INVASION OF THE SEA

THE POSSIBILITY that the wind was the first inanimate power utilized by man, has already been referred to. There are records of the use of sailing vessels in Egypt that date as far back as 6,000 years before Christ. Navigators of that early date, however, could hardly claim to have mastered the wind. They merely used wind power when the wind was disposed to help them. If the winds were adverse, they had no recourse other than to furl their sail, step the mast and depend upon oars to propel them to the desired port. It was not until thousands of years later that primitive mariners learned how to tack and pursue a zigzag course against the wind. When this knowledge was acquired we do not know, but it is certain that the Phoenicians, who rounded the continent of Africa 1,200 years before Christ, knew how to make use of the power of opposing winds. Of course they could not explain how it was that a breeze could be made to drive a vessel in a direction across and even opposed to that in which it was blowing. In order to understand this apparent paradox ourselves, we must go back to the very elements of mechanics.

ELEMENTARY MECHANICS

The popular conception of force is something that produces motion, but its true definition is “that which tends to produce or resist motion.” There are forces in existence when there is no motion. When you hold a weight in your hand there is a force tending to pull the weight to the earth, but this force is opposed by an equal force exerted by your muscles in holding up the weight. There is no motion because the two forces are perfectly balanced. If they were unbalanced, there would be motion in the direction of the greater force. If the pull of the arm is greater than that of gravity, the weight will be lifted, and if the weight is too heavy for the arm to support, it will go down despite muscular efforts to prevent it. In one case the force of gravity will endeavor to destroy motion by opposing the lift of the arm, and in the other case the arm will endeavor to resist motion by opposing the pull of gravity. A book on a table is motionless and yet it is acted upon by two forces which are opposed to each other and hence balanced. The table furnishes a force which resists and balances the force exerted by gravity. If the book were heavy enough, in other words, if the force directed downward were great enough, the table would be crushed.

When two forces are in perfect balance they must be equal and opposite. Unless the directions of the two forces are exactly opposite, there will be motion in some new direction. Suppose we use an apparatus such as shown in Fig. 56 to study the result of three coacting forces. It consists of a T-shaped frame with a pulley P at each end of the cross arm. These pulleys turn very freely on their axes, so that we need not be concerned with any appreciable amount of friction. Two fine cords running over these pulleys are knotted at O to a third short cord. Each cord is provided with a hook on which weights may be hung. Now if we put a pound weight on each cord the two A and B will raise the weight C until the angles between the cords at O are all equal. In other words each force of one pound is balanced by two other forces of one pound each pulling at an angle of 120 degrees to it and to each other. If we put a 3-pound weight at A, a 4-pound weight at B and a 5-pound weight at C, the cords will come to rest in the position shown in Fig. 57. The weight B being heavier than weight A will pull the knot O to the right until the angle between the cords running to these weights is a right angle.

PARALLELOGRAM OF FORCES

Although a force is something that cannot be pictured it can be represented graphically by means of a line, letting the direction of the line represent the direction of the force and the length of the line the strength of the force. In Figure 58 we may measure off 3 inches from O to a to indicate the 3-pound force and 4 inches from O to b to represent the 4-pound force and 5 inches from O to c to represent the 5-pound force. Now if from a a line is drawn parallel to O b and from b a line is drawn parallel to O a we shall have a parallelogram a O b d, and if we extend the line O c it will bisect the parallelogram, running diagonally from O to d, and this diagonal will be found to measure exactly 5 inches which represents the 5-pound force. This is what is known as the parallelogram of forces. It shows us the resultant of any two forces that are not directly opposite and it gives us the direction as well as the strength or magnitude of this resultant. It is only because we happen to choose the forces 3, 4 and 5 that the angle at O is a right angle. In Figure 59, where the forces are all equal, our parallelogram is lozenge-shaped and the line O d is just as long as the line O a and O b, showing that its magnitude is the same as that of the two forces that balanced it.

