THE AUTOMATIC DEPTH RECORDER AS AN AID TO NAVIGATION
By Lieutenant S. G. Lamb, U. S. Navy
As practically all the Navy knows, investigation and experiments have been carried on since the war to develop apparatus with which to locate the positions of submerged sound aids (sound transmitters) to navigation by recording their bearings, for use in foggy or unsettled weather, and further to obtain accurate depth soundings of the sea bottom without recourse to the hand lead or deep sea lead.
An apparatus which determines depth soundings by utilizing the reflection of sound waves from the sea bottom has been developed and perfected by Dr. Harvey C. Hayes, Sound Aide and Physicist of the Naval Experiment Station, Annapolis, Md., and is now in use on board the U.S.S. Stewart. The principle on which the apparatus works is, briefly, as follows: The time interval between making a sound wave on board ship, such that the wave is transmitted to the water through the skin of the ship, and the reception of the reflected wave from the sea bottom, is accurately measured and the depth is calculated from this time interval, having previously determined the velocity of sound in sea water in feet per second.
In practice an oscillator is mounted in a tank in the stern of the ship and a hydrophone is installed in the bow. Oscillator signals are made by the operator and the time between making the direct signal and the return of the reflected signal—the echo—is accurately measured by means of the sounding receiver. This time interval, referred to a scale, gives the depth in fathoms.
There follows a case in which soundings obtained as described above proved of great aid in locating and checking the position of the ship as previously observed by celestial navigation.
On August 1, 1922, while en route to the Asiatic Station in company with Destroyer Squadron 15, making passage from Aden, Arabia, to Colombo, Ceylon, a course was set to pass Farun Rocks at 9:10 A.M. on the starboard beam, distance twenty-one miles. No change of course was made during the night of July 31-August 1, nor was it noted that the course steered was other than the one which had previously been set. Reference to the chart, tracing attached, Figure 1, showed that the least depth of water on this course should be not less than 295 fathoms up until the time of passing Farun Rocks abeam, and that practically all depths to be expected were in the neighborhood of 1,100 fathoms. As the southwest monsoon was blowing during this time (and from the current descriptions in the Sailing Directions) a set to the northward of from one to three miles per hour was expected.
At 4:00 A.M., time zone description minus three, the listening operator on watch reported a sounding of 570 fathoms. Soundings were taken as shown in Figure 1, with depths as indicated. Upon comparison with the chart of this locality it was determined that the path of the ship was not that shown on the chart, line A B, but rather was that shown by line A' B', and that instead of the course previously set, 89° true, being made good, a course of 95° true had been made good during the night. This was later checked by accurate bearings and fixes obtained from navigational aids, and proved to be correct.
Another item of interest is that instead of a set to the northward being encountered, a set of three-eighths of a mile per hour to the southward was experienced.
Figure 2, shows the track of the Stewart leaving Aden. The soundings taken checked very closely with those on the chart, and with the navigational positions, and were obtained at the times expected, from previously laying off the distances on the chart. Figure 3, shows the track of the Stewart on crossing the hundred fathom curve south of British India, on the afternoon of August 5, 1922. Before reaching the hundred fathom curve it was estimated that the soundings should show close to one hundred fathoms at 15.07. At 15.00 a sounding of 115 fathoms was obtained; at 15.05 a sounding of 107 fathoms was obtained, and at 15.15 a sounding of 95 fathoms was obtained. Across the bank inside the hundred fathom curve the soundings by sound receiver do not check absolutely with those given on the chart, but it is thought that this may be due to inaccuracies in the former survey. The charted time for leaving the one hundred fathom curve was I7.20. At this time a sounding of 130 fathoms was obtained which showed an appreciable increase over previous soundings.
In Figures 2 and 3, the times are those of the twenty- four hour clock. The soundings in these two figures are underlined for the sake of clearness. In all three figures soundings obtained by sound receiver are set down opposite their respective times.
From a consideration of the soundings by chart and those by sound receiver it may readily be seen that the results obtained by this apparatus are accurate and are to be relied upon. This apparatus should prove of great value in the safe navigation of a ship, especially when cruising in waters where the bottom is well surveyed, as an accurate line of position may be obtained from soundings obtained by the sound receiver.
The United States Navy has the distinction of being the first to put this apparatus to practical use, as a line of soundings has been run to date from Newport, R.I., to Colombo, Ceylon, B.I., via the Atlantic Ocean, Mediterranean Sea, Red Sea, Gulf of Aden, Arabian Sea and Indian Ocean. This line of soundings will be continued to Manila, P.I., and when the Stewart returns to home waters will have been run around the earth.