The Boom Defence in Cairns was based on an American design and used American materials which were received during the November/December period. It was recommended that the harbor defence should consist of the American single line A/T - A/B across the channel with extensions either side to complete the obstruction. The extensions were A/T nets suspended between dolphins from the end of the flotation defence to 1 fathom L.W.O.S. and then "A" type scaffolding to each shore.

This recommendation was later modified to "A" type scaffolding for all extensions. Approval for the Boom Defence scheme for Cairns was given in February 1943.

The approved design was prepared in detail in March 1943 and arrangements were made for the additional material required from local sources. The design consisted of the American A/T – A/B defence approximately 2 cables in length across the entrance channel, 9 cables to seaward of Stafford Point with “A” type scaffolding extending in S.E. and S.W. direction to each shore, each section of scaffolding being approximately 7 cables in length.

A/T nets hung on the scaffolding each side of the channel were allowed for, to cover wharves against torpedoes fired by midget submarines operating in the shallows clear of the main channels. To efficiently join the “A” type scaffolding to the A/T – A/B Boom large pile dolphins were substituted for the two end trets? of the A/T – A/B.

A Gate 617 feet wide was allowed for hinging on the western side, opening up harbour to a turning back dolphin and tug operated.

The material which was used for the “A” type scaffolding Nets for hanging on the scaffolding and curtain nets were supplied from local sources.

In March 1943 arrangements were made for the construction of a boom depot, jetty, net chute, dolphins for ends of A/T – A/B defence and turning-back dolphin. The Depot was completed early in 1944.

After arrival at Cairns of the scaffolding material as much as possible was rigged working from punts etc and using the tides. By the end of 1943 all scaffolding work by this method had been completed. It had been intended that “KARANGI” (released from Darwin) would take over at this stage by completing the deeper part of the scaffolding and laying the A/T – A/B section. Unfortunately KARANGI suffered damage on grounding during passage from Darwin and as a consequence had to proceed South for necessary repairs. This delayed completion of the Boom until the return of KARANGI, The only Boom working vessel available.

KARANGI arrived early in April and completed her work on the scaffolding early in May. The defence was now complete except for the gate operating barge and was maintained in the “open” position. In an emergency it would have been closed at short notice by local facilities.

In February 1945 it was decided to altered strategic position to remove the flotation, i.e. the A/T – A/B section and store it in a state of preservation. Dolphins and scaffolding were removed shortly after.

A small note in the Townsville Daily Bulletin of Thursday 18 March 1946 noted:- "The boom defence at Cairns was now being demolished."

"Boom Defences" based on information supplied by Commander William Harold Thurlby, Director of Boom Defence.

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