The greater the angle between the two lifting forces the less weight can they lift. If two men are carrying a ten-pound satchel, each will be lifting five pounds, if the pull is directly upward; but this is a rather inconvenient way of carrying the bag and usually they pull at a slight angle from the vertical, and so each must carry more than half the weight. If they move so far apart that the angle between them is more than 120 degrees, each will be carrying more than the full weight of the bag.

SAILING AGAINST THE WIND

Now that we know something about the parallelogram of forces we may return to the problem of sailing across and against the wind. In Fig. 60 we are looking down on the deck of a ship and the wind is represented by the arrow. The dotted line A B represents the direction in which the boat is traveling and the line C D represents the plane of a sail. If the line E F represents the magnitude and direction of the force of the wind at the center of the sail, then we can tell how much pressure is being exerted directly against the sail, by drawing the line g F perpendicular to the sail and completing the parallelogram by drawing from E a line parallel to the sail intersecting g F at G and another line parallel to g F intersecting the plane of the sail at H. Then the length of the line G F represents the pressure against the sail. If the line G F is half as long as the line E F, then only half of the force of the wind is exerted in the direction G F. In other words, a wind pressure of one pound per square foot blowing in the direction of G F will do as much work as two pounds in the direction E F. The force of the wind has been broken up into two “components,” one (G F) at right angles to the sail, and the other (H F) edgewise to the plane, and of course the latter has no effect upon the propulsion of the boat.

If there were nothing to prevent it, the boat would sail in the direction G F; but the keel of the boat offers resistance to motion in this direction, and we must construct another parallelogram around the force G F to find the magnitude of the force exerted in the direction A B. The line K F is drawn at right angles to A B, and then the parallelogram is completed by drawing a line from G to K parallel to the line A B and another from G to M parallel to K F. We have then resolved to force G F into two components M F and K F. The former tends to push the boat along its course while the latter tends to make it drift to leeward. The length of the line M F is little more than a quarter of the length of the wind force E F and the leeward acting force K F is actually considerably greater than the forward acting force M F. Even with a deep keel there will be some drift to leeward. This is corrected by means of the rudder of the ship which is turned to head the ship further into the wind so that although the boat does not actually travel in the direction of its axis it may be made to travel along the course A B. Of course the boat cannot sail directly against the wind, but it can accomplish the same result by tacking alternately to port and starboard so that eventually it can reach a port that lies in the direction from which the wind comes.

THE SPEEDY CLIPPER

Before the advent of the steamship, sailing vessels were developed to a high degree of efficiency. The speedy clippers of 1816 to 1845 used to cross the Atlantic at an average speed of 6 to 9 miles per hour and sometimes even better, which compares favorably with a common steam freighter of to-day. The largest sailing vessel ever built was the Thomas W. Lawson, a seven-masted schooner. She was launched in 1902, but foundered in 1907 off the Scilly Islands. This great ship was a steel vessel 395 feet long, with a displacement of 10,000 tons and a cargo capacity of 7,500 tons. She had a sail spread of 40,617 feet.

FROM OARS TO PROPELLERS

Although a century has elapsed since the first steam-driven vessel made its way across the Atlantic Ocean, sails have been as yet by no means swept off the face of the sea. Nevertheless, even when sailing vessels had no competitors they did not furnish a perfectly satisfactory means of transportation. The fickleness of wind power was felt in this application as well as in that of windmills, and inventors racked their brains for some more certain means of propelling ships. Naturally, when the steam engine was a proven success, efforts were made to apply this newly discovered power to ships. How to make steam drive a ship was a problem. At first it was proposed to use a system of oars which would be moved back and forth in imitation of oarsmen and John Fitch’s first steamboat in 1786 was driven by a set of paddles operated in a manner similar to that of paddling a canoe. It was a very natural evolution from oars to paddle wheel, which consists of a series of oars mounted in a wheel so that they will come into play one after the other. The propeller, although not invented by Col. John Stevens (as has been popularly supposed), was first applied by him to steam navigation when he constructed a small steamboat on the Hudson River in 1804. But the simplicity of the paddle wheel and its high efficiency, particularly in quiet harbors and shallow inland waters, gave it preference over the propeller. In rough seas, however, the paddle wheel was far from ideal. It was too easily broken by heavy waves and between 1855 and 1865 the propeller displaced it completely for ocean-going vessels.

