Engineering Wonders of the World
Volume III

THE LAYING OF SUBMARINE CABLES. Fig. 29.—SLIPPING BIGHT AT BOWS. a level bottom, quite straight but without tension, it forms an inclined line towards the position of the bottom that Laying- Main uifjmately occupies—pre- Cable. • 1 XU X r cisely the movement oi a battalion in line changing front. Again, when paying out cable in an ocean depth of three miles, it is calculated that, with the ship steam- ing eight knots, the length from the stern of the vessel to the spot where it touches the ground is over twenty-five miles, and that it takes a particular point in the cable more than two hours and a half to reach the bottom from the time it first enters the water. As has already been indicated, in order to provide for the declivities of the bottom, a certain length of spare, or “ slack,” cable requires to be paid out beyond that which would be involved by the distance over- ground. The slack cable actually so paid out will be inversely proportional to the square of the ship’s speed, and depends, firstly, on the weight of a length of cable sufficient to reach the bottom vertically ; and, secondly, on the holding-back force. It can in fact, be varied either by regulating the brake force or changing the speed of the 373 vessel ; but the former plan is more im- mediately effective. The average slack with which the cable is to be laid is generally arranged beforehand. It is well never to let it fall appreciably below five per cent., and it should be increased to ten per cent, (or more, if necessary) over a sloping or irregular bottom. The speed of the ship during laying being usually from six to eight knots, tables are calculated in advance corresponding to dif- ferent rates of speed within these limits, giving, for about every 50-fathom depth, the load to be placed on the brake levers, in order to lay anything between five and twelve per cent, slack. With these tables the slack is readily regulated, provided we know the depth and the speed of the ship overground with sufficient accuracy. A development of this in modern practice is to pay out a small steel wire without slack, and by comparison with this to regulate the paying out of the cable. This plan was due to that dis- tinguished electrical engineer the late Werner Siemens. The soundings taken previous to laying the cable should be numerous enough to give a tolerably exact profile of the bottom between the two landing-places. The track of the cable is naturally plotted on a chart, and the positions of the ship at any time are, of course, fixed by astronomical observation as occasion offers. Recourse has also to bo made to the ship’s log and the revolutions of the propeller for estimating the distance covered by the vessel, and so also helping to give the “ dead reckoning ” position at any moment.* * Though some of the larger vessels are capable of holding upwards of 1,000 miles in each tank, it is usually necessary to perform the operation of “changing tanks ” during the laying of a long line. That is to say, the cable in one tank being exhausted, that in another has to be turned to. It would be beyond our scope to deal with the full routine of this some- what delicate operation. It was, however, described in de- tail by the author in his recent lectures to the Royal Naval War College, Portsmouth, as well as previously in those de- livered to the Royal Engineers at Chatham, since duly published.