It is not always safe to judge a thing by its name; to do so is apt to lead to sad mistakes. One would naturally suppose, from its name, that a submarine is a boat that lives under water, like a fish. But it is not a fish; it is an air-breathing animal that prefers to stay on the surface, only occasionally diving under to hide from danger or to steal upon its prey. During the war, the German U-boats did not average three hours per day under the surface! Because they were intended to run on the surface they had to be built in the form of a surface vessel, so as to throw off the waves and keep from rolling and pitching too much in a seaway. But they also had to be built to withstand the crushing weight of deep water, and as a cylinder is much stronger than a structure of ordinary boat shape, the main hull was made circular in section and of heavy plating, strongly framed, while around this was an outer hull of boat shape, as shown in Fig. 18.
PUTTING HOLES IN A TANK TO KEEP IT FULL
The space between the inner and outer hulls was used for water ballast and for reservoirs of oil to drive the engines; and, strange as it may seem, the oil-tanks were always kept full by means of holes in the bottom of them. As the oil was consumed by the engines, water would flow into the reservoir to take its place, and the oil, being lighter than water, would float on top. The false hull was of light metal, because as it was open to the sea, the pressure on the inside was always the same as that on the outside. The reservoirs of oil and the water-ballast tanks protected the inner hull of the vessel from accidental damage and from hostile shell and bombs. There were water-ballast tanks inside the inner hull as well, as shown in the cross-sectional view, Fig. 18. The water in the ballast-tanks was blown out by compressed air to lighten the U-boat and the boat was kept on an even keel by the blowing out or the letting in of water in the forward and after tanks.
A heavy lead keel was attached to the bottom of the boat, to keep it from rolling too much. In case of accident, if there were no other way of bringing the boat to the surface, this keel could be cast loose.
At the forward end, where the torpedo-tubes were located, there was a torpedo-trimming tank. Torpedoes are heavy missiles and every time one was discharged the boat was lightened, and the balance of the submarine was upset. To make up for the loss of weight, water had to be let into the torpedo-trimming tank.
A submarine cannot float under-water without swimming; in other words, it must keep its propellers going to avoid either sinking to the bottom of the sea or bobbing up to the surface. To be sure, it can make itself heavier or lighter by letting water into or blowing water out of its ballast-tanks, but it is impossible to regulate the water ballast so delicately that the submarine will float submerged; and should the boat sink to a depth of two hundred feet or so, the weight of water above it would be sufficient to crush the hull, so it is a case of sink or swim. Usually enough ballast is taken on to make the submarine only a little lighter than the water it displaces; and then to remain under, the vessel must keep moving, with its horizontal rudders tilted to hold it down. The horizontal rudders or hydroplanes of the U-boat are shown in Fig. 17, both at the bow and at the stern.
The main hull of the vessel was literally filled with machinery. In the after part of the boat were the Diesel oil-engines with which the U-boat was propelled when on the surface. There were two engines, each driving a propeller-shaft. It was impossible to use the engines when the vessel was submerged, not because of the gases they produced—these could easily have been carried out of the boat—but because every internal-combustion engine consumes enormous quantities of air. In a few minutes the engines would devour all the air in the hull of the submarine and would then die of suffocation. And so the engines were used only when the submarine was running awash or on the surface, and then the air consumed by them would rush down the hatchway like a hurricane to supply their mighty lungs.
ENGINES THAT BURN HEAVY OIL
The oil-engines were strictly a German invention. In the earlier days of the submarine gasolene-engines were used, but despite every precaution, gasolene vapors occasionally would leak out of the reservoirs and accumulate in pockets or along the floors of the hull, and it needed but a spark to produce an explosion that would blow up the submarine. But Rudolph Diesel, a German, invented an engine which would burn heavy oils.
In the Diesel engine there are no spark-plugs and no magneto: the engine fires itself without electrical help. Air is let into the cylinder at ordinary atmospheric pressure, or fifteen pounds per square inch. But it is compressed by the upward stroke of the piston to about five hundred pounds per square inch. When air is compressed it develops heat and the sudden high compression to over thirty times its normal pressure raises the temperature to something like 1000 degrees Fahrenheit. Just as this temperature is reached, a jet of oil is blown into the cylinder by air under still higher pressure. Immediately the spray of oil bursts into flame and the hot gases of combustion drive the piston down. Because of the intense heat almost any oil, from light gasolene to heavy, almost tarlike oils, can be used. As heavy oils do not throw off any explosive vapors unless they are heated, they make a very safe fuel for submarines.
