnational maritime trade union Nautilus International says that the technology at the heart of this ? the Electronic Charts Display and Information System (ECDIS), which marries GPS and seabed sonar data in one screen ? can be a problem. First, it says that the data on seabed obstacles can be out of date; second, the system generates too many alarms that can lead mariners to ignore them. "The ECDIS screens are only as good as the data that goes into them," says Nautilus spokesman Andrew Limington. "And there are major problems with their user interfaces and ergonomics."

So is that what went wrong on this occasion?
Possibly. The captain Franceso Schettino is under arrest and could face manslaughter charges, though he claims his electronic charts showed the rocks to be 100 to 150 metres away when his ship struck them. The BBC quotes his boss, Costa Cruises' chief executive Pier Luigi Foschi, saying the vessel ran aground after making an "unapproved, unauthorised" course deviation.

The ship keeled over pretty quickly. It's 13 storeys high yet only 8.2 metres of the vessel sit below the waterline. How can that be stable?
The International Maritime Organization (IMO) specifies the stability that ships must have ? and if a vessel complies with those rules it should be fine, says Staunton-Lambert. All the heavy stuff ? the engines, water ballast tanks and fuel oil ? are kept low in the hull, and the tall accommodation blocks above are largely empty space peppered with much lighter contents: people and furniture. "These cruise ships may seem high," says Staunton-Lambert. "But the trick is to ensure that the weight distribution is correct, focusing on where the centre of gravity is."

How is the degree of stability of such a ship decided?
The IMO regulations specify how quickly the vessel must return to upright when it has been upset by freak weather, a grounding or a collision ? and is has to do this even if damaged.

What is done to certify that a ship is stable and properly rights itself?
When a vessel is near completion in a shipyard, tests are undertaken in which massive steel weights or tanks full of water are moved from one side of the ship to the other while the boat is in the water. "This centre-of-gravity shifting is done to check that the ship moves only to angles theoretically predicted on paper, in computer designs and in simulations," says Staunton-Lambert. Damage to watertight compartments is also simulated, he adds, with often hundreds of cases of such damage considered.

And this vessel will have passed such tests?
Yes, it must have. The vessel would not have been been given clearance to sail unless it complied with all stability regulations, so the fact it tipped over so quickly is alarming.

So what went wrong?
"The question investigators have to ask now is: how long did Costa Concordia stay upright? And how soon did it take that large angle of heel? That's what people will be concerned with ? stability is meant to give the crew time to get people off in an orderly manner," says Staunton-Lambert.

Why was the evacuation, so close to land, seemingly so chaotic?
They lost one side of the ship for lifeboat launches when the ship listed so much the boats could not be lowered into the water. Such ships are not designed to launch lifeboats at high angles ? one reason, says Staunton-Lambert, why there are often twice as many lifeboat and life-raft places on a ship as passengers and crew.

Are evacuation tests involving real people undertaken ? like those aircraft makers have to undertake?
No ? the sheer number of people involved would make it difficult to do that for every ship ? but there has to be a solid plan to get people off within an hour of an incident if the ship cannot get to port. This 1-hour ruling dates back to the Titanic inquiry a century ago. It's hard to meet, but as New Scientist reported in 2001, efforts to make physical simulators that improve evacuation procedures are under way.

What else don't we know?
There was a power outage during the incident. Investigators do not know what caused it, how long it lasted, nor what effect it may have had on the crew's reliance on the ECDIS electronic navigation data.

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