It's no secret that China exports a lot of stuff, nor that the country's ports are the busiest in the world -- seven of the top 10 ports by container volume are Chinese. But it's harder to get information about how productive those ports are. For competitive reasons, ports themselves don't want to disclose how quickly ships are loaded and unloaded, and most national governments don't require it.

But here's who does want to know that: Shipping lines, as well as the companies that own the goods they carry. Five years ago, a shipping trade publication called the Journal of Commerce embarked upon a project to collect that data, and convinced 17 carriers representing 70 percent of global ocean transport to turn over what they knew about how quickly containers move and how long their vessels remain in their berths. The result is a white paper ranking the world's ports by how effective they are in moving cargo for their size. Surprise surprise: U.S. ports come out looking pretty dismal.

The U.S., not that great at operating ports. (Journal of Commerce)

Why are China, Japan, South Korea, and the UAE so much better at moving containers around than the United States? They're not, necessarily -- it's more a matter of down time. Chinese ports, for example, operate around the clock with gangs of dockworkers who aren't paid that much or treated that well. Most U.S. ports operate only one or two shifts a day, since longshoremen's union contracts require overtime pay for working in the middle of the night (and their pay is already higher than it is for any other blue-collar trade, reaching into the six figures). So even if they're as efficient at moving containers on a per-hour basis, they'll still be less productive overall.

That has real consequences for shipping companies, since their vessels can't simply dump their cargo, pick up another load, and move on. Instead, they have to book it to the next port of call, which is less fuel efficient than moving at a more leisurely pace, costing them tens of thousands of dollars more for gasoline. In addition, it slows down the pace of goods generally, which raises costs for consumers.

"It's a bottleneck in the supply chain, and it requires extra planning on the part of Wal-Mart, so they can get their goods in their stores when they need to be," says the Journal of Commerce's Peter Tirschwell, who oversaw the report. "All supply chain disruptions raise costs for the consumer, no question about it."

Not only that, but they can simply push business elsewhere. A relatively new port north of Vancouver, Prince Rupert, has been attracting ships that used to go to Seattle and Tacoma, since containers can get to the Midwest faster and cheaper from there via train than they would if they were snarled for days in a congested port.

Tirschwell says that even though U.S. imports and exports cooled off during the recession, they're still rising, and ships are getting bigger. If U.S. ports don't get more productive, costs will just keep going up, putting a real pinch on the stuff we're able to buy and sell overseas. The barely-avoided longshoremen strikes last year show how difficult it can be to change how ports operate. But it seems like making it easier to work during the night would be a good way to start.