To drive the U-boat when no air was to be had for the engines, electric motors were used. There was one on each propeller-shaft and the shafts could be disconnected from the oil-engines when the motors were driving. The motors got their power from storage batteries in the stern of the submarine and under the floors forward. The motors when coupled to and driven by the engines generated current which was stored in the storage batteries. The submarine could not run on indefinitely underwater. When its batteries were exhausted it would have to come to the surface and run its engines to store up a fresh charge of electricity. The electric motors gave the boat a speed of about nine knots.
In addition to the main engines and motors, there was a mass of auxiliary machinery. There were pumps for compressing air to blow the ballast-tanks and to discharge the torpedoes. There was a special mechanism for operating the rudder and hydroplanes, and all sorts of valves, indicators, speaking-tubes, signal lines, etc. The tiny hull was simply crammed with mechanism of all kinds and particularly in the early boats there was little room for the accommodation of the officers and crew. The officers' quarters were located amidships, and forward there were the folding berths of the crews. In the later boats more space was given the men. The large U-boats carried a crew of forty and as the hazards of submarine warfare increased, more attention had to be paid to the men.
FAT MEN NOT WANTED
Oddly enough, small, slender men were preferred for submarine duty, not because of lack of space, but because it was apt to be very cold in a submarine, particularly in the winter-time. The water cooled off the boat when the submarine was traveling submerged, and the motors gave off little heat; while when the vessel was running on the surface the rush of wind to supply the engines kept the thermometer low. This meant that the men had to pile on much clothing to keep warm, which made them very bulky. The hatchway was none too large and a fat man, were he bundled up with enough clothing to keep him warm, would have a hard time squeezing through.
In the center of the vessel was the main hatchway, leading up to the conning-tower, which was large enough to hold from three to five men. This was the navigating-room when the vessel was running submerged, and above it was the navigating-bridge, used when the submarine was on the surface. In the conning-tower there was a gyroscopic compass; a magnetic compass would not work at all inside the steel hull of the U-boat. And here were the periscopes or eyes of the submarine, rising from fifteen to twenty feet above the roof of the conning-tower. There were usually two periscopes. They could be turned around to give the man at the wheel a view in any direction and they were used sometimes even when the vessel was running on the surface, to give a longer range of vision.
THE BLINDNESS OF THE SUBMARINE
Now, a submarine cannot see anything underwater. The commander cannot even see the bow of his boat from the conning-tower, and until he gets near enough to the surface to poke his periscope out of water he is absolutely blind and must feel his way about with compass and depth-gage. It was always an anxious moment for the U-boat commander, when he was coming up, until his periscope broke out of the water and he could get his bearings; and even that was attended with danger, for his periscope might be seen. Of course a periscope is a very insignificant object on the broad sea, but when a submarine is moving its periscope is followed by a wake which is very conspicuous, and so the U-boat ran a chance of being discovered and destroyed before it could dive again to a safe depth. Later, telescoping periscopes were used, which could be raised by means of a hand-lever. The submarine would run along just under the surface and every now and then it would suddenly raise its periscope for an observation and drop it down again under cover if there was danger nigh. This was much simpler and quicker than having a six-or eight-hundred-ton boat come up to the surface and dive to safety. He might even collide with a vessel floating on the surface, but to lessen this danger submarines were furnished with ears or big microphone diaphragms at each side of the hull by which a ship could be located by the noise of its propellers.
In the bow were the torpedo-tubes and the magazine of torpedoes. At first there were only two torpedo-tubes, but later the number was increased to four. These were kept constantly loaded, so that the projectiles could be launched in rapid succession, if necessary, without a pause for the insertion of a fresh torpedo. In some submarines tubes were provided in the stern also so that the boat could discharge a torpedo at its enemy while running away from him.
Each tube was closed at the outer end by a cap and at the inside end by a breech-block. The tube was blown clear of water by means of compressed air, and of course the outer cap was closed when the breech was open to let in a torpedo. Then the breech was closed, the cap opened, and the torpedo was discharged from the tube by a blast of air.