WATER JET PROPULSION

Another curious form of propulsion, which dates back to the eighteenth century and is still periodically revived by inventors, is the water jet. The idea was to have the engine operate a pump which would drive a stream of water out of the stern of the boat and drive the boat by reaction. The British Government actually built two jet-propelled steamers. One of them, called the Waterwitch, was a 1,100-ton vessel and the other, the Squirt, was a small torpedo boat. The latter attained a speed of but twelve knots while a sister ship of the same steam power driven by a propeller attained a speed of seventeen knots. The Waterwitch was even less efficient. Some years ago experimental water-jet vessels were built in New York in which a jet only ? inch in diameter with a pressure of 2,500 pounds per square inch was used, but the experiment proved a failure. The propelling force of a jet is the reaction of the stream of water against the orifice from which it issues. The action is just like Hero’s reaction steam turbine referred to on page 143. The propulsion would be the same were the jet discharged in the open air or in a vacuum or against a solid stone wall.

WATER AND AIR RESISTANCE

It takes very little power to move a boat slowly because the resistance that has to be overcome is merely the parting of the water at the bow and closing in of the water at the stern and the skin friction along the sides of the hull. In addition to this there is a similar resistance offered by the air. At very low speeds the resistances of the water and the air are practically negligible. In perfectly quiet water with no air stirring the pull of a cord will move a ship weighing hundreds of tons, but the motion will be very slow indeed. Unfortunately the speed of a ship does not increase directly in proportion to the power that drives it. Doubling the power does not double the speed. If it takes ten horsepower to drive a vessel at a speed of ten knots it will take not 2 but 2³ or 8 times as much power to drive it at a speed of twenty knots. In other words, the horsepower goes up as the cube of the speed. This is an average condition for ordinary speeds. For very high speeds the horsepower may have to be increased as the 4th and even the 5th power of the speed. The shape of the bow and the stern is of utmost importance. The parting and displacement of the water at the bow and the replacement at the stern produce waves and the forming of these waves represents so much wasted energy. The swell that is kicked up by a steamer is evidence of power uselessly expended. Much of this loss can be overcome by careful design of the ship’s lines. A vessel that kicks up a high bow wave—one that sails with a “bone in its teeth”—may present a very pleasing spectacle and may seem to be traveling at high speed, but the best designed vessel—the one that slips through the water with no fuss—is much more economical of power. It is easy to understand that the bow must be carefully designed to cut through the water, but it is not so apparent that the stern must also be shaped to permit the water to flow in readily and fill in the void behind the ship. If the stern is not carefully shaped, there will be a serious drag on the vessel. The skin friction of the vessel is greatly increased by fouling of the hull with marine growths. At high speeds the wind pressure on the superstructure is considerable. Every spar and line adds its quota. A boat that is traveling in still air at a speed of twenty-two knots or twenty-five miles per hour is encountering the equivalent of a twenty-five mile wind which will exert a pressure of over three pounds per square foot of frontage.

FLYING ON WATER

Instead of cutting through the water modern speed boats are designed to ride over it. The boats have flat bottoms which are arranged as a series of flat planes known as hydroplanes. These planes form steps and are set at such an angle as to make the boat rise up on the water in the same way that a kite rises in the air. The higher the speed the higher the boat rises so that at full speed it skims on the surface. Hence there is comparatively little power wasted in displacing the water. Some of these boats are driven by air propellers so that water resistance to the propeller gear is avoided. These hydroplanes (they must not be confused with hydroaeroplanes) almost fly over the water.

