General RAN History
FIXED NAVAL DEFENCES IN DARWIN HARBOUR 1939 - 1945
Pat Forster
Boom Jetty and Yard Fort Hill
Between late 1941 and early 1942 Darwin underwent a significant metamorphosis, rapidly emptying of civilians and becoming an almost solely military town. Yet, with the end of hostilities the situation just as rapidly reversed and much of the detail of Darwin's wartime history was soon forgotten.
A number of publications tell the story of the war in Northern Australia, but they deal mainly with actions or events. Details of fixed defences have either been omitted or mentioned only peripherally along with a particular incident. This was the case with Darwin's Harbour defences.
Today, many Darwinians will proudly tell you that an anti-submarine boom net that stretched across the harbour was six kilometres long, and the longest floating net in the world. But, very few of them ever saw the flotation buoys that supported the net, and fewer still knew what was below the surface of the water. Similarly unknown were the submarine indicator loops that lay on the seabed and warned of approaching ships or submarines, and the part played by ASDIC (Sonar), fitted to ships in the defence of Darwin Harbour.
This article attempts to throw some light on the anti-submarine boom net, the indicator loops, ASDIC and the Port War Signal Station (PWSS) at Dudley Point, all of which played a vital part in the defence of Darwin.
Map of Darwin Harbour
DEFENCES PLANNED FOR DARWIN HARBOUR
In 1937 international political tensions were such that another World War seemed inevitable. Britain needed harbours for her Eastern Fleet and selected Darwin and Cockburn Sound, south of Fremantle, Western Australia, as likely bases for supplies of fuel, water and stores. Later that year HMAS Moresby surveyed Darwin Harbour, and the results of this survey revealed that it could provide a fleet anchorage suitable for 28 ships and 17 small craft in the Middle and East Arms.
In a memo circa.1939 the Admiralty announced a requirement for single berth anchorages for one battleship, one aircraft carrier, three cruisers and eight destroyers. Anchorages were also required to accommodate the numerous local defence vessels comprising four Fairmiles (small versatile wooden hulled vessels mass-produced in Australian yards and used for patrol work), four Harbour Defence Motor Launches (HDML), three minesweepers, two Boom Gate Vessels (BGV), four Boom Working Vessels (BWV), six channel patrol boats and one examination vessel.
In 1938 the Admiralty sent an expert on harbour defences, Commander Bannister RN to Australia to advise the Naval Board on necessary defence procedures. The Admiralty undertook to design the anti-submarine defences for Darwin while the Australian Naval Board initiated the construction of two BWVs that would be needed to lay the anchorage for the boom net and subsequently maintain it.
The first vessel built was HMAS Kookaburra which was based on the Royal Navy's 'Net' Class design. The second vessel, HMAS Koala, was similar to the Royal Navy's 'Bar' Class design and was 40 feet longer (178.3 feet) displacing an extra 300 tons to that of the 'Net' Class. The two vessels were completed in early 1940 with a third vessel, HMAS Kangaroo, arriving in Darwin in January 1941 followed by a fourth HMAS Karangi, in January 1942.
HMAS Kookaburra
HMAS Koala
By 1942 the boom service had also been allocated five Boom Defence Vessels (BDV) that were necessary for patrolling the boom net. These vessels consisted of former motor boats and motor yachts that were requisitioned for this purpose. The vessels were armed and put into service as HMA Ships Kuru, Kiara, Vigilant, Moruya and Larrakia, performing general boom defence and air sea rescue duties.
In 1938 Lieutenant Commander W.H Thurlby, RAN and Mr. G.D Logan, a civilian draughtsman, travelled to the UK to be present while the plans for Darwin's boom defence were being drawn up.
They also spent time at the Admiralty and visiting various boom depots. On their return to Australia in 1939, Lieutenant Commander Thurlby became Officer-in-Charge of Boom Services for Australia and he was later appointed as the Director of Boom Services. In this capacity he supervised the preparation for the laying of the Darwin anti-submarine boom net. That same year Chief Petty Officer Wright from the Royal Navy transferred to the RAN and was posted as 'Chargeman' in Darwin. In this capacity he trained civilian riggers in net making and also gave advice on steel wire rope fabrication.
