Shellness XDO Post Minster Submarine Boom Sheerness XDO Post

Since Roman times, The Isle of Sheppey, in Kent, England has played an important part of Britain’s naval defences, helping to guard the Thames Estuary — and hence London — against possible invasion.  For several centuries, it was a dockyard for the Royal Navy.  In both world wars,  the island became the sites for army and naval fortifications. This web page deals specifically with the Royal Navy's WW2 controlled mining harbour defences at Shellness at the entrance to the East Swale on the southern end of the island; and at Sheerness at the entrance to the Medway River on the northern end. The mine stations consisted of a group of electrically fired mines, mine loops and guard loops directed from the nearby Extended Defence Officer's Post. Two antisubmarine boom nets completed the Royal Navy's harbour defences.
If you have any feedback please email me:

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

Photos marked [PP] are courtesy of Paul Prior, Kent History Forum. (search "Shellness XDO"). Email:


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    Kent is a beautiful and fertile country with mild climate and flourishing agriculture. It has some of the finest farming land in England especially on the banks of the Medway and Thames Rivers. The Medway River separates into two branches either side of the Isle of Sheppey: the East Swale and the West Swale. Shellness is at the mouth of the East Swale and Sheerness is on the West Swale at the mouth of the Medway (see map below). They both flow into the Thames estuary.

    The Hamlet of Shellness is a small coastal settlement on the most easterly point of the Isle of Sheppey in the Borough of Swale in Kent, England, north-east of Harty and south-east of Leysdown-on-Sea. "Isle of Sheppey" (
    or Scaepige) means Island of Sheep and for centuries, the only access was via King's, Harty or other ferries. The beach at Shellness, not surprisingly, is made entirely of shells and visitors have told me that it feels almost too delicate to walk on as every footstep crunches the beach to a finer dust. The beach is backed by low-lying coastal grazing marsh. If you stand at the water's edge you can see across the Swale to Whitstable and Seasalter on the mainland. Shellness marks the end of Sheppey proper as you can see from the aerial photo below. If you continue along the beach you will reach the Royal Society for the Protection of Birds (RSPB) reserve on the marshy ground that covers the southern half of the island. Although Sheppey appears to be one island, the marshes comprise four other islands: Elmley, Harty, Fowley and Deadmans, the latter two small and inaccessible with Elmely and Harty making up much of the featureless side of the island that greets you as you cross the Kingsferry Bridge.

    The Swale has always been an important waterway. In his 1823 edition of New and Extensive Sailing Directions for the Navigation of the North Sea, John Norie remarks of Shellness and the East Swale: the eastern end of Sheppey Island, is an excellent harbour and a good retreat for ships losing their anchor, &c. It is about 5 cables long from east to west, and nearly three-quarters of a mile broad. The shore is, in general, soft mud: ships may run aground on any part within Shellness; but the best place is Faversham Creek. From Shellness, to about a mile above Harty-ferry, the ground is good for anchoring; and there are 3½ , 4 and 5 fathoms, at low water, spring tides; at those tides the water rises from 17 to 21 feet, and at neap tides from 12 to 14 foot.


    Kent, UK. Shellness and Sheerness are at the ends of the Isle of Sheppey.

    Ships can enter the Thames River by two routes: directly from the Thames estuary past Sheerness, or indirectly past Shellness into the East Swale and Medway Rivers and thence to the Thames. Both approaches have been used throughout history but by different enemies and for different reasons.

