A more common salvage method consists in passing cables or chains under the wreck and attaching them to large floats or pontoons. The slack in the chains is taken up when the tide is low, so that on the turn of the tide the wreck will be lifted off the bottom. The partially raised wreck is then towed into shallower water, until it grounds. At the next low tide, the slack of the chains is again taken in, and at flood-tide the wreck is towed nearer land. The work proceeds step by step, until the vessel is moved inshore far enough to bring its decks awash; when it may be patched up and pumped out. Where the rise of the tide is not sufficient to be of much assistance, hydraulic jacks or other lifting-apparatus are used.
If the salvor could always be assured of clear weather, his troubles would be reduced a hundredfold, but at best it takes a long time to perform any work dependent upon divers, and the chances are very good when they are operating in an unsheltered spot, that a storm may come up at any time and undo the result of weeks and months of labor. This is what happened when the submarine F-4 was salved. After a month of trying effort the submarine was caught in slings hung from barges, lifted two hundred and twenty-five feet, and dragged within a short distance of the channel entrance of the harbor, where the water was but fifty feet deep. But just then a violent storm arose, which made the barges surge back and forth and plunge so violently that the forward sling cut into the plating of the submarine and crushed it. The wreck had to be lowered to the bottom and the barges cut free to save them from being smashed. At the next attempt to raise the F-4 pontoons were again used, but instead of being arranged to float on the surface, they were hauled down to the wreck and made fast directly to the hull of the submarine. Then when the water was forced out of the pontoons with compressed air, they came up to the surface, bringing the submarine with them. In this way all danger of damage due to sudden storms was avoided because water under the surface is not disturbed by storms overhead; and when the wreck was floated, the pontoons and submarine formed a compact unit.
While this method of salvage seems like a very logical one for work in the open sea, one is apt to forget how large the pontoons must be to lift a vessel of any appreciable size. Not only must they support their own dead weight, together with that of the sunken vessel, but some allowance must usually be made for dragging the wreck out of the clutches of a sandy or muddy bottom. Imagine the work of building pontoons large enough to raise the Lusitania. They would have to have a combined displacement greater than that of the vessel itself, and they would have to be so large that they would be very unwieldy things to handle in a seaway. It is for this reason that submarine pontoons are not often used to take the entire weight of the vessel. So far they have been employed mainly to salve small ships and then only to take a portion of the weight, the principal work being done by large wrecking-cranes. Instead of horizontal pontoons it has been suggested that vertical pontoons be employed, so as to provide a greater lifting-power without involving the use of enormous unwieldy units.
Ships are not built so that they can be picked up by the ends. Such treatment would be liable to break their backs in the middle. Were they built more like a bridge truss, the salvor's difficulties would be materially lessened. It would be a much simpler matter to raise a vessel with pontoons were it so constructed that the chains of the pontoon could be attached to each end of the hull. But because a ship is built to be supported by the water uniformly throughout its length, the salvor must use a large number of chains, properly spaced along the hull, so as to distribute the load uniformly and see that too much weight does not fall on this or that pontoon.
The main problem, however, is to get hold of the wreck and this requires the services of divers, so that if there were no other limiting factor, the depth to which a diver may penetrate and perform his duties sets the mark beyond which salvage as now conducted is impossible.
A common diver's suit does not protect the diver from hydraulic pressure. Only a flexible suit and a thin layer of air separates him from the surrounding water. This air must necessarily be of the same pressure as the surrounding water. The air that is pumped down to the diver not only serves to supply his lungs, but by entering his blood transmits its pressure to every part of his anatomy. As long as the external pressure is equalized by a corresponding pressure within him, the diver experiences no serious discomfort. In fact, when the pressure is not excessively high he finds it rather exhilarating to work under such conditions; for, with every breath, he takes in an abnormal amount of oxygen. When he returns to the surface he realizes that he has been working under forced draft. He is very much exhausted and he is very hungry. It takes a comparatively short time to build up the high internal pressure, which the diver must have in order to withstand the pressure of the water outside, but it is the decompression when he returns to the surface that is attended with great discomfort and positive danger. If the decompression is not properly effected, the diver will suffer agonies and even death from the so-called "Caisson Disease."
We know now a great deal more than we used to know about the effect of compressed air on the human system, and because of this knowledge divers have recently descended to depths undreamed of a few years ago. When a diver breathes compressed air, the oxygen is largely consumed and exhaled from the lungs in the form of carbon-dioxide, but much of the nitrogen is dissolved in the blood and does not escape. However, like a bottle of soda-water, the blood shows no sign of the presence of the gas as long as the pressure is maintained. But on a sudden removal of the pressure, the blood turns into a froth of nitrogen bubbles, just as the soda-water froths when the stopper of the bottle is removed. This froth interrupts the circulation. The release of pressure is felt first in the arteries and large veins. It takes some time to reach all the tiny veins, and serious differences of pressure are apt to occur that often result in the rupture of blood-vessels. The griping pains that accompany the "Caisson Disease" are excruciating. The only cure is to restore the blood to its original pressure by placing the patient in a hospital lock, or boiler-like affair, where compressed air may be admitted; and then to decompress the air very slowly.
