Bribie Island, Australia Oban Bay, Scotland Nahant, Massachussetts


This is the home page for the anti-submarine magnetic indicator loop research project. Indicator Loops are long lengths of cable laid on the seafloor of harbours to detect enemy submarines.  They were developed by the Royal Navy in the early 1900s and first trialled at the end of WW1.  They were then successfully deployed in WW2 in British ports both at home, in the Dominions (Australia, Canada, New Zealand, South Africa, Kenya, Ceylon, Penang) or in allied harbours (Iceland, Holland, Dardanelles). By 1942 the United States had adapted this technology for its own needs.

If you have any feedback please email me:

Dr Richard Walding
Research Fellow - School of Science
Griffith University
Brisbane, Australia

Indicator Loops - an overview (YouTube, 70 minutes)


  • How an indicator loop works
  • Reading a US Navy loop signature
  • Who made the Loop cables?
  • Harbour Defence ASDIC (HDA)
  • John Caldecott FRS, and Alexander Crichton Mitchell FRSE - geomagneticians of note.


    Royal Navy
  • Overview of Royal Navy loop stations
  • Oban (Argyll, Scotland)
  • Isle of May (Firth of Forth, Scotland)
  • Bereshaven & Queenstown (now called Bere Is. and Cobh, Ireland)
  • Cromarty (Moray Firth, Scotland)
  • Portballintrae and Orlock (Antrim, Northern Ireland)
  • Loch Ewe (North West Scotland). Guard loops and Controlled Mining only.
  • Sheppey Island, Kent) - Shellness and Sheerness: Guard loops and Controlled Mining at  only.
  • Scapa Flow, Orkney Isles
  • Plymouth
  • The Solent: Portsmouth & the Isle of Wight - Hatherwood, Bouldnor, Bembridge, Culver, Nab Tower, Horse Sands Fort, No Mans Fort, Portsmouth
  • Portland (HMS Osprey)
  • Penang
  • Cumbrae
  • Rosyth
  • Dunoon
  • Trincomalee (Ceylon)
  • Crete (Suda Bay) Greece
  • Malta
  • Haifa Palestine (Israel)
  • Alexandria (Egypt)
  • Algiers (Algeria, North Africa)
  • Dardanelles (Turkey)
  • Bosphorus - northern end (Turkey)
  • Smyrna (now Izmir, Turkey)
  • Hong Kong
  • Singapore
  • Walcheren Island - Flushing to Breskens (Netherlands)
  • South Africa (Capetown, Durban, East London, Port Elizabeth)
  • Mombasa, Kenya: in Kilindini Harbour
  • Manza Bay, Tanga, Tanzania
  • Iceland (Hvalfjordhur or Hvalfjordur)  

    Royal Australian Navy
  • Moreton Bay Harbour Defences. These were located at several naval installations in and around Moreton Bay as follows:
    • RAN 2(A): Controlled Mining and Guard Loop Station at Tangalooma;
    • RAN 2(B): Controlled Mining and Guard Loop Station at Fort Bribie;
    • RAN 3: Controlled Mining and Guard Loop Station at Cowan Cowan;
    • RAN 4: Indicator Loop and Harbour Defence ASDIC Station at Woorim, Bribie Island
    • RAN 7: Indicator Loop and Harbour Defence ASDIC Station at Comboyuro Point, Moreton Island;
    • RAN 9: Indicator Loop and Photo-electric beam Station, Myrtletown;
    • Also of interest: Fort Bribie, including RAN 2 Controlled Mining Station.
    • Fort Skirmish, Woorim, Bribie Island (Skirmish Battery)
    • Toorbul Radar Station No. 210 run by the RAAF
  • Sydney (Loops, HDA, PE Beam)
  • Fremantle (Loops)
  • Darwin (Loops)
  • Newcastle (Loop and Controlled Minefield)
  • Port Phillip - Victoria (PE Beam)
  • Broken Bay (Loops and Controlled Minefield)
  • Port Kembla (Loop)
  • Port Stephens (Loop)
  • Botany Bay (Controlled Minefield)
  • Port Moresby (Indicator Loops); Port Moresby Guns (Gun Battery remains)

    Royal New Zealand Navy
  • Auckland, Wellington & Christchurch

    Royal Canadian Navy

  • Overview of indicator loops in Canada
  • Saint John
  • Antisubmarine School at Digby (Nova Scotia)
  • Prince Rupert (Port Edwards)

    United States Navy
  • Casco Bay (Portland, Maine): 1A Bailey Island, 1B South Portland (Cape Elizabeth), 1F Peaks Island/Fort William
  • 1C North Scituate (Fourth Cliff to Provincetown, Massachusetts); and Rhode Island (Massachusetts)
  • 1D Nahant, East Point (Boston, Massachusetts)
  • 1E Gloucester, 25 mi N of Boston (Massachusetts)
  • 1G Portsmouth (Appledore Island, Maine)
  • 1H Fort Burnside
  • 1I South Westport at Gooseberry Neck (or Westport Point); Mishaum Point HECP (Massachusetts)
  • 1X Argentia - Naval Base (Newfoundland)
  • Fort Tilden (New York) Harbor Entrance Control Post (HECP #2)
  • Fort Hancock (New Jersey) HECP #1, Sandy Hook (& Spermacetti Cove). Combined webpage with Fort Tilden.
  • Fort Wadsworth HECP, Staten Island, New York. Not a loop station but an administrative centre for HECP1 and 2.
  • Fort Wright - Fishers Island (HECP)
  • Fort Miles on Cape Henlopen near Lewes
  • Ocracoke Naval Station (North Carolina)
  • Panama
  • Woody Island, Kodiak (Alaska)
  • Cable laying ship USCG Pequot