Rolls-Royce is designing a crewless cargo ship that will be captained remotely, thousands of miles away, with the help of onboard computers, modern sensors, and GPS. It's a magnificent idea that, thanks to resistance from maritime unions and international law, won't be hitting the high seas anytime soon.

Oskar Levander, the chief of marine innovation engineering at the company, says he imagines "it will take more than 10 years before you have all the global rules in place." If the U.S. or another jurisdiction were interested in the robo-ships for use within its territorial waters, he told the Financial Times in December, it could be a question of years, not decades.

Levander, who doubles as the drone cargo ships' main proselytizer, says the unmanned vessels will be cheaper to operate, use less fuel, and hold more freight. The cost savings will come largely from getting rid of the crew, which, according to BloombergBusinessweek, eats up about $3,300 a day, or 44 percent of total operating costs. In other words, turning the ship over to robots and a remote captain means firing many of the million or so sailors worldwide who make their living shuttling cargo from seaport to seaport, and hammering a nail into one of the world's oldest professions.

It probably would be a damper on the other "world's oldest profession," too, since sailors are big financial supporters of the prostitution trade (many of those proverbial women in every port are paid professionals, even today). But as with the hit to prostitution, the slow reduction of seafarers wouldn't be all bad.

On Feb. 18, two former Navy SEALs were found dead in a cabin on the once-hijacked ship Maersk Alabama. The two men were working as guards to protect the ship in the pirate-infested waters off of East Africa. Jeffrey Reynolds was found on the bed and Mark Kennedy on the floor holding a syringe, after a night of drinking, gambling, and, reportedly, womanizing in the Seychelles' Port of Victoria. Heroin powder was discovered in the ship cabin.

It's still unclear what killed the two guards. But "boredom, both men had told friends, was the real enemy on the open sea," say Nicholas Kulish, Ian Urbina, and Mark Mazzetti in The New York Times. Sailors' unions argue that you need people on ships to prevent accidents and manage unexpected situations, but Reynolds and Kennedy were on the Maersk Alabama for one reason: Pirates.

And defeating maritime piracy is maybe the best reason to embrace crewless ships. Electronically hijacking the ships would be possible, but beyond the technological prowess of many pirate outfits and not worth the effort for others. The point of hijacking a ship is generally not for the cargo — who wants to try to unload hundreds of crates of stolen goods or hundreds of thousands of barrels of oil? — but to get millions of dollars in ransom for the boat and, especially, the crew. As piracy has declined since peaking in 2011, ransoms have risen.

Here are the top ransoms paid to Somali pirates in 2011, from Statista:

Statistic: Most expensive piracy attacks in Somalia in 2011, based on ransom sum (in million U.S. dollars) | Statista

If you take away the crew and controls to steer the ship, what is there for pirates to hijack?

As for the technological and regulatory hurdles, they seem destined to fall. If we can send unmanned drones into the air (legally, sometimes) and flirt with allowing robot-powered cars to drive on city streets, what's to keep remote-controlled ships out of the vast expanses of sea?