It is possible to relieve the pressure in a bottle of soda-water so gradually that the gas will pass off without the formation of visible bubbles, and that is what is sought in decompressing a diver. After careful research it has been found that the pressure may be cut down very quickly to half or even less of the original amount, but then the diver must wait for the decompression to extend to the innermost recesses of his being and to all the tiny capillaries of his venous system.
In the salvage of the F-4 a diver went down 306 feet, and remained on the bottom half an hour. The pressure upon him was 135 pounds per square inch, or about 145 tons on the surface of his entire body. Some idea of what this means may be gained if we consider that the tallest office building in the world does not bear on its foundations with a greater weight than 215 pounds to the square inch or only about 50 per cent more than the crushing pressure this diver had to endure.
It took the diver a very short time to go down. On coming up he proceeded comparatively rapidly until he reached a depth of 100 feet. There he found the bottom rung of a rope ladder. On it he was obliged to rest for several minutes before proceeding to the next rung. The rungs of this ladder were 10 feet apart, and on each rung the diver had to rest a certain length of time, according to a schedule that had been carefully worked out. At the top rung, for instance, only 10 feet from the surface, he was obliged to wait forty minutes. In all, it took him an hour and forty-five minutes to come up to the surface. The decompression was complete and he suffered no symptoms of the "Caisson Disease." But he was so exhausted from his efforts that he was unfit for work for several days. Yet the operations that he performed at the depth of 300 feet would not have taken more than a few minutes on the surface.
The Germans have paid a great deal of attention to deep-diving operations, and no doubt while their U-boats were sinking merchant ships German salvors were anticipating rich harvests after hostilities ended. One scheme they developed was a submarine rest-chamber which could be permanently located on the bottom of the sea close to the point where the salvage operations were to take place. This chamber consists of a large steel box which is supplied with air from the surface and in which divers may make themselves comfortable when they need a rest after arduous work. Entrance to the chamber is effected through a door in the floor. The pressure of the air inside prevents the water from rising into the chamber and flooding it. From this submarine base the divers may go out to the wreck, either equipped with the ordinary air-tube helmets or with self-regenerating apparatus which makes them independent of an air-supply for a considerable period of time. When the diver has worked for an hour or two, or when he is tired, he may return to this chamber, remove his helmet, eat a hearty meal, take a nap if he needs it, and then return to the salvage work without going through the exhausting operation of decompressing.
The work of the diver usually consists of far more than merely passing lines under a sunken hull. It is constantly necessary for him to cut away obstructing parts. He must sometimes use blasting-power. Pneumatic cutting-tools frequently come into play, but the Germans have lately devised an oxy-hydrogen torch for underwater use, with which the diver can cut metal by burning through it. This is accomplished by using a cup-shaped nozzle through which a blast of air is projected under such pressure that it blows away the water over the part to be cut. The oxygen and hydrogen jets are then ignited electrically, and the work of cutting the metal proceeds in the hole in the water made by the air-blast. A similar submarine torch has recently been developed by an American salvage company. It was employed successfully in cutting drainage-holes in the bulkheads of the St. Paul, which was raised in New York Harbor in the summer of 1918.
The diver's sled is still another interesting German invention. It is a sled provided with vertical and horizontal rudders, which is towed by means of a motor-boat at the surface. The diver, seated on the sled, and provided with a self-contained diving-suit, can direct the motor-boat by telephone and steer his sled up and down and wherever he chooses. And so without any physical exertion, he can explore the bottom of the sea and hunt for wrecks.
From time to time attempts have been made to construct a diver's suit that will not yield to the pressure of the sea, so that the diver will not be subjected to the weight of the water about him, but can breathe air at ordinary atmospheric pressure. Curious armor of steel has been devised, with articulated arms and legs, in which the diver is completely encased. With the ordinary rubber suit, the diver usually has his hands bare, because he is almost as dependent upon the sense of touch as a blind man. But where the pressure mounts up to such a high degree that a metal suit must be used, no part of the body may be exposed. If a bare hand were extended out of the protecting armor it would immediately be mashed into a pulp and forced back through the opening in the arms of the suit. The best that can be done, then, is to furnish the arms of the suit with hooks or tongs or other mechanical substitutes for hands which will enable the diver to make fast to the wreck or various parts of it.