THE TORPEDO
A torpedo is really a motor-boat, a wonderfully constructed boat, fitted with an engine of its own that is driven by compressed air and which drives the torpedo through the water at about forty miles per hour. The motor-boat is shaped like a cigar and that used by the Germans was about fifteen feet long and fourteen inches in diameter. We used much larger torpedoes, some of them being twenty-two feet long. Ours have a large compressed-air reservoir and will travel for miles; but the Germans used their torpedoes at short ranges of a thousand yards and under, cutting down the air-reservoir as much as possible and loading the torpedo with an extra large explosive charge.
We found in the Diesel engine that when air is highly compressed it becomes very hot. When compressed air is expanded, the reverse takes place, the air becomes very cold. The air that drives the motor of the torpedo grows so cold that were no precautions taken it would freeze any moisture that might be present and would choke up the engine with the frost. And so an alcohol flame is used to heat the air. The air-motor is started automatically by release of a trigger as the torpedo is blown out of the torpedo-tube. By means of gearing, the motor drives two propellers. These run in opposite directions, so as to balance each other and prevent any tendency for the torpedo to swerve from its course. The torpedo is steered by a rudder which is controlled by a gyroscope, and it is kept at the proper depth under water by diving-rudders which are controlled by a very sensitive valve worked by the weight of the water above it. The deeper the water, the greater the weight or pressure; and the valve is so arranged that, should the torpedo run too far under, the pressure will cause the diving-rudders to tilt until the torpedo comes up again; then if the torpedo rises too high, the valve will feel the reduction of pressure and turn the rudders in the other direction.
The business end of a torpedo is a "war-head" packed with about four hundred pounds of TNT. At the nose of the torpedo is a firing-pin, with which the war-head is exploded. Ordinarily, the firing-pin does not project from the torpedo, but there is a little propeller at the forward end which is turned by the rush of water as the torpedo is driven on its course. This draws out the firing-pin and gets everything ready for the TNT to explode as soon as the firing-pin is struck. But the firing-pin is not the only means of exploding the torpedo. Inside there is a very delicate mechanism that will set off the charge at the least provocation. In one type of torpedo a steel ball is provided which rests in a shallow depression and the slightest shock, the sudden stopping or even a sudden swerve of the torpedo, would dislodge the ball and set off the charge. Hence various schemes, proposed by inventors, for deflecting a torpedo without touching the firing-pin, would have been of no value at all.
GUNS ON SUBMARINES
As torpedoes are expensive things, the U-boats were supplied with other means of destroying their victims. The Germans sprang a surprise by mounting guns on the decks of their submarines. At first these were arranged to be lowered into a hatch when the boat was running submerged, but later they were permanently mounted on the decks so that they would be ready for instant use. They were heavily coated with grease and the bore was swabbed out immediately when the boat came to the surface, so that there was no danger of serious rust and corrosion. The 3-inch gun of the early months of the war soon gave way to heavier pieces and the latest U-boats were supplied with guns of almost 6-inch caliber and there was a gun on the after deck as well as forward.
The U-boats depended upon radiotelegraphy to get their orders and although they did not have a very wide sending-range, they could receive messages from the powerful German station near Berlin. The masts which carried the radio aërials could be folded down into pockets in the deck. From stem to stern over the entire boat a cable was stretched which was intended to permit the U-boat to slide under nets protecting harbor entrances, and in later boats there were keen-toothed knives at the bow which would cut through a steel net. During the war German and Austrian U-boats occupied so much attention that the public did not realize the part that the Entente Allies were playing under the sea. America, Great Britain, France, and Italy made good use of submarines, operating them against enemy vessels, blockading enemy ports, and actually fighting enemy submarines.
A STEAM-DRIVEN SUBMARINE
The British in particular did splendid work with the submarine and developed boats that were superior to anything turned out by the Germans. For instance, they developed a submarine which is virtually a submersible destroyer. It is 340 feet long and it can make a speed of 24 knots on the surface. The most remarkable part of this boat is that its engines are driven by steam. Its boilers are fired with oil fuel. There are two smoke-stacks which fold down when it submerges. Of course when running under-water the vessel is driven by electricity and it makes a speed of 10 knots. It carries three 4-inch guns, two forward and one aft, and its displacement submerged is 2700 tons as against 800 tons for the largest German submarines.