FIRST OCEAN STEAMER

It was in 1807 that Robert Fulton built the Clermont and established steam navigation by running a regular service between New York and Albany, and it was twelve years later that the Atlantic Ocean was first crossed by a steam-driven vessel. It was an American vessel, the Savannah, that made the trip. She was a 380-ton ship equipped with steam power to help her along when the wind failed. Seventy-five tons of coal and twenty-five cords of kindling wood were taken aboard to feed her furnace. This was thought to be ample for the voyage, but before the trip was completed the fuel was all gone. The log of the Savannah bears this entry the night before sighting the Irish coast: “2 A. M. Calm. No cole to get up steam.” However the captain did raise steam just before reaching Kinsale, Ireland, by burning wood. Watchers ashore beholding the smoke issuing from her stack were convinced that the vessel was afire and boats were dispatched to the rescue. The Savannah made the trip from Savannah to Kinsale in 23 days and was under steam propulsion for only 80 hours of this time.

Regular trans-Atlantic steam service was not inaugurated until 1838, but for many decades steamers were equipped with sails to assist them when the wind was favorable.

The most notable of early steamships was the Great Eastern, a combined screw and paddle-wheel ship, 692 feet long, built in 1858. She held the record for size until 1899 when the Oceanic, 704 feet long, was put into service. At present the Leviathan, formerly the Vaterland, holds the record with a length of 920 feet. It is difficult to judge of the size of a vessel out on the open water. If the Leviathan were placed in Broadway, New York, she would span nearly four blocks. Because of her 100-foot beam she would be too wide to be wedged in between the skyscrapers that border lower Broadway. If she were set up on end she would tower 158 feet above the pinnacle of the Woolworth Building. Her power plant consists of four turbines which total 90,000 horsepower and the huge vessel is driven at a speed of 25.8 knots or nearly thirty miles per hour.

The wonder of these huge floating structures lies not merely in their gigantic proportions but in the fact that they are able to weather the terrific wrenching strains of heavy ocean storms. A skyscraper is built to withstand only the steady and direct pull of gravity and the variable thrust of the wind which, except in western cyclones, seldom amounts to thirty pounds per square foot. Bridge building is more difficult because of the leverage of the parts overhanging the foundations. Wind pressures must be calculated and also the live load of objects moving over the structure. In naval architecture enter the problems of building construction combined with those of bridge building, complicated by the fact that there is no fixed foundation for the structure to rest upon. At one moment a ship may be spanning a trough in the seas and at the next it may be seesawing over the crest of the wave. Of course the bottom of the boat is seldom if ever out of the water and a certain amount of support is provided throughout the length of the vessel, but the ship is subjected to the strains of a cantilever bridge when she is passing over a wave, and to the strains of a truss bridge when spanning a wave trough. These strains are increased by the fact that the structure is in constant motion. A certain degree of flexibility is demanded of the materials which go into the structure and of the joints between the frame members.

BOATS OF ARTIFICIAL STONE

Originally wood was used for the hulls of ships; then between 1845 and 1855 iron supplanted wood, Between 1875 and 1885 steel supplanted iron and to-day efforts are being made to supplant steel with concrete. The advantages offered by concrete are cheapness and speed of construction. The first large vessel built of this material was the Faith, an 8,000-ton ship. This boat stood up very well in heavy weather despite the rigidity of her structure. It is doubtful, however, that a large boat comparable in size to the Leviathan could weather a severe ocean storm.

The proposal to build ships of cement created almost as much of a popular sensation as did the first iron boat. Although the public had accepted iron and then steel as a perfectly proper material for shipbuilding, concrete seemed too much like stone and it did not seem possible that artificial stone could be made to float. They did not realize that a cubic foot of steel weighs four times as much as the same volume of concrete. Of course concrete does not begin to have the tensile strength of steel and consequently the walls of a concrete ship must be made relatively thick. For this reason a concrete vessel is heavier than a steel vessel. She draws more water and requires a larger power plant, and because of her greater mass she is not so readily maneuvered.

SUBMARINE NAVIGATION

As was explained in Chapter VI, a body will float only so long as it is lighter than the volume of water it displaces. It is almost impossible to keep a body suspended in water unless some portion of it is exposed above the surface. If it starts sinking it will keep on going down until it reaches the bottom of the sea. There is a popular notion that at great depths water becomes dense enough to float solid iron, but water is practically incompressible and its density at a depth of five miles is only slightly greater than that at the surface. An object must therefore either float on the surface or sink to the bottom, unless its weight is exactly equal to the difference between the upward pressure of the water under it and the downward pressure of the water above it. Such an ideal balance it is practically impossible to obtain unless the object itself is compressible.

