Indicator Loops
around the World (Home Page)
How an indicator loop works
The photos and measurements were taken by Martin
Briscoe of Fort Williams, Inverness-shire, on Sunday 7th May 2000. This
page was updated on 9 September 2007.
I'd also like to thank Neil Owen
of Oban for additional information and photos.
The map below shows the disused harbour defence control huts used by the Royal Navy for its antisubmarine indicator
loop defences in World War 2. They are located at Ganavan headland near
Oban Bay - Argyll, Scotland - 95 miles North of Glasgow. The
land is currently owned by Dunollie Estates and tenanted by farmer Alaistair
MacLeod - a special constable at Oban.
Anti-submarine
‘indicator loops’ are long lengths of cable laid on the seabed in shipping
channels of strategically important ports. They rely on the production of an
induced current in a stationery loop of wire when a magnet (in this case, a
submarine or surface ship) moves overhead. Even if wiped or degaussed, ships
(including submarines) still have
sufficient magnetism to produce a small current in a loop. The current is
detected by galvanometers at a Loop Control Station on the shore. This
technology was developed by the British Royal Navy at HMS Osprey (Portland
Naval Base) and HMS Vernon starting back in 1915 and first deployed at Scapa
Flow, in the Orkney Islands at the northern tip of Scotland. In 1996, a portion
of the Oban Bay loop cable was brought to the surface by a clam dredger about
500 m offshore from the island of Lismore just to the north of Lismore
lighthouse. It was actually marked as a navigation hazard for a time.
Encrustation meant that the cable is now a pretty inflexible and bulky object.
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| Photo of Ganavan headland taken in 1945.
During the war the area was surrounded by a barbed-wire fence and the huts
were hidden in the bush and not visible from the road or the signal station.
Today, the area is grassy and the remains are quite visible. |
A BRIEF
HISTORY
In the 1930s,
Admiralty policy was that important harbours should be progressively defended by
fixed anti-submarine defences consisting of one or more indicator loops. Loops
took a fair while to survey, plan and lay and this lead time was thought
necessary. If hostilities arose, the plan was that the Royal Navy would then lay
harbour defence Asdics (HDAs) and controlled mines. However, it became
apparent that the cost of repairing and maintaining existing loops was becoming
prohibitive so the laying of more loops had to wait until conflict occurred.
By 1938, the possibility of conflict with Germany was most likely so loop laying
was stepped up. By 24th August 1938, four ports were protected (Portsmouth,
Portland, Plymouth and Firth of Forth) with the Straits of Dover being added in
Autumn 1939. Two others - at Penang and Singapore - were also under RN
control. Others, such as Falmouth, Berehaven, Queenstown, St George's
Channel, North Channel and Tynemouth were planned for 1940-41 as well as
overseas loops at Singapore, Alexandria and Malta. No loop station was
planned for Oban Bay.
In early 1940, with the imminent fall of
France and the threat of invasion, many emergency coast batteries and
harbour defence stations were constructed. On the 6th February 1940, the
Admiralty decided that Oban Bay should be used as an additional naval base
to the Kyle of Lochalsh, which was the gathering point for convoy ships but
could not provide the facilities that Oban could. The role of the navy was
to protect and administer the harbour and surrounding waters, to accommodate
merchant ships in the anchorage and organise the sailing of the convoys to
join with larger convoys to the USA, Russia and elsewhere. The naval
detachment at Oban grew as France fell and more and more convoys were
rerouted from the English Channel and South Ireland to the waters between
Northern Island and Scotland. The loss of the Irish ports was greatly felt.
The USN set up
about a dozen 'loop receiving stations'
on the East Coast, Panama and San Francisco. The Royal Australian
Navy installed indicator loops at a dozen sites around Australia and Port
Moresby from 1939 to 1942. Loops were also installed by the RNZN at Auckland Harbour
in 1942, and by the RCN at Saint John, Digby and Prince Rupert (Canada) from
1942 onwards.
GERMAN RECONNAISSANCE PHOTO
This Luftwaffe reconnaissance photo of Kerrera Sound was taken in December
1940.
It shows the naval base at Oban and aircraft of 210 Squadron RAF at their
moorings.
THE BUILDINGS TODAY
A panoramic view of the Ganavan headland - looking towards the Isle of Mull.
The harbour defence huts are on the right of the photo.
The diagram above left shows the relative location of the concrete huts and
slabs. Only buildings A and E remain standing today. The photo on the
right shows the huts as they were in 1945.
Photo 1 - A full view of the headland overlooking Oban Bay. Building
"A" - the Loop Control Hut - is on the left and building
"E" on the right. The remains of building "F" are in the
foreground. The Isle of Mull is in the background to the left and Lismore
Island is to the right.