A SUBMARINE THAT MOUNTS A TWELVE-INCH GUN
Still more remarkable is the big "super-submarine" designed by the British to bombard the forts of the Dardanelles, but unfortunately it was built too late to be used there. This submarine carries a gun big enough for a battle-ship. It is of 12-inch caliber and weighs 50 tons. Of course a big gun like that could not be fired athwart the submarine. It might bowl the little vessel over, even though it was a 1700-ton submarine. The gun is mounted to fire fore and aft, with a deviation of only a few degrees to one side or the other, so that the shock of the recoil is taken by the length instead of the beam of the submarine. It fires a shell weighing 620 pounds and a full charge is not used, so that the extreme range is only about 15,000 yards. This submarine monitor would have been a very difficult target for the Turkish gunners to hit.
When the war came to an end and the German submarines surrendered to the Entente Allies at Harwich, there was considerable public curiosity as to whether or not an examination of the U-boats would disclose any wonderful secrets. But they contained nothing that the Allies did not already know, and one British officer stated that the plans of the German submarines had often fallen into their hands long before a U-boat of the same type was captured!
CHAPTER XIII
Getting the Best of the U-Boat
The U-boat commander who sallied forth from the harbor of Wilhelmshaven in the early days of the war had nothing to fear. He was out to murder, not to fight. His prey was always out in the open, while he could kill without exposing more than his eye above water. Not even a sporting chance was allowed his victims, particularly when he chose unarmed merchantmen for his targets. He could come up boldly to the surface and shell a ship into submission. This was cheaper than torpedoing the vessel, because torpedoes are expensive. If the ship were speedy it might run away; or if the U-boat came up too close to its intended prey, the latter might run it down. That happened occasionally and it was the only danger that the Herr Kommandant had to fear.
If a destroyer suddenly appeared, the U-boat could dive into the shelter of the sea. If the water were not too deep, it could lie on the bottom for two or more days if need be. There was plenty of air in the hull to sustain life for many hours, and then the compressed air used for blowing the ballast-tanks could be drawn upon. In the U-boat there were potash cartridges to take up the carbon-dioxide, and tanks of pure oxygen to revitalize the air. If the submarine were damaged, it was not necessary for it to come to the surface to effect repairs. There were air-locks through which a diver could be let out of the boat. He was fitted with oxygen and potash cartridges, so that he did not need to be connected by an air-hose with the boat, but could walk around it freely to mend injured rudders or to clear the propeller of entanglements.
Even the small submarines of those early days were capable of taking long voyages. Setting his course at a comfortable pace of 10 knots, the U-boat commander could count on enough fuel to carry him 1600 miles, and if need be he could slow down to 8 knots and by using certain of his water-ballast tanks for additional oil-reservoirs, extend his cruising-radius to nearly 3000 miles. The big 800-ton U-boats that were built later had a radius of 5000 miles at an 8-knot speed. And so when the British closed the English Channel with nets and mines, Herr Kommandant was not at all perturbed; he could sail around the British Isles if he chose and make war upon transatlantic shipping. When harbors were walled off with nets, he could remain outside and sink vessels that were leaving or entering them.
SUBMARINE-CHASERS
A real menace came when the U-boat commander popped his periscope out of the sea and saw several little motor-boats bearing down upon him. They seemed harmless enough, but a moment's inspection showed them to be armed with guns fully as powerful as those he carried. It was useless to discharge a torpedo at so speedy and small a foe. A torpedo has to have a fairly deep covering of water, else its course will be disturbed by surface waves; and the submarine-chasers drew so little water that a torpedo would pass harmlessly under them. It was useless for the U-boat commander to come up and fight them with his guns. They would have been upon him before he could do that, and their speed and diminutive size made them very difficult targets to hit. Besides, he dared not risk a duel of shell, for he knew that if the precious inner hull of his boat were punctured, he could not seek refuge under water; and if he could not hide, he was lost. The little armed mosquito craft swarmed about the harbor entrances, ready to dash at any submarine that showed itself. They could travel twice as fast as the submarine when it was submerged and half again as fast as when it was running on the surface.
Submarines had to take to cover when these chasers were about. Herr Kommandant did not even dare to take a look around through his periscope, because the streak of foam that trailed in its wake would betray him and immediately the speedy motor-boats would take up the chase; and they had a disagreeable way of dropping bombs which, even if they did not sink the submarine, might produce such a concussion as to spring its seams. His foes had discovered one of his most serious defects. He was blind under-water and they were making the most of this handicap.
