The difficulty of attacking a surface warship steaming at right angles to the course of the submarine will be clearly understood by referring to the following diagrams. The first shows an attack on a warship travelling at 20 miles an hour, such as a big battleship or a cruiser Any increase in the speed of the surface vessel not only adds to the difficulty of the attacking submarine, but also the direction from which the attack must be made. This feature is shown in the second diagram, which illustrates a submarine attack on a vessel steaming at 30 miles an hour, such as a fast destroyer or fleet scout. On the other hand, a decrease in the speed of the on-coming surface vessel tends to either make easier the task of the attacking submarine, or else to increase the distance from which the attack can be delivered. This is shown in the third diagram, which assumes the speed of the surface vessel to be only 15 miles an hour, such as a merchantman, troopship, food-ship, collier, or old warship.
Right-Angle Attack by Submarines.
Fig. 1 represents a submarine attacking a hostile warship (or fleet) steaming at 20 (statute) miles an hour. “A” is the line of vision. The submarine sights the warship at a distance of just over 11 miles on her port bow. “B” shows the hostile vessel’s course, which is 10 miles to point marked “C,” and each division beyond equals 1 mile.
Directly the submarine, which is assumed to be lying in an awash condition, sights the object of attack, she totally submerges and steers forward at a speed of 10 miles an hour. The loss, and gain, of the submarine on the different courses, can be seen in the table above the chart.[[7]]
The spaces between the black dots show the most favourable points of attack. It will be noticed in the table that both vessels are equal at point “C,” but for many reasons this is not the best point of attack. The gain of about six minutes on the longer course enables the submarine not only to manœuvre into the best possible position for the attack, but also to discharge more than one torpedo if necessary.
Fig. 2 shows the extreme limit at which a submarine could, with reasonable chances of success, attack a destroyer, or other vessel, steaming at 30 (statute) miles an hour, having sighted her at a distance of 16 miles in the position shown by the line of vision “A.”
The distance to “C” is 15 miles for the surface vessel, and 5 miles for the submarine. Here, again, the two vessels would be equal; but the most favourable point of attack is shown by the two black dots—where the submarine has gained two minutes.
Fig. 3.—The submarine sights the object of attack at a distance of 14 ¼ miles, in the position shown by the line of vision “A.” The surface vessel has a speed of only 15 miles an hour (merchantman). In this case the surface vessel accomplishes the 10-mile journey along course “B”—arriving at point “C” 20 minutes in advance of the submarine. The table shows how the submarine, by changing her course and “throwing” the surface vessel on her beam, gradually reduces the loss, until, at the point marked with the two black dots, she is but 4 ½ minutes behind. At this distance she could fire her torpedoes at long range, with some likelihood of success.
Although these charts show approximately the extreme limits of the right-angle attack, a submarine could, of course, proceed for some distance on the surface at a much faster speed; but considering the rate at which the two vessels would be approaching each other, the submarine which attempted it would run considerable risk of being detected, and thus destroy her chances of a successful attack. Considering also the time lost in sinking from the “light” to the totally submerged condition, in coming to close quarters, the gain in speed would not amount to as much as may at first seem probable.[[8]]