Cruise Industry

How steeply can a cruise ship roll before it capsizes?

The recent incident involving Viking Sky is the latest of several weather-related events that have caused discomfort and fear for passengers, and reignited a debate as to whether modern cruise ships are top heavy, and at risk of capsizing in rough seas.

Video still from the forward lounge during the storm aboard Viking Sky

RELATED: How to treat/prevent seasickness  

RELATED: A guide to cyclone season cruising

Whenever passengers are injured aboard a cruise ship due to bad weather, it reignites an ongoing debate. Are modern cruise ships top-heavy, unstable, and therefore at risk of capsizing in bad weather? And are they even sea-worthy enough to ride out a major storm at sea?

Laymen claim they aren’t, and that ocean liners were safer, but real-world experience and naval architecture show that a cruise ship can roll to almost 60-degrees before it’s in danger of capsizing, and can ride out 50-foot seas without danger of sinking.

But that doesn’t stop ill-informed opinions proliferating online.

According to the maritime lawyer Jim Walker “if it looks right, it is right…and cruise ships don’t look right to me”. He makes this statement in a blog post in which he posits that cruise ships are dangerously top-heavy, using the opinions of laymen to back up that argument.

People incorrectly assume that cruise ships are top-heavy simple because they’re tall

But to European scholars in the Middle Ages, it looked right that the sun was orbiting the earth and that the earth was at the centre of the universe. Science showed us otherwise (unless you’re planning on taking the flat-earthers’ cruise), and it is because of scientific fact that Viking Sky did not capsize while wallowing without power in 26-foot seas.

Jim Walker’s coverage of the Viking Sky incident focuses on whether she had any valid reason to be sailing out into such conditions in the first place, which is a valid question, and his work in representing cruise passengers and crew after accidents at sea is exemplary. But the stability and seaworthiness of cruise ships is an ill-informed criticism.

Viking Sky’s problems started when she suffered engine failure in a storm off the coast of Norway. Without power, she turned beam-on to the weather and started rolling heavily. All furniture and equipment not bolted down careened across her public rooms and passengers were thrown from their feet in some instances.

RELATED: 5 things to expect on a new cruise ship’s first voyage

RELATED: What goes into building an LNG powered cruise ship?

Sound familiar? Something similar happened recently aboard Norwegian Escape when she was unexpectedly struck by strong winds equivalent to a Category 3 cyclone. The ship heeled over, sending passengers and furniture flying.

Professional ship manager Neill Conroy from the Nautical Institute says: “By itself, no wind can cause any ship to capsize.”

Norwegian Cruise Line never confirmed just how steeply Escape heeled, but it was probably around the same degree of roll experienced by passengers aboard Viking Sky.

According to a video posted on Twitter during the Viking Sky incident, the degree of roll was around 15 to 20-degress, although it looks and likely felt much steeper.

According to Richard Burke, ABS Professor of Naval Architecture and Marine Engineering at Maritime College, State University of New York, this is because of orientation.

“If you’ve ever been on a ship that’s listing 20 degrees, you almost can’t walk on the ship. Walking up a 20 degree slope is like mountain climbing,” he told CNN back in 2010 after the cruise ship Louis Majesty was smacked in the face by a 26-foot wave, breaking the glass of her forward lounge windows and killing two passengers.

That incident caused the same debate that has been reignited by the Viking Sky emergency – are cruise ships safe?

Or are they top-heavy and/or unseaworthy in severe storms? According to naval architects interviewed by the BCC as part of their documentary Freak Wave, modern ships, whether they’re merchant vessels or cruise ships, are designed to withstand waves up to 15-metres.

“The largest wave marine architects are required to accommodate in the design strength calculations is 15m from trough to crest,” says the BBC.

This is based on a mathematical system called the linear model to predict wave height that says in a storm with a significant wave height of 12m (such as that experienced by Viking Sky), there will hardly ever be a wave higher than 15m and one of 30m (a freak wave) could indeed happen – but only once in ten thousand years.

The documentary found that waves larger than 15m do actually occur far more often than previously thought – but primarily only off the South African ‘Wild Coast’, where the ocean liner SS Waratah infamously sank in 1910.

SS Waratah is believed to have been sunk by a freak wave off South Africa in 1910 – the only passenger ship since to have suffered this fate

RELATED: What happened to South Africa’s cruise ships?

RELATED: How will ‘hard Brexit’ impact UK cruise industry?

For context, some 40 or so cruise ships sail around the coast of South Africa every year, and none has ever sank due to a freak wave.

So cruise ships are designed to weather 15-metre (50-foot) waves, which are in themselves rare, and extremely unlikely to be encountered by a cruise ship. According to Harry Bolton, retired captain of the training ship Golden Bear at the California Maritime Academy, a modern cruise ship could hypothetically be capsized by a 70 to 100-foot wave if it took it directly on the beam.

