Since the financial crisis, the tide of recovery has not lifted all boats equally. But in few industries is that more true than in shipping.

Bulk carriers, which carry such things as iron ore and coal, have been hit by massive overcapacity, as Chinese demand for such commodities has collapsed.

Until the start of this year, the container-shipping business — which carries around 60 percent by value of all seaborne trade in goods — looked more like that for oil tankers — which are enjoying a boom. Rising global trade volumes, and firm steel prices that made it worthwhile for owners to scrap old ships, had kept capacity in check, and container-freight rates seemed to be steadying. As recently as August last year, demand for container shipping was so high that BIMCO, an industry association, was warning of a capacity shortage. And at the start of this year Drewry, a shipping consultant, forecast a bumper year: Owners of boxships would rake in profits of up to $8 billion in 2015, they thought, helped by low fuel costs.

But since then, the industry has been rattled by renewed weakness in freight rates, prompted by a fall in the volume of seaborne trade. The cost of sending a container from Shanghai to Europe, for instance, has almost halved since March, according to the Chinese city's shipping exchange.

Beyond their control

Some of the shipping lines' problems are because of factors beyond their control. At a time when weak trade volumes should be prompting them to scrap more old vessels, the steel price has slumped. So, 60 percent fewer boxships have been scrapped so far this year compared with the same period last year. However, some shipping groups have made a rod for their backs by taking on too much debt. This also makes it hard for them to scrap unprofitable vessels, since their balance-sheets would struggle to cope with the resulting write-downs.

Worse still, critics say, is that shipowners have embarked upon a building boom. Orders for new container ships were 60 percent higher in the first five months of this year than in the same period in 2014, according to Alphaliner, a data provider.

But for those lines that can afford it, ordering big, new ships may be a sensible reaction to falling freight rates. There are still sizable economies to be gained from increasing the size of vessels. As Hapag-Lloyd's boss, Rolf Habben-Jansen, recently pointed out, a ship capable of carrying 19,200 containers needs half as much fuel to shift each box by 1 mile as a vessel with a capacity of 4,900.

Among the winners from this flight to scale will be the world's largest three lines — Maersk, Mediterranean Shipping Company (MSC) and CMA CGM. They have the industry's lowest costs, because they have the biggest ships and the cheapest finance costs. They also have the advantage of being based in Europe: Demand to transport goods across the Atlantic has remained strong.

Maersk and MSC have also formed an alliance, 2M, to save more money by sharing space on their ships on transatlantic and transpacific routes. As the strongest lines get stronger, through fleet renewal and alliance-building, smaller lines that cannot cut their costs quickly enough or obtain cheaper finance to build bigger ships will suffer.

As falling volumes and weak shipping rates force the industry to consolidate, with fewer, bigger lines sailing ever-larger ships to fewer, bigger ports, the resulting gains in efficiency should mean cheaper transport costs, bringing benefits for consumers in many places. That is, unless the consolidation goes too far, and the surviving lines are able to jack up their rates. The 2M alliance now controls more than 28 percent of global container-shipping capacity and almost one-third on the Europe-to-Asia route.

Copyright 2013 The Economist Newspaper Limited, London. All Rights Reserved. Reprinted with permission.