In October 1939, Lieutenant Commander A.E. Fowler, RAN, at that time Darwin's Chief of Staff at the Fortress Combined Operation Headquarters, was appointed the Boom Defence Officer, Darwin. Together with four senior sailors, Fowler travelled to Singapore early in 1940 where they studied and trained in boom defence measures.
They returned to Darwin on 14 April 1940 where they were joined eight days later by Lieutenant Commander Thurlby and Mr Logan. Lectures on laying boom nets were given to officers and ratings from Kookaburra and Koala as well as the personnel who had assembled in Darwin to work on the boom defence facilities. These comprised chargemen, riggers, artisans, skilled labourers (leading seamen) and labourers (able seamen). In October 1940 approval was given for wives and families to join the riggers subject to the availability of accommodation.
Lieutenant Commander Thurlby left Darwin on 20 May 1941 and Lieutenant Commander Fowler assumed responsibility for the laying of the boom net and its ongoing maintenance until the end of the war.
The wire rope used for the nets was manufactured by the Australian Wire Rope Works at Newcastle, NSW and differed from normal wire rope, which had a core. The high tensile wire rope used for the boom nets was about one inch in diameter and consisted of all wire strands. That used for the jackstay along the top of the net was 2 inches in diameter.
The laying of the net was scheduled to be completed by the end of 1940. However, due to the preparation required for the assembly of the net in the boom yard, the laying of the moorings and unforeseen problems caused by tidal conditions, the laying of the net did not begin until the end of 1940 and was not finished until the end of 1942.
The delays significantly increased the costs of the project as the First Naval Member, Admiral Sir Ragnar Colvin, noted in a minute dated 22 May 1940:
I consider that we should now take stock of the situation with regard to our commitments on the boom defences at Darwin. The cost was originally estimated of the order of £600,000. It has now reached double that - over £1,200,000 - and the limit is not yet in sight. The extensions to the boom advised by Admiralty and the additional vessels may easily entail another £500,000 or more and the eventual total capital cost may quite likely be not less than £2,000,000. The project in my opinion has assumed dimensions of a magnitude out of all proportion not only to the other sea, land and air defences of Darwin, but to our total naval effort and expenditure.
Before work on assembling the nets could begin, a large boom shed had to be built in the boom yard next to Fort Hill.
Boom Yard, 1942 onwards
Boom Yard, 1940. Photo: Peter and Sheila Forrest
Within the Boom Yard, 1942. Sketch from 'The Navy in Darwin 1942-1943'. Courtesy of MAGNT.
Within the Boom Yard, 1950. Photo: Peter and Sheila Forrest
In April 1940 a branch line from the railway yards was run along the shoreline to the boom shed to transport the heavy components - buoys, wire rope and chains - arriving by ship at the Stokes Hill Wharf.
By 1 October 1940 work on the boom depot was virtually complete with the exception of the railway branch lines that were required to be laid across the yard. Around this time, part of Fort Hill was also demolished to make room for a road and work space at the end of the new concrete jetty being built by Hornibrook and Co. This was not completed until mid 1941. A concrete net slab on which the nets were to be made was also poured near the jetty and was ready for use by September 1940. A further necessity was the need for a concrete launching trough down to the water that allowed nets to be slid into the harbour and then towed out to the boom net location by one of the boom working vessels.
The Department of Interior in Darwin poured the concrete slab and made the large five and eight-ton concrete mooring clumps used for the anchorage of trots.
8 ton concrete clump, details of steelwork
TROTS
Trots were an assemblage of three cylindrical buoys anchored by eight 8 or 5 ton concrete mooring clumps to the seabed. Four clumps were positioned on the seaward side of the net and a further four were positioned on the harbour side of the net. The trot buoys were secured to the clumps by 1½" or 2" chain cable. In special cases a large peg top buoy was used instead of a cylindrical buoy. For the anchorage of the trots, 230 tons of 1½" chain cable arrived from America in June 1940. In all, the boom net moorings used 265 clumps.
Final Lay/Out of A/S Boom
A record of 8 May 1939 lists the cost of 70 five-ton concrete clumps made by the Department of Interior, Darwin, as being £1500.
TROTS
- Eight foot mesh - 300ft x 91ft
- Three foot mesh - 297ft x 93ft
- Net made in three depths, 25ft, 55ft and 93ft.
- Distance between Trots - 195 yards
- Number of Trots - 30
- Photographic evidence suggests there were 20 to 22 flotation buoys on each net section.