Groping along under-water by dead-reckoning was not any too safe a procedure near land, because he was liable at any moment to crash into an uncharted rock or maybe into the wreck of some submarine victim. He could not correct his bearings without coming to the surface, and, in the black depths of the sea, a slight miscalculation might send him to his doom. As was explained in the previous chapter, he had to keep moving, because he could not remain suspended under water.
He was more helpless than a ship sailing in the densest of fogs. A ship can stop and listen to sound-signals, or even to the beating of the surf on the shore, or it can take soundings to locate its position; and yet it is no uncommon occurrence for a ship to run ashore in a fog. How much easier it is for a submarine to lose its bearings when obliged to travel by dead-reckoning, particularly in the disconcerting excitement of the chase! To avoid the danger of collision with surface vessels, the commander chose to run at a depth of sixty-five feet. That was the upper limit of his safety-zone. A depth of over two hundred feet was his lower limit, because, as stated before, the water-pressure at that depth would crush in his hull or at least start its seams. If the bottom were smooth and sandy, and not too deep, he could settle gently upon it and wait for darkness, to make his escape.
But while he lay on a sandy bottom, he was still in danger. Trawlers were sweeping the bottom with nets. He might be discovered; and then if he did not come up and surrender, a bomb would let in the sea upon him.
A HINT FROM NATURE
While he could not see under water, his adversaries could. They had taken a hint from nature. The fish-hawk has no difficulty in spying his submarine prey. Flying high above the water, he can see his victims at a considerable depth, and wait his chance to pounce upon an unwary fish that comes too near the surface. It is said that the British trained sea-gulls to hunt submarines. Sea-gulls will follow a ship far out to sea for the sake of feeding on refuse that is thrown overboard. British submarines encouraged the birds to follow them, by throwing out bait whenever they came to the surface. Of course the birds could see the submarine even when it was submerged, and if they pursued it, they were always rewarded with plenty of food. The gulls drew no fine distinction between Hun and Briton, and so it came that Herr Kommandant often groped his way along in the dark sea, totally oblivious of the fact that he was attended by an escort of feathered folk who kept the British chasers informed of his presence.
In this connection it is interesting to note that the British trained sea lions to hunt submarines. The animals were taught at first to swim to a friendly submarine, locating it by the sound of its propellers. They were always rewarded with fish. These sea lions were muzzled so that they could not go fishing on their own account. Then they learned to locate enemy submarines and pointed them out by swimming directly toward them and diving down to them.
But there were human eyes, as well, that spied upon the U-boat. Fast seaplanes patrolled the waters, searching constantly for any trace of submarine. Its form could be vaguely outlined to a depth of from fifty to seventy-five feet, unless the sea were choppy, and once it was discovered, chasers or trawlers were signaled to destroy it with bombs or to entangle it in nets. Often a submarine would be discovered by a leak in its oil-tank which would leave a tell-tale trail. Sometimes when the U-boat itself could not be discerned, there would be slight shimmer, such as may be seen above a hot stove, caused by refraction of light in its wake. This was easily recognized by trained observers.
Even better aërial patrols were the small dirigibles known as Blimps. They are a cross between a balloon and an airplane, for they have the body and the power-plant of an airplane, but the planes are replaced by a gas-bag. Blimps could cruise leisurely and search the sea thoroughly. They could stop and hover directly over a submarine and drop explosives upon it with great accuracy. And so Herr Kommandant could take no comfort in hiding under a blanket of waves unless the blanket were so thick as to conceal his form completely from the eyes overhead. This made it imperative to leave the shallower waters near shore and push out into the deep sea, where the small chasers could not pursue him. But he could not shake off his pursuers. Stream-trawlers are built to ride the heaviest gales and they took up the chase out into the ocean.
There was a decided advantage for the U-boat in moving out to sea. It had a wider field of activity and could more easily escape from its pursuers. But on the other hand, its prey also had an advantage. Out in the open ocean they were not obliged to follow the usual ship lanes and it was more difficult for a submarine to intercept them. There it took more U-boats to blockade a given area.
