Why container ships probably won't get bigger

The biggest container ships ever built ply the world's oceans, but will physics and economics set some limits to how much larger these vessels can get?

When the Ever Ace, one of the largest container ships in the world, eased out of Yantian port on 14 August last year and manoeuvred gingerly into the South China Sea, she had embarked upon a record-breaking voyage.

To date, no other ship has carried such a large volume of shipping containers – the equivalent of 21,710 20ft (6m) containers.

The 399.9m-long (1,320ft) and 61.5m-wide (203ft) vessel is a true behemoth, but there are dozens of container ships of a similar size sailing today. Many more are currently under construction. Just two of them stacked vertically would be nearly as tall as the world's tallest building, the Burj Khalifa in Dubai.

If you cast your eyes over a list of the largest container ships in the world, you'll soon notice that they are all 400m (1,320ft) in length or just less than that, and about 60m (200ft) wide. It is more or less today's upper limit for these vessels. There is a surprising number of reasons as to why – and also why you're unlikely to see any container ships much larger than this, perhaps ever. But what are they?

There are around 5,500 container ships globally and together they are capable of carrying 25 million TEUs, or the equivalent of around 25 million 20ft (6m) containers. That's if they were all fully loaded at the same moment.

As George Griffiths, editor of global container markets at S&P Global Platts explains, the global order book for new container ships will increase that total collective capacity by a gigantic 25% in just a few years.

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"We're seeing a lot more of the ultra-large container ships being built," he says. "The proportion of new ships that are moving over 14,000 containers is staggering."

In the last decade alone, the average capacity of a container ship has grown from less than 3,000 TEUs to around 4,500 TEUs. And there are currently more than 50 ships with a capacity of 21,000 TEUs or more. Practically all of them were built in the last five years.

However, these vessels stretch the limits of even the world's largest ports, says Griffiths. To load and unload containers, cranes must reach across the vessels. Container ships also have to turn, pass through locks and canals – including the Suez and Panama canals, which have size restrictions.

It's crucial that vessels avoid running aground, too. In some ports, the largest ships actually sit so deep in the water that they touch the bottom and glide through the silt rather than float above it, says Stavros Karamperidis, head of the Maritime Transport Research Group at the University of Plymouth. Such a manoeuvre must be handled with extreme care.

To accommodate ships much larger than the biggest in existence today would require a huge overhaul of port infrastructure. And that would be incredibly expensive, notes Griffiths: "Why would you invest in bigger ships if it means you have to invest eye-watering amounts of money in the ports as well?"

Karamperidis adds that the largest ships are also restricted in terms of where they sail because they are so vulnerable to heavy weather. They don't tend to cross the Pacific, for example, which can hurl violent storms at vessels. Medium-sized container ships sometimes lose hundreds of containers in the Pacific.

"That's why the [ultra large] vessels come near the coast, so they don't face big waves. It's a matter of stability," says Karamperidis. He adds that most US ports are not big enough to facilitate the largest container ships. Only a handful of ships with capacities approaching 20,000 TEUs have ever called US ports.

You may be noticing a theme. The limitations on vessel size are not so much to do with the sheer engineering challenges of building really big ships but rather the economics and logistics of operating such giants.

"Physically, there's not really any limit," says Rosalind Blazejczyk, managing partner and naval architect at Solis Marine Consultants. She does, however, note there are only so many containers that one could stack on top of one another on a ship before the container at the bottom of the pile would buckle under the weight.

Containers are also lashed to hold them in place and such systems have upper limits for the number of boxes they could secure safely, notes John Simpson, Blazejczyk’s colleague at Solis.

Another issue is how these very large and wide ships handle waves. When sailing directly into a sequence of waves, ships can experience a phenomenon called parametric rolling. It happens because, as the waves pass along the length of a very broad container ship, her bow and stern may clear the water whenever the peak of the wave is at the midpoint of the ship. This leaves the upper part of the bow and stern lacking the support of the water below. The variation in this support as waves continue to pass can cause the ship to bob weirdly from side to side.

"You get very large roll angles with not very large wave heights," says Blazejczyk.

Supersized ships are more at risk of this. And container ships also have very large hatches on their decks, which means their overall structure is weaker than some other vessels and more susceptible to torsion, or twisting.

"They're kind of like a shoebox with no lid," Blazejczyk adds. No problem in calm seas but, again, limiting when or where a ship sails likely also limits its usefulness.

Besides all of the above reasons, there is economic weather to contend with. Griffiths notes that the price of oil is exceedingly high at the moment and the largest container ships require huge volumes of fuel.

Investing in even bigger vessels might not be the wisest financial choice in the future though, having said that, at the moment these costs are being more than covered by the astronomical freight rates worldwide. Such is the present demand for moving goods around.

Karamperidis says that, for container ships with a capacity much higher than 25,000 TEUs to become viable, the economics of operating them would have to change. The Suez Canal will probably always act as a bottleneck for ships travelling from Asia to Europe but it's not impossible to imagine a 30,000 TEU vessel or similar one day traversing a route from China to, for example, a booming African port, says Karamperidis.

"Maybe we'll see those kind of vessels going from Asia to Mombasa," he adds, referring to the largest port of Kenya.

The 24,000 TEU ceiling on container ship capacity we see globally today is, more or less, a reflection of economic limits as much as it is about port infrastructure, the shape of the world's busiest waterways and engineering.

But there's always that possibility that, given the right conditions, someone somewhere with enough money will one day commission a vessel that dwarfs even today’s gargantuan ships. It would certainly be a wonder to behold.

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