Monday, January 22, 2018

Indicator Loops

On a small hill on the southeastern edge of Ocracoke village, obscured by cedars and other vegetation, is the crumbling concrete base of the "Loop Shack," a control hut where, during WWII, signals from underwater magnetic loops were monitored to detect off shore German U-boats by electromagnetic induction.

"Indicator loops," as they were called, were long lengths of cable laid on the seafloor. Developed by the British Royal Navy in the early 20th century, they were tested at the end of WWI, and employed in various locations around the world, including Ocracoke Island, during WWII.

When a submarine (which acted as a magnet) passed above the indicator loop, an induced current was produced and recorded on a paper chart, thus signaling its presence to the operator in the loop shack who could then detonate mines to destroy the submarine.

During the first half of 1942 more than 200 merchant vessels were sunk by German U-boats along the Eastern seaboard, more than 60 off the North Carolina coast.

In the summer of 1942 the Navy began construction of a U.S. Navy Section Base in Ocracoke village (where the NPS Visitors Center, NC Ferry Operations, OPS Museum, and parking area are located today). The Base was commissioned October 9, 1942. At the same time an indicator loop was laid off shore, and a loop shack was built on what was then a large dune separated from the ocean by tidal flats. Various support buildings were added nearby.

Loop Shack Hill, courtesy OPS, W. E. Cochran Collection

By the time the Base was completed the Navy was aware that U-boat attacks had already stopped. In 1944 the Base was converted to a top-secret Amphibious Training Station. In 1945 the Base became a Combat Information Center.

Dr Richard Walding, Research Fellow - School of Science, Griffith University, Brisbane, Australia, has compiled a thorough history and explanation of indicator loops, including the one on Ocracoke Island. To read more, click on the links below:

This month's Ocracoke Newsletter is about Old Christmas in Rodanthe. You can read it here:


  1. Anonymous8:24 AM

    By the time the base was completed.... so X number of dollars had been spent on indicator loop technology that was not ever used at the location. Is that not what sonar and radar do but not using a magnetic field but the physics of the Doppler effect. The Navy knew that the loop technology was obsolete-- was there ever an investigation as to When they Knew and what they knew when? It seems to me that money was thrown at the war effort but the women that worked in the factories the Rosie the riveter
    of the day was promptly let go when Johnny came Marching Home. The Greatest generation includes the Bandana Babe Rosie the Riveter.

  2. I was unaware of the loop cable on Ocracoke.
    There was a similar method using a cable detector from the naval base at Buxton. It was well after WW2, so probably relates to the cold war instead.
    The history around our islands is deep and always fascinating.

  3. Anonymous4:55 PM

    Anon 8:24 Don't you find this interesting? I think we all do.
    Are you mad about something?
    Please seek medical help.
    As usual good stuff Philip. Thank you

  4. Anonymous9:51 PM

    anon 4:55 maybe it was always meant to be a top secret training center but publicly funded as a indicator loop shack. a top secret training center now that is the story. why top secret -- was this the first navy SEAL training on the east coast... Anon 8:24 here, I can see the big picture-- I have a nose for news and I am a truth seeker mister. I hope you have a nice day, fella.

  5. Anonymous4:32 PM

    8:24 SONAR and RADAR depend on transmitted signals. Sound freq. for SONAR. Radio freq. for RADAR. The passive (nothing being transmitted) loop detection of ferrous metal is a regular ordinary metal detector. A technology still in use to day. The Doppler effect is used for the measuring the speed of an object based on a known freq. An increasing freq.,from the object, means it's coming towards you. Decreasing freq. means it's moving away.

  6. Anonymous7:48 AM

    Wait a minute, Anon 4:32 what is your definition of frequency? Is not frequency a measure of the oscillation of a wave and Hertz the wave cycle frequency , ( hence radio station on the dial at 96.5 ) Now an FM radio wave is frequency modulation and AM is amplitude modulation. Now Doppler measures distance by Bouncing a signal off of an object. In astronomical studies Doppler measurements detected Bi-nary stars. In any event Anon 4:32 your Doppler definition is a bit flawed how does the frequency changed hoe does it increase or decrease? I think the measured time to transmit the signal to bounce off the object and receive it determines distance D=RxT Oh dear this is a bit early to explain this on one cup of cold coffee.