A GAME OF HIDE-AND-SEEK
Then, it ceased to be quite so one-sided a game when merchantmen began to carry guns. That made it necessary for the submarine commander to creep up on his victims stealthily, and depend upon his torpedoes. He had to get within a thousand yards of the ship and preferably within five hundred yards, in order to be sure of hitting it. If the ship could travel faster than he could, he had to do this without betraying his presence. But ship-captains soon learned that their safety lay in zig-zagging. When Herr Kommandant reached the point from which he had planned to attack, he would raise his telescopic periscope out of the water, expecting to see his victim within good torpedo range, only to find it sailing safely on another tack. Again, he would have to take observations and make another try, probably with no better luck. It was a game of hide-and-seek in which the merchant ship had a good chance of making its escape, particularly when blotches of camouflage paint made it difficult for him to get the range, as described in Chapter XI.
Slower ships could be attacked without all this manœuvering, provided the submarine's guns outranged those of the ship. And so U-boats were provided with larger and larger guns, which made it possible for them to stand off and pound the merchantmen while out of reach of the vessel's guns. But ships found a way of hiding on the surface of the sea. A vessel would spout forth volumes of dense black smoke which would obliterate it from view. (See Fig. 19.) If the wind was quartering, the ship would change its course and dodge behind the sheltering pall of smoke. Not only was the smoke produced on the vessel itself, but smoke-boxes were cast overboard to form a screen behind the vessel. These smoke-boxes contained a mixture of coal-tar and phosphorus and other chemicals which would produce incomplete combustion. They were ignited by the rubbing of a phosphorus compound on a priming-composition, and then cast adrift to pour out dense volumes of heavy smoke. (See Fig. 20.) Behind this screen, the ship could dodge and zig-zag and if her speed were greater than that of the submarine, her chances of escape were very good.
Another annoyance that Herr Kommandant experienced was, when he lifted his periscopic eye above water, to find it so smeared with a sticky substance that he could not see. His foes had strewn the water with tar-oil that had spread in a thin film over a surface miles in extent. This blinded him at first, but before long he was equipped with a jet for washing off the periscope glass and that little annoyance was overcome.
But the craft most dreaded by the U-boat commander were the destroyers. These light, high-powered, heavily armed vessels could travel twice as fast as he could on the surface and three times as fast as he could submerged. Shells were invented which would not ricochet from the surface of the sea, but would plow right through the water, where they struck and hit the submarine below water-level.
DEATH-DEALING "ASH CANS"
However, it was not shell-fire that he dreaded, but the big "ash cans" loaded with TNT which were timed to explode far under water, and which would crush his boat or start its seams. It was not necessary for these bombs to hit the U-boat. When they went off they would send out a wave of pressure that would crush the boat or start its seams even if it were a hundred feet and more from the point of the explosion. Within limits, the deeper the explosion the wider would its destructive area be.
The timing-mechanism of some depth bombs consisted merely of a float on the end of a cord. When the bomb was thrown overboard this float remained on the surface until the cord was pulled out to its full length, when there would be a yank on the firing-trigger and the charge would explode. In other depth bombs there was a valve operated by the pressure of the water. When the bomb sank to the depth for which the valve was set, the pressure of the water would force the valve in, exploding a cartridge which set off the charge. So powerful were these depth bombs that the destroyer had to travel at high speed to get out of range of the explosion.
Depth bombs were rolled off the stern of the destroyer and also thrown out from the sides of the vessel by means of mortars. Some of the mortars were Y-shaped and held a depth bomb in each arm of the Y. When a blank 3-inch shell was exploded at the base of the gun, both bombs would be hurled from the ship, one to port and the other to starboard. In this way the destroyer could drop the bombs in a "pattern" of wide area. Herr Kommandant gained a wholesome respect for these terriers of the sea. It was suicide to show himself anywhere near a destroyer. In a moment the speedy boat would be upon him, sowing depth bombs along his course. His chances of escaping through this hail of high explosives were remote indeed.
The ships that he was most eager to destroy were either too speedy for him to catch, unless they happened to come his way, or else they were herded in large convoys protected by these dreaded destroyers. The convoy proved a most baffling problem for Herr Kommandant. He dared not attack the convoy by daylight. In a fog he might take a chance at picking off one of the ships, but even that was very risky. He could trail the convoy until dusk and then under cover of darkness draw near enough to discharge a torpedo, but in the daytime he must keep his distance because there were eyes in the sky watching for him. At the van and rear of the convoy there were kite balloons high in the sky, with observers constantly watching for periscopes, and for U-boats that might be lurking under the surface.