How then can a submarine navigate under water without sinking to the bottom?

A SUBMARINE OF THE SEVENTEENTH CENTURY

The first solution to this problem dates back to the seventeenth century. Doctor Cornelius Van Drebel, a Netherlander, who was a guest at the court of King James I of England, built three submarines between 1620 and 1624. These were rowboats covered over with a water-tight deck and propelled by twelve oarsmen. It is recorded that Van Drebel discovered a means of holding the boats submerged by observing some fishermen towing baskets full of fish up the Thames. The barks to which the baskets were attached by cables were weighted down by the load they were towing, but when the cables slackened the boats rose a little bit. Van Drebel’s method of applying this principle was evidently to attach a weight to the boat which trailed along the bottom. When the oarsmen propelled the boat, she was pulled down under the surface by the drag, but when the rowing ceased the boat would float up to the surface. King James himself is said to have made a journey of several hours’ duration in one these boats, which was kept at a depth of twelve to fifteen feet below the surface. Progress must have been very slow because the range of the submarine was given as five or six miles.

During the Revolutionary War David Bushnell built a submarine with which attempts were made to sink a British frigate lying in the Hudson River. This submarine was driven by a hand-operated screw propeller. The boat was provided with water ballast tanks, and by pressing a valve with one foot he could let in water enough to submerge the boat while with the other foot he could operate a pump to empty the tanks and bring the boat to the surface. When the boat was ballasted so that she would barely float, a vertical screw propeller was operated to raise or lower her as much as desired. A 200-pound lead weight was attached to a long cable which passed up through the bottom of the boat, and by letting out this cable the submarine could be made to rise instantly in case of an accident.

FULTON’S HAND-PROPELLED SUBMARINE

To Robert Fulton, however, belongs the credit of building the first submarine operating on the principle that is now universally used. His boat was also driven by a hand-operated screw propeller and was furnished with water tanks which could be filled or pumped out at will, but after the submarine was weighted until only the conning tower showed above water, she was submerged or raised by means of horizontal rudders or hydroplanes which could be tilted to any angle desired. Of course these rudders, like any other rudders, would not operate unless the boat were in motion. Such is the case with modern submarines. Like bicycles, they must keep on going or they will fall. If they are heavy, they will fall to the bottom, and if light they will “fall” to the surface. When in motion the hydroplanes will either hold them down or lift them up according to the angle to which these horizontal rudders are tipped.

Robert Fulton’s Nautilus had a fish-shaped hull of copper plating on iron ribs and was twenty-one feet three inches long by six feet five inches at her greatest diameter. The screw propeller was operated by two men. When the boat was on the surface a sail was raised to assist in driving the boat. This sail could be folded up like a fan when it was desired to submerge.

Fulton deserves full credit for anticipating so many of the essential features of the modern submarine, but of course the Nautilus was a mere toy compared to the marvelous machines which swim under the surface of the sea to-day.

The German U-boats at the outbreak of the war were 150 feet long and could make only nine knots submerged and twelve knots on the surface, but later they grew to a length of 300 feet with a submerged speed of twelve knots and a surface speed of eighteen knots. The British in the meantime developed a submarine that was 340 feet long and had a displacement submerged of 2,700 tons as against 800 for the largest German U-boats. The speed of this big British boat is twenty-four knots on the surface and ten knots submerged.

SUBMARINES AS SURFACE BOATS

Despite their name, submarines are really surface boats. Only when necessity demands are they submerged. During the war even the U-boats did 90 per cent of their sailing on the surface. Origially submarines were built primarily for submerged travel and consequently they were given the form of a fish or of a fat cigar, but such a shape was not adapted for surface sailing. Water piled up on the nose of the boat and tended to bear her down. To overcome this, submarines are now shaped more like a boat with a bow high enough to part the waves without burrowing into them.

