CDBOP & CASL Directing Station

Close Defence Battery Observation Post

While a Battery Observation Post was already located on Lookout Hill, approximately 1.2 kilometres west of Bickley Battery near the site of the Wadjemup Lighthouse, the Close Defence Observation Post (CDBOP) played a critical role during night operations. At night, the CDBOP was manned by the Gun Position Officer, largely because visibility was significantly reduced after dark. This limitation was further compounded by the restricted range of the coastal searchlights, which could illuminate targets only out to a distance of approximately 7.3 kilometres.

This meant that close-range monitoring of nearby waters became increasingly important after sunset. The CDBOP provided a vantage point from which threats approaching under the cover of darkness could still be detected and responded to.

The Control Tower

The control tower for the CDBOP was located on the first floor of the northern side of the building. From this elevated position, it offered uninterrupted 360-degree views across the Rottnest Island landscape and surrounding waters, allowing observers to maintain constant situational awareness.

On the southern side of the building, at ground-floor level, was the Coastal Artillery Search Light (CASL) Directing Station. From here, the Battery Commander could issue direct instructions to the Search Light Officer using a four-inch voice tube. This simple but effective communication system allowed the commander to specify which targets were to be illuminated by searchlights during night operations (Glyde).

Depression Range Finders (DRF)

Inside the CDBOP were two instrument pillars, each fitted with a Depression Range Finder (DRF). Together, these instruments provided full 360-degree coverage around the site.

Depression Range Finders, also known as Depression Position Finders, resembled a telescope mounted on a fixed base. They were used to determine the position of a target at sea by measuring the angle between the observer and the waterline of the target vessel. Once a target was sighted, the operator would read a scale on the instrument, which automatically converted that angle into a distance. The necessary calculations were built into the internal scales and gearing of the DRF and were carefully configured when the instrument was installed for a specific site.

The calculated range and bearing were then transmitted mechanically by dials to the gun positions, allowing the guns to be aimed accurately without the gun crews needing to see the target directly.

2014 refurbishment works

In 2014, Stantec Pty Ltd was awarded a $1.8 million contract to refurbish the Coastal Defence Battery Observation Post and the Coastal Artillery Search Light Directing Station.

The refurbishment works were undertaken with an emphasis on retaining the integrity of the original structure. As part of this process, the structural team:

treated existing steel elements
installed new reinforcements
repaired deteriorated concrete
replaced damaged glass
refurbished masonry
carried out steel preservation treatments on the blast-resistant doors to maintain the original painted signage

Significant attention was given to the roof structure. Due to the risk of collapse, extensive concrete repairs were undertaken to address advanced spalling and corrosion of the internal steel reinforcement.

January 2023

The experience

The interior of the CDBOP was relatively free of graffiti and wall carvings, aside from a small number of artistic drawings. Along the outer edge of the ceiling, numerous wasp nests were present, most of which appeared to have been abandoned some time earlier. Fragments of wasp skins were scattered near the entrances to several of the nest chambers.

At the time of entry, a single intact orange spider wasp was visible near the top centre of the CDBOP. Apart from occasional movement, possibly related to nest maintenance, it showed no signs of aggression. At times it appeared to pause, almost as if observing its visitor, before continuing with its activity. On later inspection of one photograph, however, it raised the question of whether the wasp had in fact been alive, as its thorax appeared to be separated from its abdomen.

Aside from occasional gusts of wind, the distant sound of small waves breaking nearby and the brief passage of single-engine aircraft overhead, the silence within the tower was striking. This quiet amplified the historic presence of the structure, particularly given the absence of crowds and the lack of modern interpretive overlays often associated with tourist sites. This sense of isolation is a key reason why visits to such places are often most rewarding when undertaken alone.

The experience felt rare and immersive, allowing time to absorb the space and document details at a leisurely pace. As time passed, the heat inside the structure became increasingly intense, especially with the metal shutters on the southern side bolted shut. Stepping back outside brought immediate relief as a cool breeze swept through, though this quickly gave way to the full force of the day’s heat, which hovered around 34 degrees.

References

The Coast Defences of Western Australia 1826–1963: A Study. R. K. Glyde (printed 1991).