As the destroyers gained in experience, the difficulties of the U-boat attack grew greater and its work grew more and more perilous. The crew grumbled and grew mutinous. The morale of the men was shaken. We can imagine the horror of plunging hurriedly into the depths of the sea, and rushing along blindly under the surface, dodging this way and that, while terrific explosions of depth bombs stagger the submarine and threaten to crush it, and there is the constant expectation that the next explosion will tear the thin shell of the U-boat and let in the black hungry water. The tables were turned. Now, if never before, Herr Kommandant, the hunter, knew what it felt like to be hunted.
It takes an exceptional man to go through such a harrowing experience with unshattered nerves. On at least one occasion, a submarine that was being depth-bombed came suddenly to the surface. The hatch flew open and the crew rushed out, holding up their hands and crying, "Kamerad." The U-boat was uninjured, but the shock of a depth bomb explosion had put the electric-lighting system out of commission, and the crew, unnerved by the explosion and terrified by the darkness, had overpowered their officers and brought the boat to the surface.
EYES IN THE SEA
There were other craft that Herr Kommandant had to look out for. His were not the only submarines in the sea. His foes also were possessed of submarines. They could not see under water any better than he could, but they could fight on the surface as well as he, and they could creep up on him even as he crept up on his prey. As a French submarine commander puts it: "The U-boats used to enjoy the advantage of remaining themselves invisible while all the surface and aërial craft which were sent in pursuit of them were boldly outlined against the sky and visible to them. This is one of the reasons we used submarines to ambush U-boats." Submarines were also used to accompany the convoys, so that the U-boat commander had to watch not only for the eyes of the ship's lookouts and the eyes in the kite balloons, but also for the periscope eyes that swam in the sea.
TRAILING U-BOATS BY SOUND
The troubles of the submarine-commander were multiplying. All over the world inventors were plotting his destruction. As long as we depended upon our eyes to ferret him out, the sea was a safe refuge, provided he dived deep enough, but when we began to use our ears as well, he found himself in a very serious predicament. Although light is badly broken up in its passage through water, sound-waves will travel through water much better than in air. The first listening-devices used were crude affairs and did not amount to much, particularly when the U-boats muffled their motors and engines so that they were virtually noiseless. But the French invented a very sensitive sound-detector. It consisted of a lot of tiny diaphragms set in a big hemisphere. There were two of these hemispheres, one at each side of the boat. When sound-waves struck these hemispheres, the diaphragms would respond. At the focus of each hemisphere there was a megaphone receiver; one of these carried the sound to the operator's right ear and the other to his left. He would turn a megaphone around until he found the diaphragm that produced the loudest sound. This gave him the direction of the sound-wave. Then the boat would be steered in that direction. He knew that it was aimed properly when the sound coming to his right ear was just as loud as that which came into his left ear.
A still better hydrophone was developed by a group of American inventors. The details of this cannot yet be disclosed, but we know that it was adopted at once by our allies. A very sensitive receiver was used which could detect a U-boat miles away and determine its direction accurately. Under ideal conditions the range of the device was from fifteen to twenty-five miles, but the average was from three to eight miles. If two or more boats fitted with sound-detectors were used, they could determine the position of the U-boat perfectly. One drawback was that the vessel would have to stop so that the noise of its own engines would not disturb the listener, but this was largely overcome by trailing the detector a hundred feet or more from the stem of the ship. The sounds were then brought in by an electric cable to the listener in the ship.
These sound-detectors were placed on Allied submarines as well as surface vessels and they were actually tried out on balloons and dirigibles, so that they could follow a U-boat after it had submerged too deeply to be followed by sight.
Many U-boats were chased to their doom by the aid of the American hydrophone. Fig. 21 illustrates a very dramatic chase. The full line shows the course of the U-boat as plotted out by hydrophones and the broken line the course of the submarine-chasers. The dots represent patterns of depth bombs dropped upon the U-boat. Try as he would, the Herr Kommandant could not shake off his pursuers. At one time, as the listeners stopped to take observations, they heard hammering in the U-boat as if repairs were being made. The motors of the submarine would start and stop, showing clearly that it was disabled. More depth bombs were dropped and then there was perfect silence, which was soon broken by twenty-five revolver-shots. Evidently the crew, unable to come to the surface, had given up in despair and committed suicide.