Pulling weeds out of a backyard is a chore. Doing it on the seabed, by hand, in scuba gear, dragging a suction hose across the sand, is a different kind of grind.<\/p>\n
For the past couple of years, that was New Zealand’s main defense against an invasive seaweed quietly swallowing its coastline up north. A diver working that way could clear about 70 square metres a day, roughly 750 square feet. The weed grows back faster than that.<\/p>\n
So the country built a machine instead. A genuinely strange one.<\/p>\n
On June 22, a bright red, 16-tonne submersible about the size of a couple of tiny homes was lowered into Om\u0101kiwi Cove, in the Bay of Islands, for its final sea trials. Its job is to crawl along the seafloor and shave the seaweed off the bottom, plant, roots and all, like an enormous underwater wood planer.<\/p>\n
Northland Regional Council and the engineers behind it call it a world first, and so far nobody has produced another machine of its type.<\/p>\n
The thing is basically a lawnmower and a vacuum cleaner bolted together and sent underwater, as 1News<\/a> described it after the launch.<\/p>\n At the front sits a planing head that works on the exact same principle as a carpenter’s wood planer. As the machine creeps forward, the head shaves off the caulerpa, its plant, its stolons and its rhizomes, the runners and roots that let it grow back, along with about seven centimetres of the surface sediment underneath, roughly three inches.<\/p>\n That whole slurry gets sucked up a pipe to a barge floating overhead. On deck, the sand and water are spun out and dropped back into the sea, and only the seaweed gets carted off to shore.<\/p>\n Taking the top layer of sediment too is the point, not an accident. Caulerpa hides its regrowth machinery just under the surface. Leave the rhizomes and you’ve accomplished nothing. Andrew Johnson, the marine engineer who designed the machine, says the aim is one clean pass with nothing left behind: “100 percent complete removal in a single treatment.”<\/p>\n It doesn’t roll on wheels or tracks. It walks on two giant steel corkscrews.<\/p>\n Underneath the machine sit a pair of seven-metre Archimedes screws, long steel cylinders wrapped in a spiral blade, about 23 feet each. As they spin, they grip the soft sediment and push the whole rig forward, spreading its weight so it doesn’t sink into the muck. It moves at about eight metres a minute, which is roughly 26 feet. Nobody’s racing it anywhere.<\/p>\n Ballast inside the screws can be tweaked to change how hard the machine presses on the seafloor, trading grip for a lighter touch on the bottom. Big inflatable yellow bags help float it up and down off the seabed when it needs to reposition.<\/p>\n For the dimensions, it’s 13 metres long and 2.5 metres high, about 43 feet by 8, with a footprint around 52 square metres. That’s where the “couple of tiny homes” comparison comes from. Sixteen tonnes of it, give or take, roughly 17 and a half US tons.<\/p>\n The strangest part of this machine is that a lot of it is second-hand.<\/p>\n The heart of the rig, the electric hydraulic power unit that drives the pumps and moving parts, is repurposed kit from a retired Scottish North Sea oil and gas ROV, hardware originally built to work at depths of up to 350 metres, about 1,150 feet. It’s the same North Sea where a generation of aging offshore platforms and wind farms is now being argued over<\/a>, and some of that industrial leftover has found a second life off the coast of New Zealand. Other parts come from elsewhere in New Zealand, plus Australia and Germany.<\/p>\n Power, control signals and camera feeds all run down a single 650-metre umbilical cable to the machine, around 2,100 feet of it. Inside, about 250 metres of hydraulic line do the heavy lifting. Operators drive the whole thing remotely from a control room on the barge, watching camera feeds, sonar and GPS, with no divers in the water at all.<\/p>\n That “build it ourselves out of spare parts” approach is the whole point. New Zealanders have a phrase for it, “number-eight-wire,” the idea that a Kiwi can fix nearly anything with a length of fencing wire and some stubbornness. Johnson, who holds an ocean engineering degree from the University of Tasmania, told the New Zealand Herald<\/a> the build was a genuinely Kiwi fix to a problem already reshaping coastal life. He reckons putting it together anywhere else would have run into the hundreds of millions of dollars. The whole project came in at NZ$5 million, a little under US$3 million.<\/p>\n The numbers are the whole story here, and they’re worth laying side by side.<\/p>\n When the cleanup started, crews were vacuuming the weed up by hand with a small nozzle. An earlier mechanical dredge working off a surface barge pushed the daily haul up to around 2,500 square metres. The new submersible is built to clear about 10,000 square metres a day, roughly two and a half acres, or close to one and a half rugby fields, and it’s built to do it in a single pass.<\/p>\n Dr Kaeden Leonard, the regional council’s marine biosecurity manager, called the jump a potential game changer for tackling the dense underwater meadows the weed forms, as RNZ’s Local Democracy Reporting<\/a> covered. “This takes us from manual harvesting to large-scale mechanical removal,” he said.<\/p>\nTwo seven-metre corkscrews do the walking<\/h2>\n
Most of its guts came from the North Sea<\/h2>\n
From 70 square metres a day to 10,000<\/h2>\n