An Investigation on Suitable Position of Transducer for the Echo-sounding Equipment

Abstract

The Precision Depth Recorder (PDR) manufactured by Nippon Electric Company of Tokyo was installed in usual manner on MS HAKUHO MARU, an ocean research ship delivered by the Shimonosheki Shipyard of Mitsubishi Heavy Industries, Ltd. to the Ocean Research Institute of the University of Tokyo, in March, 1967. She is 3, 226 tons gross and 87 meters in length. In spite of its capability of sounding as deep as 10, 000 meters, we found that the equipment could not show the sea bottom at such depth when her speed exceeding 10 knots at the scale of the sea state class 3, being disturbed by the flow-noise. The flow-noise is generally attributed to bubbles in the current of water in the front of transducer. We have, therefore, investigated how to obtain the optimum performance of the equipment by installing temporarily four hydrophones under her keel between the transducer of the PDR and her FP, and we observed noises of each hydrophone. By this investigation it was verified that the layer of water current containing bubbles is generated under the bottom of ship. Though this layer is very thin near the stem but it becomes thicker as the distance from the stem increases towards the stern. We found that its thickness at the frame No. 87 was about 40cm, where the transducer has been installed. The speed of ship was about 10 knots. From the results obtained, we installed a new transducer under keel at the point of 7.1 meters from FP on the side of stern. By this reformation the aggregate performance of the equipment is greatly improved. In scheduled survey trip from November, 1968 to March, 1969, she crossed the Tonga Trench and sounnded the sea bottom. The tests were very successful. When she crossed the trench at the speed of 10 knots toward east against head wind, the echo signals were never masked by flow-noise. The maximum depth recorded is 10, 450 meters. On her westward course with the speed of 14 knots in fair wind, similar tests were performed ; the maximum depth recorded was 9, 800 meters. The meteorological conditions during the investigation were: 1) the scale of wind was class 4, 2) those of waves and swells were both classes 3. The draught of the ship was 3.80 meters at the stem and 6.40 meters at the stern.