Obviously a torpedo fired at a range of 8,000 yards having a speed of 30 knots an hour, or, in other words, of 50 feet per second, is not comparable to a projectile from a gun which has a velocity at 8,000 yards of say 2,000 feet per second. The torpedo may run perfectly straight after discharge, but unless the speed and course of the target have been determined with considerable accuracy, the torpedo will not hit. Let us assume that the target ship X at position A is steaming at 15 knots, and that the destroyer attacks from a favourable position on the bow so that the torpedo with its speed of 30 knots is discharged on a line at right angles to the course of the target at a distance of 8,000 yards (_see_ diagram). The target ship will advance 4,000 yards along the line A B whilst the torpedo is running 8,000 yards along the line C D. The time occupied in each case is eight minutes. It will be seen that if the course of the target ship has been misjudged very slightly, or had been altered during the passage from A to B, the torpedo will pass ahead or astern of it. In that case it might hit instead a ship Z ahead or one Y astern of X. There are no means available on board a destroyer for determining with any real accuracy either the speed or the course of a ship at a distance of four or five miles. Hence the difficulty, and the reason why torpedoes are fired at a ship a little way down a line of ships, in expectation that _one_ of the ships in the line will be hit. [Illustration] The object in view is thus rather to “brown” the enemy, and the chances of achieving this object are naturally proportional to the target presented by a ship as compared with the space between adjacent ships. In the case of a British line of eight battleships attacked “beam on,” the chances of a hit for torpedoes which reach the British line may be assessed roughly at seven to nine, taking the length of a ship as 600 feet, and the distance from the bow of one ship to the bow of her next astern as two and a half cables, that is 1,500 feet, thus giving a total length of _ships_ of 4,800 feet, and the total of the _interval_ between them as 6,300 feet. A German destroyer usually carries six torpedoes, and at long ranges one may calculate the chances of hits _on the above reasoning_ at between three and four per destroyer, provided all the torpedoes are correctly fired at such a range as to ensure that they reach the British battle line, and provided that the British ships can take no effective steps to avoid the torpedoes.[O] [O] Few British destroyers carried more than four torpedoes up to the year 1917, although they mounted a much heavier gun armament than their enemies.