POWER PLANT OF A SUBMARINE

The gasoline engines that were formerly used for propelling the boats have now given way to heavy oil or Diesel engines, because of the greater efficiency of these engines and the greater safety of handling heavy oil. Some submarines have been equipped with steam power plants. Such is the power used on the big British submarines above referred to. Of course any power that involves the burning of fuel can only be used on the surface. As has already been explained, an internal combustion engine burns seven to nine times as much air as oil and it would be impossible to store enough air on board to keep the engines going very long; hence they can be operated only while the hatches are open to the atmosphere.

Coupled to the shafts of the engines are dynamos which generate electricity and feed it to storage batteries. A dynamo will serve as a generator when turned by some mechanical power and on the other hand when current is fed into it, it serves as a motor, so that when traveling submerged and fuel can no longer be used, the generators are disconnected from the engines but remain connected to the propeller shafts and driven by the very current they previously stored in the batteries.

THE COLLAPSIBLE “EYE” OF THE SUBMARINE

Amidships there is a bridge from which the vessel can be navigated when on the surface and a conning tower from which she can be navigated when running awash. Most of the submerged travel is maintained at a comparatively shallow depth so that the submarine can keep an “eye” on the surface. The eye is the periscope of which there are two so that in case of damage to one the other may be used. Periscopes date back to the “Fifties” when they were used on some experimental European submarines. In the Civil War, when the river monitor Osage ran aground in the Red River, her captain, now Rear-Admiral Thomas O. Selfridge, rigged up a periscope with which he could look over the high banks of the river and direct the fire of his guns upon a Confederate force that was attacking him. This periscope consisted merely of a three-inch pipe with a hole at each end cut in opposite sides of the pipe. Small mirrors were set in the pipe so that the light coming in through the upper hole was reflected down through the pipe and out of the lower hole. This crude periscope is the same in principle as the modern submarine periscopes except that the latter are provided with lenses to gather and focus all the light possible on the eyepiece so that the operator will have a perfectly clear view. Periscopes extend fifteen to twenty feet above the roof of the conning tower. While periscopes are insignificant objects on the broad seas when a submarine is moving very slowly, they are made conspicuous by the wake of foam that follows them when the boat is traveling even at a moderate speed. For this reason during the war the Germans developed a telescoping periscope which could be shot up to the surface whenever desired in order to give the commander a glimpse of his surroundings.

The submarine is submerged by letting water into the ballast tanks and then turning the hydroplanes to a diving angle. There is an after as well as a forward pair of diving rudders.

When running completely submerged the submarine is blind and solely dependent upon the chart and compass. It is impossible to see far under water. Searchlights are of no value at all. They will not make visible an object a hundred feet away. The submarine commander cannot see even the bow of the boat he is piloting. The ordinary magnetic compass will not operate when entirely incased in steel as it is in the hull of the submarine and so a gyroscope compass has to be used instead.

While the compass serves as a guide for travel in the horizontal plane the depth gauge must be watched to see that the boat does not dive too deeply. The pressure of the sea increases at the rate of 64 pounds per foot of depth. Two hundred feet below the surface the pressure amounts to about six tons on every square foot of the surface of the submarine. Few submarines can stand a greater pressure than that without being crushed or at least springing serious leaks. When coming to the surface the hydroplanes are used and if the boat is to remain on the surface water is blown out of the ballast tanks.

THE “EARS” OF A SUBMARINE

Although submarines may be blind under water they are not deaf. Sound detectors are used which enable them to locate other vessels by the throbbing of their engines or the beat of their propellers and so they can avoid collisions when coming up to the surface.

Despite the perfection of the submarine the sea is still a great mystery to us. We know only its surface and its shallows. We have sounded a few deep holes and brought up samples of deep-sea life, but we have not been able to penetrate in person its profound depths and explore with our eyes and hands the world that is buried beneath its waves. Three quarters of the globe is covered by water and if we are to claim complete mastery of the earth we must find some way of descending into the heart of the ocean and exploring its deepest valleys.

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