"'Wouldn't it have been fine,' said Madden, 'if they had had on board a mine so designed that, when dropped overboard, it would have exploded when it reached the depth at which the submarine was lying?'"
"That remark," continued Admiral Jellicoe, "gave us the germinal idea of the depth charge. I asked the Admiralty to get to work and produce a 'mine' that would act in the way that Admiral Madden had suggested. It proved to be very simple to construct—an ordinary steel cylinder filled with TNT; this was fitted with a simple firing appliance which was set off by the pressure of the water, and could be so adjusted that it would explode the charge at any depth desired. This apparatus was so simple and so necessary that we at once began to manufacture it."
The depth charge looked like the innocent domestic ash can, and that was the name by which it soon came to be popularly known. Each destroyer eventually carried twenty or thirty of these destructive weapons at the stern; a mere pull on a lever would make one drop into the water. Many destroyers also carried strange-looking howitzers, which were made in the shape of a Y, and from which one ash can could be hurled fifty yards or more from each side of the vessel. The explosion, when it took place within the one hundred feet which I have mentioned as usually fatal to the submarine, would drive the plates inward and sometimes make a leak so large that the vessel would sink almost instantaneously. At a somewhat greater distance it frequently produced a leak of such serious proportions that the submarine would be forced to blow her ballast tanks, come to the surface, and surrender. Even when the depth charge exploded considerably more than a hundred feet away, the result might be equally disastrous, for the concussion might distort the hull and damage the horizontal rudders, making it impossible to steer, or it might so injure the essential machinery that the submarine would be rendered helpless. Sometimes the lights went out, leaving the crew groping in blackness; necessary parts were shaken from their fastenings; and in such a case the commander had his choice of two alternatives, one to be crushed by the pressure of the water, and the other to come up and be captured or sunk by his surface foe. It is no reflection upon the courage of the submarine commanders to say that in this embarrassing situation they usually preferred to throw themselves upon the mercy of the enemy rather than to be smashed or to die a lingering and agonizing death under the water. Even when the explosion took place at a distance so great that the submarine was not seriously damaged, the experience was a highly disconcerting one for the crew. If a dozen depth charges were dropped, one after the other, the effect upon the men in the hunted vessel was particularly demoralizing. In the course of the war several of our own submarines were depth-charged by our own destroyers, and from our crews we obtained graphic descriptions of the sensations which resulted. It was found that men who had passed through such an ordeal were practically useless for several days, and that sometimes they were rendered permanently unfit for service. The state of nerves which followed such an experience was not unlike that new war psychosis known as shell-shock. One of our officers who had had such an adventure told me that the explosion of a single depth charge under the water might be compared to the concussion produced by the simultaneous firing of all the fourteen-inch guns of a battleship. One can only imagine what the concussion must have been when produced by ten or twenty depth charges in succession. Whether or not the submarine was destroyed or seriously injured, a depth-charged crew became extremely cautious in the future about getting anywhere in the neighbourhood of a destroyer; and among the several influences which ultimately disorganized the moral of the German U-boat service these contacts with depth charges were doubtless the most important. The hardiest under-water sailor did not care to go through such frightful moments a second time.
This statement makes it appear as though the depth charge had settled the fate of the submarine. Yet that was far from being the case, for against the ash can, with its 300 pounds of TNT, the submarine possessed one quality which gave it great defensive power. That was ability to make itself unseen. Strangely enough, the average layman is inclined to overlook this fairly apparent fact, and that is the reason why, even at the risk of repeating myself, I frequently refer to it. Indeed, the only respect in which the subsurface boat differs essentially from all other war vessels is in this power of becoming invisible. Whenever it descries danger from afar, the submarine can disappear under the water in anywhere from twenty seconds to a minute. And its great advantage is that it can detect its enemy long before that enemy can detect the submarine. A U-boat, sailing awash, or sailing with only its conning-tower exposed, can see a destroyer at a distance of about fifteen miles if the weather is clear; but, under similar conditions, the destroyer can see the submarine at a distance of about four miles. Possessing this great advantage, the submarine can usually decide whether it will meet the enemy or not; if it decides that it is wise to avoid an encounter, all it has to do is to duck, remain submerged until the destroyer has passed on, entirely unconscious of its presence, and then to resume its real work, which is not that of fighting warships, but of sinking merchantmen. The chief anxiety of the U-boat commander is thus to avoid contact with its surface foe and its terrible depth charge, whereas the business of the destroyer commander is to get within fighting distance of his quarry.