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|
Photo 4 - Building "E", viewed from the South East.Three rooms: two are 2270 mm x 2070 mm, and the third
is 3670 mm x 3670 mm. |
Photo 5 - Building "E" viewed from the top of the steps
that lead East from the Loop Control Hut ("A"). |
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|
Photo 13 - The Loop Control Hut ("A") viewed from the West.Two rooms: 2070 mm x 3860 mm; 2480 mm x 3860 mm. |
Photo 15 - The Loop Control Hut ("A") viewed from the East, seaward
side. |
Photo 16 - Loop Hut "A" viewed from the seaward side.
The white building on the hill behind it is the Naval Signal Station.
The floorplan of the Loop Control Hut (Building "A"). Walls are
double thickness brick (230 mm wide) and the windows are 1200 mm wide and 900
mm deep. On the right there is a small hole in the wall through which cable
(probably power) runs. Above it are four pieces of dowel embedded in the wall to which some
equipment was affixed.
The rear wall of the right hand side room (the Indicator Loop Room) of the Loop
Control Hut as it is today. The rear and right-side windows are now open to the
elements. A doorway leading to the left room (Binocular Room) can be made out.
This photo of a naval rating in the Indicator Loop room of the Control Hut
was taken during WW2. It is the same room as shown directly above. The rear and
side observation windows and frames are clearly discernable although the
equipment being monitored by the rating is not clear. The wooden door, door
jamb and cement-rendered brick is visible.
These small side-by-side images show the observation room in the Loop Control
Hut as it was in 1940 and as it is today (right hand photo).
Photo 20 - Building "E" viewed from the rear.
Building "E" - floorplan. The walls are double layer bricks about 230
mm wide. Because this is down in a bit of a hollow and has no view of the loop
area, it is suspected that this building may be a generator room or store.
LOOP CABLE LOCATIONS
There were two loops at Oban Bay. Loop 1 went from Ganavan headland (just a little bit
north of Maiden Island) across to the beach to Gorten just north of Loch Dun on the
Island of Mull. Loop 2 stretched from Craignure (on Mull) across the Sound of
Mull to the tip of the Morven district (Rubha an Ridire). See map below.
LOOP CABLE SAMPLES
Coming ashore at Gorten on Mull (see map
above) are some cables that are most likely WW2 "tail" cables for the No. 1 Indicator Loop.
The cables were photographed by Geoff Twibell. He describes them as follows:
There are some six steel-sheathed cables in all, three of them having frayed
down to the inner cores. One contains four rubber-covered cores - possibly "loop
tails". Another contains a single, larger core. The remaining cable is post WW2
and contains four PVC insulated cores one red, one blue and two white.
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| The six armoured cables at
Gorten |
Gorten shoreline with hut in
background. Cables run up this rocky shore. |
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| Probable Indicator Loop "Tail"
cables |
The telegraph hut - post WW2 |
R.A.F. BASE AT GANAVAN SANDS
At Ganavan Sands beach (on the north side of the headland where the loop huts were
located) there was an RAF flying-boat base where maintenance and training was
carried out. A hanger and several engineering sheds were built there to house
the operations.
The photo above on the left shows the Number 302 Ferry Training Unit based at
Ganavan Sands where they trained crews to fly their aircraft over long journeys
to such destinations as Africa, Asia and even Australia. The photo on the right
is of the Ganavan Sands Caravan
Park today. Little remains of the RAF base, however, the steel runners of
the hangar doors and the former generator shed still remain. The indicator loop
huts were on the headland at the rear of the photo. RAF Base image taken from
Paul Burns' RAF
Oban web page.
OBAN - MINEFIELD CONTROL TOWER
There is a minefield control tower at Gallanach overlooking the Sound of
Kerra; it lies on the hillside on the south side of a caravan park.
For more details, have a look at Nick Catford's photos and text on the Subterranea Britannica web
site.
HELP!
If you know of other WW2 indicator loop stations around the world, please
contact me (Richard Walding) with
some of the details so we can correspond.
I'm particularly interested in getting information from officers and ratings
who worked in the loop stations or laid the loop cables. Also, many scientists
were involved in the research at the Royal Navy's "Underwater Detection
Establishment". Are you still out there?
"Wren's Eye View"
- The Adventures of a Visual Signaller by Stephanie Batstone. Published by
Parapress Ltd, 5 Bentham Hill House, Stockland Green Road, Tunbridge Wells,
Kent. ISBN 1-898594-70-8. The first edition (1994) was revised and launched on
5th May 2001 with an
extra chapter added in which she describes the reunion with USN Signalman Jack
Campbell she got to know at Oban.
The following
extract talks about the loop station at Oban Bay (Little Ganavan Beach). The
author, Stephanie Batstone, was a 21 year old visual signaller working at the Royal Navy signal
station to the south. In a letter to me (8th September 2001) she said that she never actually saw the loop huts as
they were completely fenced off and quite invisible from the coast road and
from the signal station. She said no photos were taken of any of the huts:
'penalty of death I should think'.
