Wednesday, September 19, 2018

Anechoic Tiles

In its quest for ever quieter submarines, the Navy began affixing sound absorbing tiles (anechoic tiles) to submarines in 1980.  The tiles are an inch or so thick and are made of different layers, materials, and void spaces designed to deaden specific frequencies of sound. (1) 

Interestingly, anechoic tiles were first used on German U-Boats at the end of WWII.

While effective, the tiles have a disturbing tendency to fall off during patrols.  Tile loss reduces the effectiveness of the silencing and increases flow noise due to the “holes” left on the submarine’s surface.  The rough edges of these holes likely create additional turbulence and noise just as any rough edge or projection on a hull would.  Addressing a photo of a ragged looking USS Mississippi on return from patrol, former submariner Bryan Clark stated,

Bryan Clark, a senior fellow at the Center for Strategic and Budgetary Assessments and a former Navy submariner, said the amount of acoustic coating missing on the Mississippi "could create enough flow noise to be a sound problem at even relatively slow speeds. Also, there is enough tile missing that it could reduce the coating's ability to absorb sonar energy and make the submarine easier to find with active sonar." (3)

The Navy has formed various study groups to address the problem and have claimed success multiple times but tile loss remains a problem as this photo of USS Virginia in 2018 shows.


USS Virginia - Note the patches of missing tiles.


One alternative approach to anechoic tiles is being developed by researchers at the University of Michigan.  They are developing a “superhydrophobic (water repellent)” paint-like coating presumably filled with bubble voids which reduces drag thereby increasing speed, fuel efficiency, and quietness. (2)  The challenge, as with anechoic tiles, is to make the coating durable.

Similar research at the Universit√© Paris Diderot in France has examined the use of microscopic bubbles in thin coatings. (1)  The bubbles dissipate acoustic energy.

In underwater experiments, the scientists bombarded a meta-screen placed on a slab of steel with ultrasonic frequencies of sound. They found that the meta-screen dissipated more than 91 percent of the incoming sound energy and reflected less than 3 percent of the sound energy. For comparison, the bare steel block reflected 88 percent of the sound energy.

To make submarines invisible to the sound frequencies used in sonar, larger bubbles are needed. Still, the researchers predicted that a 0.16-inch-thick (4 millimeters) film with 0.08-inch (2 millimeters) bubbles could absorb more than 99 percent of the energy from sonar, cutting down reflected sound waves by more than 10,000-fold, or about 100 times better than was previously assumed possible. (1)

While the experimental results are encouraging, the challenge, again, is to produce an easily applied, durable coating.  This is technology that is worth keeping track of in the future.



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(1)Live Science website, “Thin 'Bubble' Coatings Could Hide Submarines from Sonar”, Charles Q. Choi, 4-Feb-2015,

(2)Popular Mechanics website, “Navy Testing Superhydrophobic Hull Coatings For Submarines”, Kyle Mizokami, Jul 5, 2018,

(3)Military.com website, “Navy Subs Still Show Issue with Stealth Coating”, The Honolulu Star-Advertiser By William Cole, 2018,

13 comments:

  1. The tiles appear to be much thicker than one inch:
    http://www.staradvertiser.com/wp-content/uploads/2015/10/20101003_newssub3.jpg

    Delamination was already an issue with the rubber coating of German submarines during WW2 ("Alberich"). Alberich, BTW, wasn't tiles; it was 1 m wide and several metres long 4 mm thin perforated rubber.

    I suppose the delamination issue is not going to be solved until boatbuilders stop trying to apply tiles to smooth amagnetic steel hulls and instead design the steel surfaces to actually offer good-enough connections to the anechoic coating. It might also help to use a scales-like shaping instead of tiles where the loss of one tile immediately exposes others to increased mechanical stress and risk of delamination.

    A rubber paint as you mentioned would likely have delamination issues as well.

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    1. Another photo, Virginia class:
      http://nextnavy.com/wp-content/uploads/2013/11/subonedebondeddetail.jpg
      This clearly shows that the steel hull has no protruding features to manually keep the 'tiles' (or strips) in place. The bonding appears to be all-chemical, and that just doesn't work reliably.

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  2. I did find one link that suggests a updated manufacture and affixing 'patch'

    http://www.nmc.ctc.com/index.cfm?fuseaction=stories.newsDetails&id=368

    The Russians use the same concept any ideal if they have the same issues.