But if a diver feels helpless in the bag of a suit now commonly worn, what would he do when encased in a steel boiler; for that is virtually what the armored suit is! A common mistake that inventors of armor units have made is to fail to consider the effects of the enormous hydraulic pressure on the joints of the suit. In order to make them perfectly tight, packings must be employed, and these are liable to be so jammed by the hydraulic pressure that it is well nigh impossible to articulate the limbs. Again, the construction of the suit should be such that when a limb is flexed it would not displace any more water than when in an extended position, and vice versa. A diver may find that he cannot bend his arm, because in doing so he would expand the cubical content of his armor by a few cubic inches, and to make room for this increment of volume it would be necessary for him to lift several hundred pounds of water. The hydraulic pressure will reduce the steel suit to its smallest possible dimensions, which may result either in doubling up the members or extending them rigidly.
But these difficulties are not insuperable. There is no reason why a steel manikin cannot be constructed with a man inside to direct its movements.
Other schemes have been devised to relieve the diver of abnormally high air-pressure. One plan is to construct a large spherical working-chamber strong enough to withstand any hydraulic pressure that might be encountered. This working-chamber is equipped with heavy glass ports through which the workers can observe their surroundings in the light of an electric search-light controlled from within the chamber. The sphere is to be lowered to the wreck from a barge, with which it will be in telephonic communication and from which it will be supplied with electric current to operate various electrically driven mechanisms. By means of electromagnets this sphere may be made fast to the steel hull of the vessel and thereupon an electric drill is operated to bore a hole in the ship and insert the hook of a hoisting-chain. This done, the sphere would be moved to another position, as directed by telephone and another chain made fast. The hoisting-chains are secured to sunken pontoons and after enough of the chains have been attached to the wreck the pontoons are pumped out and the wreck is raised.
It is a pity that ship-builders have not had the forethought to provide substantial shackles at frequent intervals firmly secured to the framing. A sunken vessel is really a very difficult object to make fast to and the Patent Office has recorded many very fantastic schemes for getting hold of a ship's hull without the use of divers. One man proposes the use of a gigantic pair of ice-tongs; and there have been no end of suggestions that lifting-magnets be employed, but no one who has any idea of how large and how heavy such magnets must be would give these suggestions any serious consideration.
But, after all, the chief obstacle to salvage in the open sea is the danger of storms; months of preparation and thousands of dollars' worth of equipment may be wiped out in a moment.
However, there has been another recent development which may have a very important bearing on this problem of deep-sea salvage work. It has often been observed that a submerged reef, twenty or thirty feet below the surface, may act as a breakwater to stop the storming waves. An inventor who studied this phenomenon arrived at the theory that the reefs set up eddies in the water which break up the rhythm of the waves and convert them into a smother of foam just above the reef. Thereupon he conceived the idea of performing the same work by means of compressed air. He laid a pipe on the sea bottom, forty or fifty feet below the surface, and pumped air through it. Just as he had expected, the line of air bubbles produced exactly the same effect as the submerged reef. They set up a vertical current of water which broke up the waves as soon as they struck this barrier of air.
The "pneumatic breakwater," as it is called, has been tried out on an exposed part of the California coast, to protect a long pier used by an oil company. It has proved so satisfactory that the same company has now constructed a second breakwater about another pier near by. There is no reason why a breakwater of this sort should not be made about a wreck to protect the workers from storms. Where the water is very deep, it would not be necessary to lay the compressed-air pipe on the bottom, but it could be carried by buoys at a convenient depth.
Summing up the situation, then, there are two serious bars to the successful salvage of ships sunk in the open sea—the wild fury of the waves on the surface; and the silent, remorseless pressure of the deep. The former is the more to be feared; and if the waves really can be calmed, considerably more than half the problem is solved. As for the pressure of the sea, it can be overcome, as we have seen, either by the use of special submarine mechanisms, or of man-operated manikins or even of unarmored divers. We have reached a very interesting stage in the science of salvage, with the promise of important developments. Fifty fathoms no longer seems a hopeless depth.
Even in times of peace the sea exacts a dreadful toll of lives and property. Before the war the annual loss by shipwreck around the British Isles alone was estimated at forty-five million dollars. But the war, although it was frightfully destructive to shipping, may in the long run save more vessels than it sank; for it has given us sound-detectors which should remove the danger of collisions in foggy weather, and the wireless compass, which should keep ships from running off the course and on the rocks. And now, if salvage engineering develops as it should, the sea will be made to give up not only much of the wealth it swallowed during the war, but also many of the rich cargoes of gold and silver it has been hoarding since the days of the Spanish galleon.