Chapter 3 - The Loops (page 53)
Next morning Carol and Rosemary reported at 0800 to take over our first
watch officially from Fred and Alf. Joan and I relieved them at 1300. We
thought what a beautiful romantic walk it was, though the autumn sunshine,
better than sitting at a typewriter in London. Past Dunollie Castle on the
right we went, along the shore of the little bay with a stupendous view of sea
and mountains opening up on our left through a kaleidoscope of shimmering
russety leaves, round the headland with its cliff face on the right, over the
barbed wire fence, and then a stiff climb up the far side.
There were blankets hanging oyer the balcony railings. In the bright sunlight
they looked very grey. Carol and Rosemary had their sleeves rolled up. 'We've
been cleaning,' they said, 'it's filthy. Only one signal. There doesn't seem to
be any more coal just the bucket Fred left. Best of luck,' and they were
offdown the hill.
'There's a hole in the kettle,' Joan said, 'and it's awfully rusty. I wonder if
Fred took the good kettle?'
'What did Dave say about that little phone with the handle on the box?' I said.
I couldn't understand,' said Joan. I hoped you had. Something to do with
loops.' At that minute it rang huskily.
Joan snatched it up and said in a quavering voice, 'WSS' I could hear a male
voice the other end say, 'Little Ganavan - Bert here our DO is on the way up to
check your CBs'. The line went dead.
We looked at each other thoughtfully, registering DO as Duty Officer and CBs as
Confidential Books. But where were they, and why did he want to check them, and
what had Dave said Little Ganavan was? Anyway, the place looked fairly clean.
We rolled our shirtsleeves down and put our jackets on to be on the safe side.
Ten minutes later a rather elderly Sub-Lieutenant RNVR (thirty at least) with a
propitiating expression came panting up the steps. It's OK,' he said, 'they're
in that little grey tin box on the wall, and the key is under the telephone.
Did Dave tell you I have to come and check them once a week?'
'No - oh, yes,' Joan said. It was probably politic not to let Dave down. We
stood respectfully while he checked the signal and code lists.
'Dave explained about the loops, did he?' he said. 'Oh yes,' I said. 'Good,
good,' he said. 'Well, remember we're not far away, if you ever want a hand. '
'Thank you,' we chorussed, as he executed a deft descent of the outside ladder,
missing several steps (a skill which we made a mental note to acquire as soon
as possible). 'Don't forget the Ensign,' he shouted up, 'and the gale cone.'
At that moment the husky phone rang again. He rapidly reversed his ladder
drill, coming up three at a time, and picked up the phone. 'WSS' 'Little
Ganavan, sir,' said a male voice. 'Crossing on one.' He shot a practised eye
over the expanse of sea. I indicated a telescope but he waved it away and
snapped back, 'Fishing boat, Maid of Mull, outward'.
He flashed us a dazzling smile, said rather apologetically, 'She always goes
out about this time,' and was away, galloping down the side of the clifflike an
antelope.
It seemed odd to us that at Cabbala, where it didn't really matter, everyone
had gone to such trouble to explain everything. Here, where it patently did
matter, nobody thought any explanations necessary. We tried to piece it
together.
'There are two loops,' said Joan, 'one stretching across the Firth of Lorne and
one across the Sound of Mull, under the sea.' 'And every time a boat crosses
one of them it gives off a bleep. ' 'Where?' In Little Ganavan!' 'But where is
Little Ganavan?'
'Could it be all that barbed wire and notices saying no admittance, RN
property, at the bottom of the cliff? That's the only thing that's quite near.'
'You can't see any buildings. They must be right on the beach.' 'Then they sit
there watching a radar screen - have you ever seen a radar screen?'
'Yes - a little round window in the dark - there was one on the battleship we
got on in Liverpool docks. It's awfully new - the latest thing. Sort of green
and blurry, with a wiggly line going across the middle. When it sees anything,
the line jumps up and down.'
'So when it does that, they phone us up and say "crossing on one" or
"crossing on two" and we look at the sea and tell them what ship it
is.' 'Supposing we don't know.' 'We flash it up, you clot, and get its name.'
'How do we see it at night?' I suppose it'll have some lights.' 'And supposing
we can't see anything?' 'Then we tell them, and it must be a submarine.' We
looked at each other with widening eyes, visualising a sleek sinister shape
gliding into Bonnie Oban Bay to blow it to bits, or to land Conrad Veidt in
dark glasses at Dunollie Point to mastermind a plot to assassinate Churchill.
It would all be our fault. I looked again at the sea. I don't know how he could
see that fishing boat,' I muttered. 'And we've got to know whether it's naval
and we give it the code of the day challenge, or international and we give it
the unknown ship call.'
* Note: The green radar screen referred to above was at the Radar Station not inside the Loop
Hut. An indicator loop uses pen and ink to record a line on a continuous
roll of paper about 8cm wide. The photo below was taken at the Pennyfuir Gates. The radar towers are visible in the background.