    ReplyDelete
    Replies
    1. The Navy has claimed, multiple times, to have solved the problem but it keeps coming back so I'd take any claim of a fix with a huge grain of salt!

      I have no information on the Russians.

      Delete
  3. Off subject - Appreciate your thoughts

    Tuesday DefenseNews on the LSC - "The US Navy is going to need a bigger boat, and it’s getting ready to buy one" (revelation that Navy want new large missiles requiring bigger VLS)

    The office of the Chief of Naval Operations Director of Surface Warfare, OPNAV N96, Rear Adm. Ron Boxall is running the “large surface combatant requirements evaluation team” to replace the aging Ticos.

    Boxhall mentioned planning to fit a new and larger VLS cells to accommodate larger missiles, presumably hypersonic cruise missiles, maybe naval versions of the Air Force missiles, the Hypersonic Conventional Strike Weapon and the Air-Launched Rapid Response Weapon for which initial R&D contracts of $1.3B recently awarded to Lockheed.

    My first thoughts is additional $Bs will be needed when the Navy has many other higher priorities, just the Navy wanting some of the pie surrounding hypersonic missiles.

    Air Force with their B-52, B1 and B2 more suitable vehicles to launch hypersonic missiles or even use the C-17 as they have done with the Air-Launched Intermediate Range Ballistic Missile target used in Dec. 2015 test of the SM-3 1B (a failure) and the Army ground launched option as with the Russian Iskander-M.

    Raises the question does not the DoD exercise overall control and set boundaries to avoid duplication between Air Force, Army and Navy.

    From

    ReplyDelete
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    1. You ask/raise a couple of good questions.

      "new and larger VLS cells to accommodate larger missiles"

      Wasn't "new and larger VLS" what the Mk57 Peripheral VLS cells on the Zumwalt were supposed to be? Of course, they were only inches bigger so it never really made sense. I don't know whether this reference is to the Mk57 or yet another new and larger VLS. Looks like we may have wasted effort on the Mk57.

      "Navy wanting some of the pie surrounding hypersonic missiles."

      You're spot on about this. Now, whether the hypersonic weapons craze has any combat validity remains to be seen.

      "boundaries to avoid duplication"

      This is post-worthy. The degree of duplication/overlap between the various services is out of control. The Marines are leading the charge with trying to become our third air force. The Army is aggressively trying to expand into deep strike (300+ mile ATACMS, for example). It's all in pursuit of greater budget slices. We desperately need someone to rein this in. Lacking any other responsible group, the Secretary of Defense ought to be controlling this but, instead, seems content to let the services run wild.

      Delete
  4. Thanks for talking about this CNO!!! I've always wondered about anechoic tiles and there really isn't a lot of declassified info on them.

    Why do they shed? Is it because of ship diving/rising numerous times and the pressure increase/decrease? Is it speed? Maybe depends on salinity of the water? Any particular reason acknowledged why this happens?

    Could it be something that's similar to a aerodynamic problem on a jet fighter and you just try to avoid that specific condition/limit the flight envelop to avoid the problem?

    ReplyDelete
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    1. The closest technical analogy may be space shuttle heat tiles.

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    2. "Why do they shed?"

      I have no information on this. My speculation is the same as theCase, below - that the compression/expansion cycle of the sub's hull as it dives and surfaces is the likely problem.

      Delete
  5. I believe one of the main reasons form tile failure is the compressibility of the hull. At test depth the hull is probably 3" to 6" smaller in diameter.

    The "small bubbles" in the tiles are subject to drastic compression expansion cycles too. Finding adhesives to deal with all this is tough....

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  6. The Navy just took delivery of South Dakota SSN-790 which is the lead ship in the Navy's Acoustic Superiorty Program. Navy Recognition reports, "The modifications include new acoustical hull coatings, a series of machinery improvements inside the hull and the addition of two new large vertical sonar arrays—one on each side." There might be some good news ahead.

    ReplyDelete
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    1. There might be but the Navy has a history of making unfounded claims that are routinely disproved by actual DOT&E testing. For example, the Navy has claimed, multiple times, to have solved the tile shedding problem and yet it persists.

      So, one can hope but that hope should be greatly tempered until actual tests or experience demonstrate it to be true!

      Delete

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