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Patent Spotlight: Ribbon Fairing Apparatus’

By Benjamin McKnight III, NSWCCD Public Affairs | April 22, 2020

Accounting for water resistance on below-surface vehicles can, at times, make or break the system. When attachments are added to the vessel, those too should be optimized to address resistance concerns.

Last May, Naval Surface Warfare Center Carderock Division engineer Dr. David Coakley of the Maritime Systems Hydromechanics Branch tackled that issue head on and was awarded a patent for a “Chevron Ribbon Fairing Apparatus” (U.S. 10,286,983) a device purposed to reduce hydrodynamic drag on marine cables that are used to tow items underwater. While the Navy does not have a relatively large amount of towed systems, the ones in use are often carrying out significant tasks such as mine hunting that Coakley believed could benefit from an improvement on the cable fairings.

Coakley’s research didn’t begin at the chevron ribbon fairing idea. In 2014, he and others were given funds to explore the general concept of drag reduction using fairings. Within a year, the group built a mechanism that allowed them to test fairings at multiple angles based on the various speeds and flows.

“After the first battery of tests, we realized the existing fairings probably had some shortcomings when towed at an angle to the flow,” Coakley said. “That’s what made me think of the chevron fairing.”

To test the idea, Coakley took cable samples to Carderock’s 36-inch water tunnel and measured the vibration produced from the water running over the cables. About 40 samples were tested and from those, Coakley was able to determine three to take to the 12-inch water tunnel for further evaluation. The vibrations group at NSWC Philadelphia assisted Coakley with finding the most effective way to measure the cables’ response to water speeds between 0-20 knots.

The chevron ribbon fairing, according to the patent, is designed to accommodate tow angles between 10 and 90 degrees. If the vertex angle of this fairing is double that of the cable’s towing angle to the flow, the chevron concept can operate at its highest efficiency. A variety of pliable materials can be used to make the fairings, which are woven through the cables at the chevron’s vertex so its legs are free to move and align with the flow of movement.

By itself, though, the chevron ribbon fairing was not as effective as Coakley hoped it would be. “In a couple cases it turned out to be a little bit better, but in other cases a little bit worse than existing fairings,” he said. What he did discover was that combining the chevron concept with a second type of fairing – a Sandwich Ribbon Fairing – would yield more favorable results for reducing hydrodynamic drag.

“Around the same time, I put in a patent application for the Sandwich Ribbon Fairing,” said Coakley of the second fairing, which is still in pending status. “What this does is come up with a simple means by which one can have a ribbon fairing with a variable stiffness and increased stiffness. If the fairing stiffness is increased the right amount, then you can eliminate the flapping and waving in the flow at realistic speeds.”

Coakley also said that funding for fairing improvements have since ceased as the Navy determines if such will be necessary at a large scale. Should the decision be made to go forward, he is confident that a combination of enhancements including those he investigated can provide positive outcomes for the systems that would benefit.

“It’s always going to be a niche for the Navy because we don’t have that many towed systems,” Coakley said. “In cases where fairings are used, it turns out they’re very important. They can make or break a system.”