Two attacks on suspected Japanese submarines by U.S. Navy Subchaser SC 648 of the U.S. Navy's 7th Fleet in Australian waters during WW2

TWO ATTACKS ON SUSPECTED JAPANESE SUBMARINES
BY U.S. NAVY SUBCHASER SC 648
OF THE US NAVY'S 7TH FLEET
IN AUSTRALIAN WATERS DURING WW2

Subchaser SC 648 was attached to the 7th Fleet, US Navy and operated in Australian waters from 8 May 1943 to 18 August 1943. It operated out of Brisbane for 100 days

On 23 May 1943, in company with HMAS Vendetta, SC 648 made a submarine attack outside Moreton Bay, but with no results. The depth charge attack was at Lat. 23 degrees, 32', 30" S x Long.151 degrees 26' E. They thought they had something. They fired a round of mousetraps and three of them exploded underwater (mousetrap projectiles exploded only on contact). They then made a full depth charge attack. The only result was a good-sized air bubble and a thin trickle of oil. They were later told that no enemy submarines were within 500 miles at the time. Ted Treadwell 's assessment was that they must have attacked a sunken vessel and in their excitement merely thought it was moving.

On the 7 July 1943 when they were in a convoy enroute from Gladstone to Brisbane, one of the escorts, HMAS ML 429, made a depth charge attack on a suspected enemy submarine and SC 648 and HMAS Lithgow left station to assist. The convoy did some radical turning and avoidance manoeuvres but nothing resulted. They were not far from Lady Elliott Island lighthouse when this action occurred.

Both of the above actions was based on underwater detection of a possible enemy submarine. They picked up a distinct echo and began tracking it, ultimately ending in depth charge attack. SC 648 had underwater electronic echo-ranging sonar gear. During their escorting trips the gear was in use around the clock, manned by a sonarman on all watches. The sonar equipment send out "pings" under water which would bounce off a ship's hull or a submarine hull, or a whale, a school of fish, a coral formation, or even a buoy. A good sonarman was able to distinguish by the sound of the echo. The time lapse between a ping and an echo from that ping would give the range to the target. They were also able to get a bearing also. If the bearing changed from one echo to another they would know the object was moving. The echo from a submarine's hull was clear and sharp. Echoes from fish etc. tended to be more muffled.

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