148 CHALLENGER 



Challenger and the trawlers were fitted with gear called the 

 Oropesa sweep, normally used for the sweeping of moored mines, 

 but also used by surveyors to sweep for obstructions at various 

 depths below the surface of the sea. The merit of the apparatus 

 is that by means of steel 'otter boards' the sweep rims out to 

 port and to starboard of the sweeping vessel at a set depth and 

 thus the ship does not need a partner to sweep a wide swathe of 

 the channel. Challenger worked in the somewhat deeper water to 

 the eastward using this gear, but so shallow was the water of the 

 Prince of Wales Channel in which the trawlers were sweeping 

 that the otter boards themselves, which hang five or six feet 

 below the sweep wire, were fouling the bottom. So the trawlers 

 worked in pairs, dragging a chain bottom sweep between 

 them. This could not be done however where the sea-bed was 

 rough and so in such areas a wire sweep was rigged between two 

 vessels. The light wire used for the sweep is held down below 

 the stern of each ship by passing it through a block secured to 

 a heavy weight suspended from the stem of the sweeping craft. 

 Wires with floats attached to them are secured at intervals along 

 the sweep to keep the whole 1000 feet of sweep w^ire at the 

 correct set depth. Such work had to be taken very slowly: the 

 vessels did not sweep at random, for in one of these craft, each 

 swathe cleared was plotted upon the sounding boards by a sur- 

 veyor who repeatedly fixed his own ship by reference to the 

 survey marks which Challenger had now erected and co-ordinated 

 on the numerous small islands and reefs along both sides of the 

 channel. The second trawler maintained its careful distance of 

 1000 feet from the fixing vessel. Thus was the progress of the 

 work recorded day by day and gaps in the sweeping filled. 



Challenger's work consisted of the erecting and fixing of the 

 marks, the sounding out of the more intricate parts of the straits 

 with her echo sounding boats in order to locate the deepest 

 passages, and, together with the Royal Australian Naval mine- 

 sweeper Echuca, which had now joined the Task Unit, sweeping 

 the North-East Channel with Oropesa gear. Each time this equip- 

 ment fouled an obstruction the ships would have to be hauled 

 back and every effort made to salve as much of the sweep gear as 

 possible. There were many obstructions and the supply of sweep 

 gear arriving at Thursday Island barely kept pace with these un- 



