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From Thinner Coating, a Stealthier Sub

Friday, January 9, 2015

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The images of sound-absorbing rubber tiles peeling off a fairly new $2 billion U.S. nuclear sub made the technology's shortcomings painfully apparent.

Now, nearly five years after photos of the tattered skin of the USS Virginia emerged, researchers are reporting a more durable sonar-cloaking technology.

U.S. Navy via

In 2010, the anechoic coating on the USS Virginia peeled off in "large sections up to hundreds of square feet," the Pentagon's director of testing admitted. A new, thinner coating may be an option.

The team, comprised of French and Canadian scientists, says it has developed a coating that can offer the same protection as the conventional perforated rubber tiles, but at a fraction of the thickness.

A Few Millimeters

Rather than tiles that are several centimeters thick, the team predicts that "a few millimeters of soft material containing regularly spaced air pockets can absorb well over 99 percent of the acoustic-wave energy impinging on it," explains Physics, where the research ("Superabsortpion of acoustic waves with bubble metascreens") was published Tuesday (Jan. 6).

The polymer tiles now in use, known as anechoic tiles or anechoic coatings, were developed by Nazi Germany's Navy during World War II. The rubber sheets were drilled with air holes that reduced Allied sonar detection and helped to dampen sound from the U-boat engines.

CC BY-SA 3.0 / NZSnowman

Anechoic tiles were developed by Nazi Germany's Navy. The sound-absorbing holes were drilled in prescribed patterns.

The basic technology—a thin elastic layer embedded with cylindrical, air-filled cavities—has proved "an excellent sound absorber, but figuring out how to optimize such materials for a particular frequency or application has involved time-consuming numerical simulations," Physics reports.

What Bubbles Can Do

Led by Valentin Leroy of Paris-Diderot University, the team modeled the cavities as spherical bubbles, "each with a springy response to a pressure wave and a resonant frequency that depends on its size and the elasticity of the surrounding material."

From there, the team developed an equation that ties sound attenuation to the material's properties, the cavity size, and the cavity spacing.

Underwater tests showed promise with a 230-micrometer-thick polymer filled with air cavities and applied to a steel slab.

The material still must be tested at sonar frequencies, but the team calculates that a 4-millimeter film containing 2-millimeter bubbles could attenuate reflected waves by more than 10,000-fold—"about a hundred times better than what was previously assumed possible," according to Physics.

Bonding Failures...

The Virginia had been in service just two years when its anechoic coating began to debond while the sub was on patrol. The same problem also plagued two of the next three subs in that class, the Navy admitted.

Beyond My Ken / CC BY-SA 3.0

The Royal Navy's HMS Triumph shows large patches of missing anechoic tile in this undated photo.

"Clearly, we had problems on the early ships," Vice Adm. Kevin M. McCoy, commander of Naval Sea Systems Command, told reporters at the time.

The Virginia Class anechoic coatings, applied by Northrop and Electric Boat, were the first to use a then-new procedure called "mold in place" rather than the tile method. The process was supposed to be faster, cheaper and more durable.

Instead, the coatings tore off in "large sections up to hundreds of square feet," the Pentagon's director of test and evaluation admitted later in a report quoted widely.

The Virginia's coatings were the most damaged. But all told, that ship and its classmates, North Carolina and Texas, each lost 5 percent to 7 percent of their coatings, the Associated Press reported, citing the Navy.

...And Navy Assurances

The hull coatings were repaired during their normal dry-dock maintenance periods, and the Navy insisted that the problem was minor.

The failures were not "a real big deal for us," McCoy said.

Alan Baribeau, NAVSEA's spokesman for the Virginia Class program, told Inside the Navy in a statement that the failures had not caused any "fail to sail" events.

“The debonding issue has been aggressively pursued since its recognition in 2006,” the statement said.

“The problem was largely due to immature application processes, which have been corrected on later ships. Because of the parallel construction process, [the hull treatment] was applied to several ships before the first at-sea testing of Virginia."

The Navy's coatings application processes are classified.


Tagged categories: Coating / Film thickness; Coating Materials; Disbondment; EMEA (Europe, Middle East and Africa); Marine; Marine Coatings; North America; Protective Coatings; Research

Comment from Tony Rangus, (1/9/2015, 11:03 AM)

I would sure as hell keep my application processes classified, especially when they don't work! I wonder how much the initial work cost versus the rework cost? Lets hope it is not the common bugaboo of poor surface preparation!

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