As already said, the Otranto Strait was an ideal location for this type of anti-submarine craft. It was so narrow--about forty miles--that a force of moderate size could keep practically all of the critical zone under fairly close observation. Above all, the water was so deep--nearly 600 fathoms (3,600 feet)--that a submarine, once picked up by the listening devices, could not escape by the method which was so popular in places where the water was shallow--that of sinking to the bottom and resting there until the excitement was over. On the other hand, this great depth made it very difficult to obstruct the passage by a fixed barrier--a difficulty that was being rapidly overcome by a certain Franco-Italian type of torpedo net. This barrage, after the arrival of our chasers, was so reorganized as to make the best use of their tactical and listening qualities. The several lines of patrolling vessels extended about thirty-five miles; there were vessels of several types, the whole making a formidable gauntlet, which the submarines had to run before they could get from the Adriatic to the Mediterranean. First came a line of British destroyers; it was their main duty to act as protectors and to keep the barrage from being raided by German and Austrian surface ships--a function which they fulfilled splendidly. Next came a line of trawlers, then drifters, motor launches, and chasers, the whole being completed by a line of kite balloon sloops. Practically all these vessels, British as well as American, were provided with the American devices; and so well did these ingenious mechanisms function that it was practically impossible for any submarine to pass through the Otranto barrage in calm weather without being heard. In fact, it became the regular custom for the enemy to wait for stormy weather before attempting to slip through this dangerous area, and even under these conditions he had great difficulty in avoiding detection.