    When the Roman army under the command of Claudius landed in Britain, the troops were stationed at the Isle of Sheppey but the "cold, crude, turbid, and unwholesome water and moist, thick air" of the marshes was blamed for them developing the mysterious disease scurvy. However, the plant Herba Britannica growing at Shellness was given to the soldiers by the natives and their lot improved. The strategic importance of Sheppey was apparent to the Romans and was thus fortified to become an important military station. The Thames was one of the major strategic objectives of the Vikings in the 800s. In 835 Sheppey was ravaged by Vikings during one of their hit-and-run raids on England and they began to moor their fleet there. In 857 the Viking army overwintered on Sheppey and then in 893, the Viking Hastings invaded England in an attempt to dethrone the monarch and directed his fleet to Kent where under his own command he took eighty ships of his fleet of 250 into East Swale past Shellness and made an intrenchment near Milton. Sheppey was once a much bigger island yielding an abundance of corn and feeding vast flocks of sheep and of great strategic importance. It was at Elmley, on the East Swale, in 1688 that the oppressive and misguided monarch King James II, while trying to embark in a smack (fishing boat) for his escape into France, ran ashore and was boarded by the local fishermen who took his money, watch and coronation ring and took him as a prisoner to Faversham. The King's supporters never forgave the Shellness fishermen who treated the king, even after they were told who he was, with the utmost indecency.
    Sheerness: is a commercial port and main town of the Isle of Sheppey and owes much to its origins as a Royal Naval dockyard town. The Royal Navy Dockyard was established in the 17th century by Henry VIII, who required the River Medway as an anchorage for his navy and so ordered that the mouth of the river should be protected by a small fort. Garrison Fort was built in 1545. Sheerness was attacked by the Dutch navy in June 1667, when seventy hostile ships captured the fort and occupied the town.  In the years leading up to 1780, the waves reached the site of the Roman camp and undermined them for several years until they fell into the sea in 1780. In 1804 the waves reached the churchyard (which was once one mile from the shore) and several houses were carried away. The church was to be dismantled but an artificial barrier of stone and wood was raised for its protection. Mariners used the twin spires as a well-known landmark. In the early 1800s the island was losing about 25 acres per year due to erosion and it was predicted that within 50 years the island would be annihilated. But it is still there!  The Garrison Point Fort is the most well known of Sheppey defences. It was constructed in 1872, on the site of earlier block houses, on the peninsular at Sheerness.

    The land at Shellness has been owned by the Ministry of Defence since before WW2 and has been used for training purposes and as a firing range. There is still quite a lot of unexploded ordnance in the area. It also incorporated a coastal Battery consisting of 2 x 6" BL MK II, 2 x DELs, 1 x 20mm, 2 x MGs and a small Barracks. In WW2 it was an Emergency Coastal Defence Battery of which Observation Post, Pillboxes and Gun Emplacement survive. The Barracks was used for officers accommodation for the local Airbase. There is also an XDO (Extended Defence officers) post which is in very good condition (see photos below). It is rumored that it was also used as a "Z" rocket testing station but there is no evidence for this. It is also part of a large nature reserve that includes Harty and Elmley. A map of the various military installations has been prepared. Click this link Sheppey Military to see the map.

    The Royal Navy had four defensive installations on the Isle of Sheppey:
    1. Sheerness Boom: a small antisubmarine boom on the northern end at Garrison Point, Sheerness stretching across the Medway to Grain Tower Battery on Grain Island.
    2. Minster Boom: a large boom at Royal Oak Point, Minster stretching across the Thames estuary to Shoeburyness on the Essex coast.
    3. Sheerness
    Extended Defence Officer's Post: on the northern end of Sheppey Island at Sheerness; responsible for the Sheerness and Minster antisubmarine booms and Medway minefield.
    4. Shellness Extended Defence Officer's Post: on the southern end of Sheppey at Shellness; responsible for the East Swale minefield .

    1. The Boom defences at Sheerness
    The Medway Boom was built at the beginning of WW1 (about 1914) and consisted of a vast chain attached at one end to Grain Tower Fort (Martello Battery) on Grain Island across to the southern side at Garrison Point Fort at Sheerness. It was raised and lowered by a winch on the wharf at the Sheerness dockyards. See the map below. The chain was removed at the end of WW1 and the Grain Tower Fort placed into 'care and maintenance'. In WW2 twin 6pdr QF guns were installed as defence against the fast and very manoeuverable German E-Boat.

    Grain Tower Fort from satellite. Boom net chain at bottom is clearly visible. The boom attached by chain to Grain Tower Fort on Grain Island across the Medway from Sheerness. Thanks Kyn for the photo.

