A new US law and legislation planned in Colombia aims to sink what has become South American drug traffickers’ latest favoured means of getting massive loads of cocaine to Mexico and Central America – vessels the size of whales that glide just beneath the surface of the water.

On the rare occasions that these home-made ‘semi submersibles’ are spotted, the Coast Guard dispatches an armed team to board the craft. But instead the crew open sea cocks aboard their vessel, sending it and its cargo to the bottom. They then wave and jump into the sea to be rescued.

Colombia has yet to make a single arrest in such scuttlings because the evidence sinks with the boat. However, 12 people have been arrested under the US Drug Trafficking Vessel Interdiction Act of 2008 since it came into effect in October last year. It outlaws unregistered craft plying international waters ‘with the intent to evade detection’. Those convicted face up to 15 years in prison.

Though semi-subs aren’t new to cocaine trafficking, a more sophisticated variety averaging about 60 feet (18 meters) in length has emerged of late. With just over a foot of above-water clearance and V-shaped prows designed to leave minimal wakes, ‘semi-subs’ – hand-crafted in jungle clearings from fibreglass and wood – are nearly impossible for surface craft to detect visually or by radar outside a range of about 10,000 ft (3,000m). This accounts for their relatively high success rate.

They are propelled by 250 to 350 horsepower diesel engines and take about a week to reach Mexico’s shores averaging seven knots, investigators said. Fuel tanks carry about 3,000 gallons of diesel, so no refueling is needed on the 2,000-mile journey from Colombia north.

Engines and exhaust systems are typically shielded to make their heat signatures nearly invisible to infrared sensors used by US and allied aircraft trying to find them. The cooling system of one semi-sub seized off Costa Rica in September piped engine exhaust through the hull and discharged it at ambient temperature. Unfortunately for the crews, such design sophistication does not extend to their quarters.

‘The conditions are terrible,’ said Captaiin Luis German Borrero, the navy chief in the Pacific port of Buenaventura. ‘They don’t have bathrooms. The beds are two mattresses draped over the fuel tanks, and the pilot can barely see through very small windows in mini-cabin. The noise and heat must be something infernal.’

With cocaine in Mexico fetching $6,500 per kilo ? about triple the Colombian price, according to the US Drug Enforcement Administration ? an average seven-tonne shipment yields $30 million. On arrival at their destination, sub crews simply off-loading their cargo then scuttle their boat, investigators say. The roughly $1 million spent on each craft is simply written off as the cost of doing business.

Though authorities caught 11 semi-subs last year in international waters off the Pacific ? with seven tonnes of cocaine seized in one off Mexico in September ? they estimate from intelligence and interdiction that another 60 delivered their cargo, Nimmich said.

So far this year, sub crews have sunk five of their vessels off Colombia’s coast after being pursued by drug enforcers.