This tremendous spider web was sixty miles in diameter. It allowed for the searching of four thousand square miles of sea, and was right across the path of the submarines. A submarine ten miles outside of it was in danger of being spotted, so at cruising speed it took ten hours for a U-boat to cross it. Under ordinary conditions a boat could search two sectors—that is, a quarter of the whole web—in five hours or less. The tables were turned on Fritz the hunter; for here he was the hunted, the quarry, the fly that had to pass through some part of the web. The flying-boat was the spider.
The Spider Web Patrol was based on the North Hinder light-vessel, which was used as a centre point, and allowed for a thorough searching of the sea in a forty-mile radius. It was an octagonal figure with eight radial arms thirty sea-miles in length, and with three sets of circumferential lines joining the arms ten, twenty, and thirty miles out from the centre. Eight sectors were thus provided for patrol, and all kinds of combinations could be worked out. As the circumferential lines were ten miles apart, each section of a sector was searched twice on any patrol when there was good visibility.
A chart was kept showing the positions, dates, and times of day that submarines were fixed by wireless, and it was from this chart that the sectors which would pay for searching were determined.
The pilots were to boom out from Felixstowe to the North Hinder, a distance of fifty-two sea-miles, fly out a radial arm as instructed, and then proceed along the patrol lines in the sectors to be searched, sweeping from the outside to the centre, returning to the North Hinder and so to the base.
Navigation over the sea, where one square mile of water looks exactly like every other square mile, is more difficult than finding the way over land. The only fixed objects by which a pilot can check his calculated position are light-vessels and buoys, but in war-time these are shifted about, and there are large areas without any such marks.
The difficulty of navigation is due to the fact that unless there is absolutely no wind, the compass, after the corrections for variation and deviation are made, only shows the direction in which the head of the flying-boat is pointing and not the direction in which it is travelling, and the air-speed indicator only gives the speed of the machine in relation to the air.
For an aircraft is completely immersed in the air, so that besides its movement in relation to the air caused by its own mechanism, it moves with the air over the surface of the earth, the speed and path of the machine being the result of the two movements.
If the pilot of a flying-boat had to go to a light-ship sixty miles due east from his station when a twenty-knot wind was blowing from the north, and he flew at sixty knots due east by his compass, at the end of an hour he would not fetch up at his object, but twenty miles to the south of it. If, instead of flying on 90 degrees, which is east, he flew on 71 degrees on his compass, he would fetch up at the light-ship in sixty-three minutes, having travelled due east over the surface of the sea. To a man in a ship he would appear to be flying sideways.
Similarly, if a pilot flew into a sixty-knot wind with his air-speed indicator showing sixty knots, he would not be moving over the surface of the sea, and to the man in the ship he would appear to be standing still.
The Chaplain of the station, the Rev. W. G. Litchfield, produced for us a simple table with which the pilot, knowing approximately the force and direction of the wind, could quickly work out the compass correction for drift and the time correction for the air-speed indicator.