    2. The Boom defences at Minster

    The second, and much larger boom, was installed across the Thames estuary between Royal Oak Point at Minster and Shoeburyness on the Essex coast. The boom consisted of a row of pylons stretching out into the sea with nets in between and supported by large barges called lighters weighing some 200 tons. The nets were opened and closed by three ships; one in the middle, one to its left and one to its right. Incoming ships sailed between the left and middle ships, outgoing ships sailed between the right and middle ships. The right hand side ship was HMS Pallisade which was stationed five miles out of Sheerness and, like the other boom vessels, was fitted with Horlicken guns; every 5th shell was a tracer - they lit up the area. The Boom vessels were also fitted with ASDIC (now known as sonar) and if a German U-Boat was detected within half a mile of the Boom Defence a signal was sent to Garrison Point (Sheerness XDO), for a destroyer to be sent out to sink the submarine. You can visit the Shoebury boom at the far end of the Ministry of Defence controlled East Beach between Shoebury Garrison and Pig's Bay. The beach is owned by the MOD, who once a year, close off public access and fire a shot out onto the Maplin Sands to retain their firing rights.
    Boom pylons at Royal Oak Point, Minster. The original hulk mooring poles are to the right. Many of the pylons were removed in the 1960s as they were a danger to shipping. [PP]. Boom at the Essex side - at Shoeburyness. It was built in 1944 to replace one built in 1940, 100 m to the east. It was upgraded with concrete in 1954. Photo courtesy of Julieanne Savage

    Operation of the Shoebury-Minster Boom. Referring to the figure below right, there are three anchored ships in the shipping channel, designated as Left, Mid and Right. There is a single cable that runs from the "Left" ship around a pulley on the fixed pylon "C" and to the "Mid" ship. The boom net is attached to the cable at point "B".  When the incoming ship has been cleared for entry the "Left" moored ship would winch in cable "A" around the pylon "C" and at the same time "Mid" ship would let the cable run out. This would draw the boom net end (point "B") towards the pylon "C" and they would then allow the cable to slacken and fall to the seabed. Thus the boom gate would be open for the incoming ship. To close the gate, the reverse is done with Mid ship winching in the cable as Left ship allows it to run out. The end of the boom net "B" is drawn right up to the side of the Mid ship and the gate is thus closed. A similar operation is done for outgoing ships. For smaller booms or where shipping is less frequent, a single boom gate is used.

    Probable arrangement for Shoeburyness-Minster Boom Schematic for operation of the boom nets

    3. Sheerness
    Extended Defence Officer's Post and Controlled Mining defences
    On 27th July 1939, the Admiralty finalised its minelaying program for ports at home and abroad for the early stages of the war should it occur. The Admiralty policy was that minefields should only be laid at the outbreak of hostilities and not before as the cost of maintaining the mines and the minefield was prohibitively expensive. Minefields were planned for Hong Kong, Singapore, Alexandria, Malta, Tynemouth and sites around the UK.  Upon outbreak of the war with Germany in September (1939) the minelaying program began in earnest, including those at Sheerness (medway) and Shellness (East Swale). Personnel were trained at the Controlled Mining School at HMS Vernon (Portsmouth, England).

    The Sheerness XDO Post and mine watch station is located on the northern end of Sheppey Island at Sheerness. The XDO was responsible for the Sheerness and Minster antisubmarine booms and Medway controlled minefields. As stated before, the Thames naval defences go back two hundred years. But the army defences go back further; the fortification of Sheerness began in the 1670s. Originally, many Martello towers were built to defend the Kent coast against a French invasion between 1805 and 1808 and are a familiar sight to visitors to the south coast around Folkestone, Shorncliffe and Dymchurch. However, less well known are the three other Martello towers in Kent built between 1850 and 1915 as part of the Medway defences at Sheerness. They were designated the 'Centre Bastion Battery' and were several hundred yards from the bigger guns at the Garrison Point Battery at Sheerness.  In 1937, it was proposed to transfer the heavier 6" guns from Garrison Point Fort on the towers in place of the 4.7-inch guns, but this proposal was never implemented. Instead the guns were removed at the beginning of WW2, and the three buildings taken over by the Royal Navy for Thames and Medway defences (see photo below). Both towers (far left and far right) had additional concrete structures built on the old gun platforms, one of which was a standard pattern square shaped Observation Post probably for control of nearby searchlights. The other structure on the tower (rightmost) nearest Garrison Point Fort was an hexagonal Observation Post with a raised dome-shaped top and was built for use by the Extended Defences Officer (XDO) who supervised the firing of the controlled minefield laid at the entrance to the Medway. This type of OP was known as 'Observation Minefield Control Tower-Type E' and on the lower floor of the OP were the mine firing panel, electric batteries, charging panel and tail resistance balancer together with a 4 feet 6 inch (1.38 metre) Barr & Stroud rangefinder and telephone connected to XDO above. The upper observation dome contained an open sight bearing indicator. Between the two towers was the battery command post (BCP) which was square in shape with a domestic-style hipped roof and chimney stack with three chimney pots, all helping to disguise its military role. The command post was approximately 14 m high with two doors and two windows at the first floor level at the front of the building. All three buildings were camouflaged during WW2. By 1946, the towers had finally been abandoned and today they sit in all their loneliness like Easter Island warriors thinking of times passed. They are now flanked by a new promenade on the seaside while at the rear Sheerness commercial docks encroach more and more. Bill Clements' Medway Martellos webpage has more details on the history and structure of the Centre Bastion Battery. In Bill Clements words "Little has been done to protect these structures and it can only be a matter of conjecture as to how long it will be before further expansion of the docks brings about their demolition".

