IN OCTOBER 2013 the Seaman Guard Ohio, a Sierra Leone-flagged ship, was intercepted just under 11 nautical miles off the coast of India by the local coastguard. The grey-hulled vessel looked like a naval ship—bristling with antennae and radar—but was chartered by AdvanFort, a private security firm based in Washington, DC. It had 35 crew and carried 35 guns and thousands of rounds of ammunition. On January 11th this year all those aboard—among them Britons, Estonians and Ukrainians—were convicted of entering Indian waters with illegal weapons. They were sentenced to five years in prison.

The case offers a glimpse into a world not often seen by landlubbers. The Seaman Guard Ohio was a “floating armoury”, a ship that loiters semi-permanently in international waters, acting as a hotel and base for private security guards hired to protect ships from Somali pirates. They are typically stationed in waters off Sudan, Sri Lanka or the United Arab Emirates, waiting for their customers—merchant ships in need of protection—to pass by.

Guards hop aboard a client’s ship with their guns, then ride it through the piracy “high risk area” (HRA). Since armed guards first started protecting ships against Somali pirates about a decade ago, no ship with them aboard has been successfully hijacked. Now about 40% of ships rounding the Horn of Africa carry armed guards, according to IHS Jane’s, a research company.

Once the ship has passed back into safe waters the guards disembark to another armoury. Then they fly home or jump aboard the next ship going the other way. This arrangement keeps guns out at sea, avoiding bothersome and inconsistent national laws. When they stray too close to land, as the Seaman Guard Ohio allegedly did, they can run into legal trouble. Armoury operators market their services online. Some vessels feature wi-fi, television rooms and gyms to keep guards happy, along with safes to store weapons.

No official register of floating armouries exists, so it is impossible to count them reliably. But at least 15-20 lurk in and around the Indian Ocean, according to one seasoned guard. He reckons thousands of military-grade weapons are stored aboard the vessels.

The British government has tried to regulate the industry. It has issued licences for “private maritime security companies” to use certain armouries that it deems safe and professionally run. Among its rules are that weapons should only be used in self defence. Tom Frankland, a director at Sovereign Global UK, a firm that runs two floating armouries (but unlike AdvanFort does not itself guard cargo ships) says his firm’s craft are regulated by the governments of Britain and Djibouti, whose government representatives control the transfer of weapons at sea.

Armouries have done brisk business since governments and marine insurers first demarked an official HRA in 2010. At the peak of Somali piracy in 2012, shipowners would pay about $45,000 per trip for armed guards. Insurers often insisted they have them, reckoning that this would reduce the risks of them having to pay out millions in ransoms if a ship were hijacked. Yet competition has driven down rates charged by security firms and standards are falling as less highly-regulated companies enter the market. A boss of one British-owned armoury worries that the use of such firms could lead to weapons entering the black market.

“You used to see teams of Royal Marine Commandos and Navy Seals guarding ships,” says one naval officer involved in patrolling the waters off the Horn of Africa. “Now you get three [untrained men] sharing a rusty AK-47.” Moreover, the industry itself is facing tougher times. On December 1st the HRA was shrunk, thanks to a steep drop in the number of pirate attacks—itself the result of more guards as well as patrols by mainly western navies. That is good news for shipowners, but bad news for their guards. Adding to the uncertainty is a chance that Somali piracy will make a comeback in 2016. IHS says strife in Somalia, coupled with a forecast for months of clement weather, have put Somali pirates in its “top 10 risks” for 2016. If piracy rebounds, some old sea dogs of war may get another lease of life.

CORRECTION: This article has been amended to make clear that Sovereign Global UK does not itself use weapons at sea.