“I guarantee you’re never going to be in those kinds of waves anyway,” he said. “[Cruise ships] avoid bad weather like the plague. They don’t want the passengers in peril, they don’t want to risk any injury or accidents,” he told the BBC.

In situations where cruise ships do find themselves in bad weather, it’s because the cruise ship has been unable to avoid it, such as Viking Sky, which was sailing in the North Sea at the tailing edge of winter specifically to give passengers a chance to the see the Northern Lights (it could be argued she should have stayed in port, though), or because of a misjudgement in forecasting, such as when Royal Caribbean’s Anthem of the Seas found herself caught short by Cyclone Hermine in the Atlantic.

The captain was forced to ride out a major storm and although it was uncomfortable for passengers, with the ship rolling and pitching heavily, and leaking due to rain water pushed through deck doors by the wind, she was never in any danger, according to Royal Caribbean.

“Safety is our highest priority and ships are designed to withstand even more extreme circumstances than Anthem of the Seas encountered. While the weather was unpleasant, the ship remained seaworthy at all times,” it said in a statement at the time.

The incident was the first time a brand new mega cruise ship had been subjected to such seas in real-world conditions

RELATED: Unstable weather ruins port call in Dubai for three ships

RELATED: Sea Princess runs from cyclone on cruise to Dubai

This is backed up by Burke, who made the following comments to CNN long before the Anthem of the Seas incident. “If a ship heels more than [20 degrees], your real problem is that you’re going to get thrown off your feet and a lot of equipment and furniture is going to break loose and go flying around. So the possibility of injury is very high when that happens. But the ship should right itself without any problem.”

In fact, in extreme cases, a ship can actually list 60 degrees and recover, according to Burke. An angle of 90 degrees would be the ship lying on its side. This figure is not arbitrary, it’s based on complex computer modelling and wave pool tests.

Modern cruise ships are designed to be able to roll up to 60-degrees and recover, in this picture, Costa Concordia has partially sunk and is lying at a 65-degree angle.

For context, the picture above shows Costa Concordia after she partially sank in January 2012. In this picture, she is lying at a 65-degree angle. It’s worth noting that Concordia didn’t actually capsize, despite the massive gash in her hull, she sank onto her starboard side.

It may appear that cruise ships are top heavy visually, but naval architects design them in such a way that all of the heavy liquids, machinery and the main engine are positioned very low, said Burke. So the ship’s center of gravity is also low even though the superstructure is very high.

Because of this, cruise ships have a shorter roll period than ocean liners, which were actually more top-heavy to make them more comfortable for passengers before stabilisers were invented. When a cruise ship rolls, it rights itself faster than an ocean liner would because of all that weight (the bilge, the fuel, the ballast tanks, the engines, the food stores etc) that’s all kept down below the waterline.

This was apparent in CCTV footage captured aboard Pacific Sun while sailing in a storm near New Zealand back in 2008. More than 40 passengers were injured by the intense rolling motion.

In the video, you can see her roll period is less than 10 seconds, causing that snap-back action due to her low centre of gravity. It was uncomfortable for the passengers, but never dangerous to the ship. Like Viking Sky and Anthem of the Seas, this incident also occurred in seas of around 26 feet (still well below the 50-foot threshold ships are designed to weather).

The opinion of professional naval architect Rick Spilman can be applied to Viking Sky, Pacific Sun, Norwegian Escape and any other modern cruise ship. “The recent encounter between a modern cruise ship and major storm was a test, not a calculation or a simulation but a full-scale blowout trial in highly dangerous conditions,” he wrote in reference to Anthem of the Seas.

“It was a test that probably could and should have been avoided, but proved interesting and revelatory, all the same.”

Anthem of the Seas, of course, weathered the severe storm without anything but superficial damage and a lot of interior mess due to furniture on the move.

Similarly, Viking Sky, although much smaller, but with the same so-called top-heavy design, weathered equally extreme seas taking it directly on her beam, and survived with only superficial damage.

Yet this doesn’t stop laymen speculating that cruise ships are top-heavy and unstable. In the (now-defunct) blog Made in America Blogging, the author ‘Teddy Sheperd’ says that cruise ships are unstable and therefore unsafe because they have a shallower draft than ocean liners.

According to the New York Times 16 passenger ships have sunk since 1980, most of them ferries (not cruise ships).

In fact the last cruise ship that sank purely due to flooding in rough weather was the MTS Oceanos off South Africa’s Wild Coast, back in 1990 – and she was an ocean liner-turned cruise ship.

Leave a Reply