“Now the shore stations have the longest range; they’ll spot a sub nearly a thousand miles away, but since their work is long-distance triangulation, they’ll place her only roughly. This will serve very well to tell the destroyers and chasers where to go to look for her, but by the time the information is transmitted and the hunting craft have got there, the sub may be too far away from the designated spot to find her with hydrophones. It is in this intermediate stage that the radio compasses on the destroyers themselves will come in. These have less range than the shore stations, but more than the hydrophones. Cruising in line, the destroyers may, by quick work, get cross-bearings on a sub sending signals anywhere within fifty miles, and thus locate her to within a mile. Then there’s a fair chance of picking her up with the hydrophones or magnetic detectors after she has submerged; and once that is done there’s a fair chance of covering her with a pattern of depth bombs, if the team-work is good.
“Now that sounds all very pretty, here by the fireside. But the pirate will have sense enough to fear these tactics, and will cut his messages down perhaps to one or two letters at a time, which won’t give the boys much chance for accurate bearings, no matter how quick they are. Then, he too has hydrophones, and when he hears us coming, he’ll slow down till you can’t hear him unless you’re almost on top of him. Still, with all these difficulties, it’s worth while playing that game for all there is in it. It may get us a few subs, if we keep it up on a big enough scale; even if it doesn’t, it will bother the sub and cramp her style, and it will give our boys the finest kind of drill.
“Now, there’s another matter which the difficulties of hunting with destroyers brings to my mind. In the war with Germany the chasers used to run their drifting patrols and hunts with all their listening gear, and once in a while, probably even less often than they thought, they picked up the sound of a sub. Then it was ‘up tubes’ and give chase; then ‘down tubes’ and listen again; a short burst of speed, then a dead stop, for they couldn’t listen under way. The chases had to be short, for Fritz could change course and speed, and then, while they were rushing to where they thought he was, they’d lose him. It was a sort of a hare and tortoise race, and you remember who won that. Our chasers to-day are somewhat faster and have much better hydrophones than those of 1918, but the problem is still much the same.
“Now, there’s a group of young officers in the Bureau of Engineering that have studied the history of those chaser maneuvers of 1918, and have put their heads together and worked out what looks to me like a real solution of that difficulty. More than one of the chaser captains of that time have said that, if they could have continued to listen instead of having to attack, they would have kept track of the sub’s whereabouts much longer than they could in the intermittent pursuit. Taking their cue from this fact, these officers have worked out a scheme which enables the listeners to keep on listening, and then to delegate other vessels to go to the spot they designate and surround the sub as a seiner surrounds a school of mackerel—with a string of nets. Their plan is to develop a fleet of ships like those that laid the great North Sea mine barrage—ships that will be fast and will have machinery for laying a net at very high speed. A sub can cut through an ordinary net and escape, so they have worked out plans for a net that will give notice if she tries it. It is to be studded with small telltale bombs just big enough to raise a fountain, but not big enough to add much to the weight and size of the net. These bombs, each in a section a hundred yards long, will be exploded by an electrical device if the sub touches any part of that section. It won’t hurt the sub, but it will tell you where she is, and you’ll have boats on hand with depth charges to do the hurting. The point is to establish quickly a barrier around the sub through which she can’t pass without giving away her position.
“These young officers have been fearfully keen on this thing and have worked out practical plans for machinery that will pay out the net at a speed of six knots, and the electrical gear for setting off the bombs.
“You can’t anchor the net, for the ocean’s too deep out there; it must be suspended from buoys. It needn’t reach down more than about three hundred feet, for no sub could dive under it at that depth without being crushed by water pressure. The tactical scheme is to have these boats, eight or ten of them, lie in wait at a central point with groups of drifting listeners scattered all around them, say, twenty miles away. When a sub is heard and located, the listeners call the net-layers by radio, and they go to the spot at a thirty-knot speed. As they approach they soon drown out the sound of the sub, but their speed and number enables them to surround the area in which she is known to be, for she won’t have time to go far from where she was last heard. Paying out their nets, they hem her in and keep the ring of nets closed round her till the area is raked with trailing wires and other electromagnetic detectors which will enable the chasers to concentrate their depth charges with the certainty of a kill.”
“That sounds like the most constructive—or rather artistically destructive—proposition I’ve heard yet,” said Mortimer with enthusiasm. “What are they doing about it? Have they submitted it to the Bureau Chief?”
“They’ve tried to, but they couldn’t get much attention paid to it.”
“Why not?”
“I’ve tried to find out why, but I can’t quite make it out. On the face of it the trouble seems to be the usual conservatism and inertia about taking up anything new. Did you ever read the article Sims wrote on Military Conservatism?”