Sticking to sound principles

Tom Shelley reports on a clever way to produce a controlled amount of suction, without requiring excessive consumption of energy



A small remotely operated vehicle (ROV) can create a vortex to attach itself to the side – or even underside – of a ship and drive around to inspect it.
Developed for the US Navy, this hovercraft in reverse with wheels is proving to be of interest to navies around the world. But it also has potential in many other spheres, including the inspection and maintenance of civilian vessels, and the insides of storage tanks and reservoirs.
“The US Navy wanted to inspect a ship’s hull in black water, but a conventional ROV is like a helicopter and thus difficult to manoeuvre into position and keep it there”, says Jesse Rodocker, president of Seabotix, San Diego, explaining how the project came about. So Seabotix partnered with another company, Vortex Holding Company, based in New Jersey, which had a patented device known as a suction cup vortex attractor.
According to US Patent 6881025, the apparatus comprises an impeller, housed within a shell: “The vortex attractor generates a vortical fluid flow in the form of a helical or spiral shaped flow. The fluid flow creates a low pressure region extending from the impeller end of the device. This low pressure region is contained by the walls of the fluid flow, thus directing the attractive forces toward a surface and minimizing effects of ambient fluid on the system.”
What can be seen underneath one of the actual ROV devices, designated as a ‘Little Benthic Crawler’, or LBC, is a centrifugal impeller, which sucks in water and pushes it out of a skirt. The ROV is also equipped with rubber-tyred wheels, so it could motor along the ship hull, while the suction kept it in contact. Rodocker confides that they had thought of using magnets, or ducted fans, to bring the device to the side of a ship, but these were considered to be very large and inefficient. The impeller in this device, on the other hand, “doesn’t go round very fast,” he states. “If it sucks stuff in, it just spits it out. It will work in 5 knots of current, where even a $5 million dollar ROV won’t work.”
Specifications are 150m depth rating, 175m umbilical, and five brushless CD motor-driven thrusters give it a forward speed of three knots. The four-wheel drive enables the device to move over hard surfaces at up to 30m/minute and it can be used, according to the makers, on the surfaces of ship hulls, concrete dams, reservoirs or tank surfaces.
Seabotix has so far produced 550 ROVs of various kinds, for customers including the Royal Navy.
The inventors of the Vortex Attractors also see other uses for the device. In the patent, they suggest toys, lifting devices, swimming pool cleaners and actuators to move things. The suction can just as well be an air vortex, as one based on water. Assisting downforce on racing cars is also a possibility, but likely to be immediately legislated against as illegal, if anyone were to introduce it.

Seabotix is a SolidWorks user

Pointers

* Gentle suction from a low-speed impeller keeps this wheeled ROV in contact with a ship hull or other surface requiring examination or maintenance

* Detritus is passed into the impeller and out again; one of the other applications suggested for the idea is cleaning systems

* The basic idea works with air, as well as with water, and could, in theory, be used to assist the downforce on racing cars

Author
Tom Shelley

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