Ordinarily, conditions favour the U-boat in this game, simply because the ocean is so large a place. But there is one situation in which the destroyer has more than a fighting chance, for the power of the submarine to keep its presence secret lasts only so long as it remains out of action. If it makes no attempt to fight, its presence can hardly ever be detected; but just as soon as it becomes belligerent, it immediately reveals its whereabouts. If it comes to the surface and fires its guns, naturally it advertises to its enemy precisely where it is; but it betrays its location almost as clearly when it discharges a torpedo. Just as soon as the torpedo leaves the submarine, a wake, clearly marking its progress, appears upon the surface of the water. Though most newspaper readers have heard of this tell-tale track, I have found few who really understand what a conspicuous disturbance it is. The torpedo is really a little submarine itself; it is propelled by compressed air, the exhaust of which stirs up the water and produces a foamy, soapy wake, which is practically the same as that produced by the propeller of an ocean liner. This trail is four or five feet wide; it is as white and is as distinct as a chalk line drawn upon a blackboard, provided the weather is clear and the sun is in the right direction. Indeed, it is sometimes so distinct that an easily manœuvred ship, and even sometimes a merchantman, can avoid the torpedo which it sees advancing merely by putting over the helm and turning out of its course. But the chief value of this wake to the submarine hunters is that it shows the direction in which the submarine was located when the torpedo started on its course. It stands out on the surface of the water like a long, ghostly finger pointing to the spot where the foe let loose its shaft.
As soon as the destroyer sees this betraying disturbance, the commander rings for full speed; and one of the greatest advantages of this type of vessel is that it can attain full speed in an incredibly short time. The destroyer then dashes down the wake until it reaches the end, which indicates the point where the submarine lay when it discharged its missile. At this point the surface vessel drops a depth charge and then begins cutting a circle, say, to the right. Pains are taken to make this circle so wide that it will include the submarine, provided it has gone in that direction. The destroyer then makes another circle to the left. Every ten or fifteen seconds, while describing these circles, it drops a depth charge; indeed, not infrequently it drops twenty or thirty in a few minutes. If there is another destroyer in the neighbourhood it also follows up the wake and when it reaches the indicated point, it circles in the opposite direction from the first. Sometimes more than two may start for the suspected location and, under certain conditions, the water within a radius of half a mile or more may be seething with exploding depth charges.
It is plain from this description that the proceeding develops into an exceedingly dangerous game for the attacking submarine. It is a simple matter to calculate the chances of escaping which the enemy has under these conditions. That opportunity is clearly measured by the time which elapses from the moment when it discharges its torpedo to the moment when the destroyer has reached the point at which it was discharged. This interval gives the subsurface boat a certain chance to get away; but its under-water speed is moderate, and so by the time the destroyer reaches the critical spot, the submarine has advanced but a short distance away from it. How far has she gone? In what direction did she go? These are the two questions which the destroyer commander must answer, and the success with which he answers them accurately measures his success in sinking or damaging his enemy, or in giving him a good scare. If he always decided these two points accurately, he would almost always "get" his submarine; the chances of error are very great, however, and that is the reason why the submarine in most cases gets away. All that the surface commander knows is that there is a U-boat somewhere in his neighbourhood, but he does not know its precise location and so he is fighting more or less in the dark. In the great majority of cases the submarine does get away, but now and then the depth charge reaches its goal and ends its career.
If only one destroyer is hunting, the chances of escape strongly favour the under-water craft; if several pounce upon her at once, however, the chances of escaping are much more precarious. If the water is shallow the U-boat can sometimes outwit the pursuer by sinking to the bottom and lying there in silent security until its surface enemy tires of the chase. But in the open sea there is no possibility of concealing itself and so saving itself in this fashion, for if the submarine sinks beyond a certain depth the pressure of the water will crush it.
While the record shows that the U-boat usually succeeded in evading the depth charges, there were enough sunk or seriously damaged or given a bad shake-up to serve as a constant reminder to the crews that they ran great danger in approaching waters which were protected by destroyers. The U-boat captains, as will appear, avoided such waters regularly; they much preferred to attack their merchant prey in areas where these soul-racking depth charges did not interfere with their operations.
It is now becoming apparent why the great battle fleet, which always sailed behind a protecting screen of such destroyers, was practically immune from torpedo attack. In order to assail these battleships the submarine was always compelled to do the one thing which, above all others, it was determined to avoid—to get within depth-charge radius of the surface craft. In discharging the torpedo, distance, as already intimated, is the all-important consideration. The U-boat carries a torpedo which has a much shorter range than that of the destroyer; it was seldom effective if fired at more than 2,000 yards, and beyond that distance its chances of hitting became very slight. Indeed, a much shorter distance than that was desirable if the torpedo was to accomplish its most destructive purpose. So valuable were these missiles and so necessary was it that every one should be used to good advantage, that the U-boat's captain had instructions to shoot at no greater distance than 300 yards, unless the conditions were particularly favourable. In the early days, the torpedoes which were fired at a greater distance would often hit the ships on the bow or the stern, and do comparatively little damage; such vessels could be brought in, repaired in a short time, and again put to sea. The German Admiralty discovered that in firing from a comparatively long distance it was wasting its torpedoes; it therefore ordered its men to get so near the prey that it could strike the vessel in a vital spot, preferably in the engine-room; and to do this it was necessary to creep up within 300 yards. But to get as close as that to the destroyers which screened the battleships meant almost certain destruction. Thus the one method of attack which was left to the U-boat was to dive under the destroyer screen and come up in the midst of the battle fleet itself. A few minutes after its presence should become known, however, a large number of destroyers would be dropping depth charges in its neighbourhood, and its chances of escaping destruction would be almost nil, to say nothing of its chances of destroying ships.