    Sheerness XDO Post. Photographer Paul Prior was kind enough to reshoot the XDO Post (on the very top) in April 2009. Thanks Paul, we owe you a beer. Sheerness XDO Post (left) from road side. You can see how the road corridor is encroaching on this heritage site. Thanks Kyn for the photo.


    Close-up of Sheerness XDO Post (top) and mine control room (bottom). Photographer Paul Prior thought it looked more arty to crop the top heavily but we think he should change to Foster's Light (or get his eyebrows trimmed). Satellite photo of XDO Post and Battery Command Post (BCP). The old 4.7" gun emplacement is on the right. The promenade and roadway are clearly visible.

    Possible layout of Medway Boom Net and Guard & Mine Loops between Grain Fort Island and Sheerness. Further information is required.

    4. The Shellness XDO Post and Controlled Mining defences
    consisted of a Extended Defence Officer's Post which doubled as a Mine Watch Station, square in construction and made from reinforced concrete about 8 ft. high. It has an entrance door in the east corner, which is angled slightly - presumably for blast protection. It has two rooms: a large observation room (9' x 9') and a smaller generator room. In the generator room, the concrete mounting pad is still there with a door leading to the observation room and exhaust vents to the outside. The observation room has holes in the wall for equipment and a ladder (now gone) reaching up to the observation turret (cupola)  measuring about 3 ft. by 3 ft., and about 3 ft. higher than the roof of the building; a cube, effectively. There is a small loop-hole in each of the four faces. To reach this turret from inside, a short iron ladder was fixed to the floor directly beneath it, high enough (four or five rungs) to enable observers to peer through the turret loop-holes with ease. There may have been a small platform attached to this at some time. The observation window (shown below) allowed direct observation and use of a Barr & Stroud telescope. Russell Barnes' website "XDO Posts" is a useful comparison on XDO Posts at Workington Dock and Kingswear Castle (Devon).

    View North East through the Shellness XDO Post and Mine Watch Station observation window overlooking the Thames Estuary [PP].

    The layout of the Shellness minefield is shown below. Please note: the layout is only indicative of the position; and the number and arrangement of loops is an estimate.

    Typically, a mine loop was 640 yds long by 25 yds wide with four to six buoyant mines moored in the centre and attached to a firing cable. A guard loop is usually 2000 yds x 25 yds wide. The Guard Loop was usually positioned about 1000 yds on the seaward side of the minefield. Tail cables (4 core) ran from each loop to the shore and thence underground to the XDO Mine Watch Station on the beachfront. The "L" Mark 2 mine units were the most common being used at that time. They consisted of two 40" diameter mild steel hemispherical pressings 3/16" thick joined with 1/4" thick bolts. The charge was 500 lb of amatol with an electric detonator (No. 28, Mark 2) inserted into a 1 lb charge of primer. The complete mine and charge weighed 3151 lb (1430 kg) and could disable a submarine within a radius of 40 feet. They had a positive buoyancy of 468 lb meaning they would float. To keep them moored, a sinker ("L" Mark 2) consisting of a cast iron base with a drum in the centre housing a cable that could moor mines in water up to 25 fathoms deep. Cables used for controlled mining were Admiralty Pattern 660,  Pattern 7048 and Pattern 9610 and for tail cables the four-core Pattern 2865/4 was used. Details of the cable types can be found on my Cablemakers website. It is interesting to compare this XDO Post with that of the Royal Navy's Cromarty (Scotland) Mine Watch Station.

