TIDAL STREAMS.

Putaruru Press, Volume VII, Issue 294, 27 June 1929, Page 8

TIDAL STREAMS.

A FIELD FOR RESEARCH. A question vitally important to all seafaring men is tliat of tidal streams and currents round Australia (states Commander H. T. Bennett, in the Argus). Tlie* surveys of the coast of Australia are all very old, and though Admiralty charts are revised, as far as possible-, by corrections from the latest information available, there are still many inaccuracies to be rectified. Recently it has been found that islands off the coast of Queensland are charted very much, out of position—the Capricorn Group, Lady Elliott Island, North Reef, etc. The islands of ■Whitsunday passage was not in their proper position, which meant that a mariner who used objects on this chart for fixing h.s position would get an inaccurate fix. If he attempted to check hi&domrqr bg the^ simple methods & a > eak ,n line 'with fUi a rock, he would

find that By jising different objects as he passeJfthfbugh the channels of Whitsunday he was getting totally Afferent compass errors, although his ship might be still heading in the same direction. If he be' fixing his position as he is passing through this area, using different objects, he will find that he is not proceeding on a straight line, and at once he thinks that he has encountered a current cither with, against or sideways. It is therefore necessary, in the interests of navigation, that surveys of the Australian coast be energetically pursued. Little is really known of tidal’ streams round Australia, and the original surveys have been shown to be very cursory. Tidal streams can be determined only by months of ■work, the longer the better, so as to -obtain an exact idea of the time of turning before or after high or low water at any spot, with reference to U fixed place where the heights and times of t de are being observed and recorded continuously. This can be done only by an efficient surveying service, of which there is the skeleton in H.M.A.S. Moresby. The other ship of the Australian Navy, H.M.A.S. Geranium, which was employed on thin important work was, for economy’s sake, put out of commission, much against the wishes of the Naval Board, and just at a time when her staff was fully competent to carry out any hydrographic work asked of it, leaving a survey of the approaches to, Darwin uncompleted, on the main track of shipping in an area where the tides are paradoxical. The tidal streams in Van Diemen’s Gulf are most erratic, and five -months’ study by the Geranium did not solve by any means the directions of streams and other data required. In one spot the ship could be, anchored in a stream frunning north east air more than three knots, and if he were a quarter of a mile away from this spot the tide at the same time would be running south west at the same rate; none of this information was on the existing charts.

In Clarence Straits, which is the approach from Dundas Strait and Van Diemen’s Gulf to Darwin, the tide runs up to seven knots, and after many days of strong south easterly winds ten knots have been experienced on the wind dropping, the reason being that the water has been heaped Up to *.he west of the Clarence Straits and as soon as the wind abates it is free to flow back to the gulf. This abnormal increase in the usual tidal stream may last for a few hours or longer, much depending on the configuration of the ocean floor. That the Kanovma encountered such an abnormal “set,” caused by abnormal winds, is certain. This led to her destruction. Other ships wrecked, such as the Coons and Riverina, reported similar abnormal currents or tidal streams. Data to prevent similar accidents can only be ob+ained by survey. Ships fitted for this purpose specially should be provided. ‘ There is enough surveying work round the Australian coast to keep two ships employed for 300 years. Though aerial photography will help the surveyor with his coastline. and though electrical sonic methods of obtaining soundings will expedite this branch of hydrographical work, a ship will always have to be Employed to carry this electrical apparatus over the area to he sounded, fixing her position as she goes along by angles to points determined by trigonometrical triangulation. A sur’-pv’ng ship costs about £60,000 a year to maintain. Surely the federal Government should not close its eyes to the urgent need to improve the exi sting charts, and to m ake new cherts of large areas, many of which have not been touched. The study of tidal streams, which is a most important part of a surveying ship’s work, is essential to the safety of shipping.