ECHO-SOUNDING

Evening Star, Issue 21004, 19 January 1932, Page 10

ECHO-SOUNDING

MATAI'S APPARATUS EQUIPMENT NOW IN ORDER SUCCESSFUL TESTS In the last annual report of the Secretary of Marine, it was mentioned that the only feature of the new lighthouse steamer Matai which had not proved entirely satisfactory since the vessel was placed in commission at the end of 1930 was the echo depth-sound-ing apparatus. It is explained (says tho ‘Post’) that the apapratus was in a transitional stage when it wb fitted on the Matai, and this, added to the inexperience of tho ship’s officers regarding its working, accounted for the unsatisfactory results. When Captain H. R. Hughes, who recently retired from the command of the cable steamer Iris, was in England last year he made a study of the apparatus, and when he returned by the Romuera in September he brought with him a set of the latest equipment, which has since been fitted to the Matai, and, after extensive trials, which were completed in Cook Strait yesterday, has’proved entirely satisfactory. .Prior to the war, vessels relied almost wholly on a weighted line for depth-finding, but considerable progress has since been made in Nie development of new systems of sounding at sea. Those new methods all employ sound waves as a means of measuring depths, thereby saving considerable time and trouble, and are steadily replacing the old lead and wire systems. A sonneting in 4,000 fathoms can be made in about lOseo by the new “echo” methods, whereas many hours of labour are involved to obtain a somewhat doubtful result by the old “wire” system. Further, observations can be made when tbe ship is travelling at speeds up to fifteen or twenty knots. MEASUREMENT BY SOUND. Tho echo method involves the measurement of distance in terms of the time interval between the initiation of a sound impulse and the reception of an echo. Sound is reflected from the sea bed in the same way that it is reflected in air from building and cliffs, with the difference that in air sound takes about ssec to travel a mile, whereas in water the time is only about Isec. Actually, in sea water, sound travels at the rate of 4,800 ft per second. . Various methods have been devised in America, France, Germany, apd Great Britain of using echoes for depthsounding purposes, the differences lying in the manner of producing and receiving the* sound impulse. In one system, the Behm system, a small detonator is fired under the water, thereby operating a microphone and relay, and setting a graduated disc in motion; the arrival of the echo stops the disc, which is graduated in “depths,” and the angle through which it has revolved is a measure of the time interval, and consequently of the depth. The method developed by the British Admiralty depends upon the indirect measurement of the time interval. The transmitter which, on tho Matai, is fitted aft on the starboard side of the hull, consists essentially of a steel diaphragm which is struck a powerful blow by an electro-magnetic hammer, thereby emitting a strong sound-wave. The receiver, or hydrophone, is an ordinary “ button ” carbon granule microphone, mounted on_ a small diaphragh, and enclosed in a watertight container. The Matai’s receiver is situated forward, on the port side. The apparatus is electrically operated so that at the moment the sound is transmitted the hydrophone is not ‘‘ listening,” but it is ready to receive the echo when it arrives. RANGE OF 4,000 FATHOMS. There are two types of apparatus, a shallow water set, such as that on the Matai,' which records up to 133 fathoms, and an oceanic apparatus recording up to about 4,000 fathoms, which is sufficient to ascertain the depth of practically any part of the sea. For ordinary navigation purposes the 133-fathom typo is sufficient, as it enables vessels to pick, up tho 100fathom lino on approaching the coast, but it is felt that in a vessel such as tho Matai, which is largely used for investigation purposes, the oceanic type would he of value. The difference in cost is about £l5O. _ Both types are in common use in British and foreign commercial vessels, and in the survey ships of the British Navy. The cable steamer Iris (recently renamed the Recorder) uses a line for depth-sounding purposes, hut on other British and Italian cable steamers the echo-sound-ing apparatus is generally used. One inventor has claimed to have evolved an-apparatus, which can detect icebergs up to a distance of two and ahalf miles ; but in some quarters this is considered surprising and remarkable, to say the least, in view of the relatively small size of icebergs and the poor reflecting properties 'of ico under writer NEW EQUIPMENT. When Captain Hughes, who is the New Zealand representative of Henry Hughes and Son Ltd., London, makers of the Admiralty typo of echo-sounding apparatus as fitted on tho Matai, arrived in New Zealand, tho new machine was installed and tested in the Haurki Gulf. As the Matai’s forward hold was full of coal, however, it was not possible to _ fit tho new hydrophone until the_ ship returned to Wellington. The ship has profited from the delay since she was built by being now equipped with the most up-to-date apparatus. The transmitting apparatus emits three beats per second and takes fortyfive soundings each minute. The soundings, which are electrically Recorded on sensitised paper, can also be heard on earphones, either independently of or by using a different circuit, in conjunction with the record on the paper. The paper has to be used wet, and trouble is sometimes as the result of its breaking while it is being wound from one cylinder to another. CONTOUR OF SEA BED. When tho ship is steaming at a speed of one knot, the soundings are made 2.25 ft apart, and it is thus possible to calculate the distance between them according to the speed of the ship. The paper chart is marked in minutes, so that when tho time at any given point is known, the time at another point can be found without difficulty, and tbe depths found can be correctly mapped. The Matai was out in the Strait all day making the tests, which were carried out under the eyes of the jjhip’s master (Captain Burgess) and his officers, Captain Hughes, and a representative or the Post and Telegraph Department. Tho course followed the telegraph cable to Scddon, and a distance of between sixty and seventy miles was covered at a speed of about nine knots.

A glance at the records of the soundings is exceptionally interesting, ahd would bo oven more so but for the fact that the depth sometimes drops right off the chart. The irregularity of the Cook Strait locality is revealed by a number of submarine “craters,” one of tho deepest of which is located just outside tho Heads.