    Possible orientation of the Guard and Mine Loops showing how the controlled mines may have been positioned. It seems likely that there would have been two overlapping Guard Loops and four Mine Loops arranged in an echelon pattern. The Coastal Battery is also shown (but there's not much left today); and the Coastal Artillery Search Light (CASL) emplacement is also marked.

    The XDO Post at Shellness. The front face (on right) faces north-east. See floor plan at end. [PP]. Looking through the viewing slot south east over The Swale to Whitstable. [PP].

    Rear entrance. Wall of generator room. [PP]. The entrance. The white things to the left are just old fence posts. Nothing important. [PP].

    Hatch up to turret. Ladder bolts can be seen. The ladder was quite short with four or five rungs to enable a watchkeeper to peer through the slots in the top. [PP]. Generator slab and usual graffiti. [PP].

    Observation window facing SE. The brackets supported a steel hatch that could be closed in the event of attack. [PP]. The structure in 1980. It was being used as a marker for a motor boat race. Note the steel hatch on the front. [PP].

    Note the holes in the wall where electrical equipment was mounted. Four galvanometers are likely to have been used. [PP]. Holes in the entrance wall for tail and power cables. [PP].

    Pit near the XDO MWS hut. Dave Wood has informed me that this is a small magazine for two inch rockets. It was part of a system named 'Radiator'. There are a number of these in Suffolk and there should be a large block of concrete nearby which the rocket firing rig/weapon was situated. [Photo by PP]. Armoured tail cable emerging from the "shelly" beach. [PP].

    Two projectiles found at Shellness by Paul Prior. He says that the bad stuff is taken away or blown up by the Royal Engineers. Averages about 2 unexploded AA shells,1 mortar shell a week and 1 UXB and or 1 Mine a month. [PP].  

    Floor Plan of Shellness XDO Post.

    OPERATION OF The Extended Defence Officer's POST & Mine watching STATION. 
    The XDO was the naval officer responsible for all naval activity in a given area. He had the responsibility for the minefield, boom nets and signals and was the liaison between the RN and army at the Garrison Point Fort at Sheerness. If the XDO's visual sightings and communication with the army revealed the likely presence of an enemy U-Boat, the XDO would give orders to the mine watch officer on duty. The orders and firing sequence were the same throughout all CM stations around the world. The men in the MWS had to perform daily and weekly tests on the equipment to make sure it was in good working order and had to record perturbations ("perts" - the swings of the galvanometer spot due to the tides) every 5 minutes to the hour. These had to be recorded in the log book. At the end of each watch, the Handbook of Controlled Mining Confidential Book (CB) had to be mustered and had to be locked in a CB chest and then placed in a locked cupboard. The guard loops were 1000 yds to the seaward side of the mine loop so that if a submarine entered the bay it would cross the guard loop first and alert the CM operators. If a swing was detected on the guard loop, the operator in the MWS would close the control switch to start the 300V generator and then ring one long bell to the power hut. The operator would then advise the XDO  "Mines to Active" and would say the number of the guard loop (if more than one) giving the swing. He would then check to see if the correct voltage was showing on the voltmeter. Then they had to wait. If a swing was detected on the mine loop after a swing on a guard loop it meant that a submarine had passed over the loops. The operator would then report to the XDO "Mines to Active" again, shift the selector switch to the number of the mine loop involved and fire the mines as the galvanometer spot crossed it's zero after it's first displacement. He would then say to the XDO "Mines to Active, mine number XX fired". After 5 seconds, the firing release lever would be released and the selector switch replaced. If there was just a swing on the guard loop they took no chances. The operator would fire anyway. After a firing took place, the fuses in the mine cable would be replaced. Of course, if a boat was sweeping for mines in the area, they should have told the XDO of the fact.

    If you have any further details of Shellness harbour defences or antisubmarine harbour defences in general (Indicator Loops and Harbour Defence Asdic) that may help with this research project please email me at the address at the top of the page.