|Breaking the Water Barrier||page 1|
There was a song popular in America in the 1930's called "Slow Boat to China." The lyrics glamorized the leisurely pace of the cruise ship and its passengers as they crossed the Pacific Ocean. The fact is that the captain had not cut engine power for romantic reasons. The "slow boat" was going as fast as it possibly could - which was only twice the speed of 100 years before. Cars, trains and airplanes had increased their speeds by 10 or 20 fold from the moment they were first introduced, but the commercial ship lagged far behind. It had to contend with a medium other than land or air: it had to deal with the water barrier.
In this unit we will learn about the differences and similarities of air and water, and how studies of both led to great improvements in the speed of travel by water.
Air and water are both fluids. A fluid is a substance that has a low resistance to flow and the tendency to assume the shape of its container. Wind and ocean, for example, move with ease and are in constant motion. And if you pour milk (a liquid fluid) from a carton into a bowl, the milk fills and takes the shape of the bowl - it doesn't retain the shape of the carton. Neither would air, although you can't see it. Air (a gaseous fluid) is invisible. We know that air takes the shape of its container by being able to see the container, a balloon for instance.
One difference between air and water that has a major effect on travel by ship is density. Density is a way to express the amount of matter - physical "stuff" - contained in a substance. Water is 815 times more dense than air. This means that a ship encounters a great deal more resistance than air vehicles. Add to this the fact that the faster a ship travels, the greater the resistance - large increases in power provide relatively small gains in speed. Finally, ships that ride deep in the water are effected by ocean waves that lower the comfort and safety of passengers and craft.
Naval architects thought the problem could be solved by reducing the amount of hull immersed in water. Craft with shallow or flat V-shaped bottoms were designed so that dynamic lifting forces would take over as the vessel gathered speed, thus raising more of the hull out of the water. Many of these designs failed because they were unable to achieve sufficient speed. Those that did were very unpleasant in waves - the craft continuously rose high on one wave and crashed down on the next. River shores also suffered from the wakes created.
In order to provide both speed and comfort, it seemed necessary to lift the hull of the craft completely out of the water, and also to uncouple it from the continual motion of the waves. The solution was eventually provided by two new classes of water craft - air cushion vehicles and hydrofoils.
Riding on an Air Bubble
In 1716 Emmanuel Swedenborg, a Swedish philosopher and designer, built the first air cushion vehicle. It looked like an upside down boat with a cockpit in the center. Oar-like scoops pushed air under the boat on each downward stroke, raising the hull out of the water to ride instead on the compressed air. The idea was valid, but Swedenborg quickly realized that a human could not generate the sustained energy needed to power the oars. As with other forms of transportation, significant progress had to wait until a lightweight motor was developed.
In the late 19th century, when such a power plant was a reality, experiments with air cushion vehicles resumed in earnest. Realizing that pressurized air reacts against the water surface and causes a vessel to skim over rather than through water, naval architects patented several designs intended to solve the first problem - that of water resistance, or hydrodynamic "drag." Fans on board the craft forced compressed air into a chamber beneath, lubricating the hull from stem to stern and allowing it to rise slightly above the water. Not until 1916, however, was the first working example demonstrated. Dagobert Muller von Thomamhul, an Austrian engineer, designed and built an air cushion torpedo boat for the Austrian navy. According to sketchy reports of the time it achieved a speed of 40 knots. Further development was abandoned as World War I destroyed both the navy and empire.
Over the next 40 years experiments continued but were hampered by a lack of financing. International interest was ignited on July 19, 1959, when early morning strollers in Dover, England were startled by the approach of a full-scale craft that was literally floating over the water. The ship came right upon the beach, blowing sand about as it maneuvered clumsily but successfully around and back out over the water. This science-fiction apparition had crossed the English Channel from France - 50 years to the day of the French aviator Bleriot's first flight from Calais. The difference between the two "flying" vehicles was that this one looked like a boat, and its height above the surface was only two feet. It was the brainchild of English inventor Christopher Cockerell, who called his ship a "hovercraft."
Cockerell had initially imitated other designs that used fans to force air directly down from the deck into the chamber below, and continued to pump air into the base to replace that which leaked out. His new system used jets attached to the sides of the craft that aimed down and in to create a continuous air current. This minimized leakage and provided a much greater clearance in relation to the power required. The improved aerostatics design, which kept the air pressure stable, allowed the vehicle to hover at a height greater than previous designs.
But a second obstacle - ocean waves - still had to be overcome. The clearance provided by Cockerell's peripheral jets required a considerable amount of power, and the ride was turbulent in waves greater than one foot. Without improved clearance the hovercraft would have been practical only as a means to move heavy machinery over land. But another English inventor, C.H. Latimer-Needham, had read of Cockerell's experiments and anticipated its drawbacks. He designed a rubber "skirt" that attached to the jets that extended downward from the sidewalls. Air entered between the two walls of the skirt, which inflated and discharged the air into the cushion. The skirt was very flexible: when it came into contact with waves, rocks or other obstacles, it would simply collapse momentarily and then return to its normal inflated shape.
The introduction of the skirt created a spectacular improvement. In 1959 a skirtless craft could only operate in calm seas at low speeds. In 1962 a craft fitted with 4 foot skirts moved at 50 knots in calm seas, 40 knots in seas with waves of 4 to 5 feet, and at reduced speed it could cope with 6 to 7 foot waves. Just as important, the craft was operating at twice its original weight with no increase in lift power. Just 10 years after the introduction of Cockerell's hovercraft its descendants, 50 times heavier and 3 times faster, would carry a third of all passengers and cars across the English Channel.
Air cushion designs have proved useful in climates that would otherwise be difficult or impossible to navigate. Swamps, marshes, ice and snow have been overcome by tractors and barges that use the pressurized air design to carry or pull heavy loads.
On land, tracked air cushion vehicles - "super trains" - operate at higher speeds (250-300 mph) with less air and noise pollution than their wheeled predecessors.
As you might expect, specialized aircraft also utilize this principle. Actually, pilots during World War II discovered that by flying very low over the ocean a "surface effect" seemed to buoy them up and reduce fuel consumption. The increased air pressure created between the water and plane created a cushion. The air cushion design on today's aircraft minimizes runway requirements and allows it to take off and land from any flat surface - open fields, ice, snow, sand. These aircraft can bear heavier loads than those with conventional multi-wheel undercarriages, and can service areas that cannot be reached by land or water borne air cushion vehicles.
The air cushion principle has been adapted for sheer pleasure as well as mass transit. A typical sport size vehicle can be strapped to the roof of a car, assembled in an hour, and will carry a family of four at 40-60 mph. Hovercraft rallies now occur every weekend throughout the world.
Flying Through Water
The air cushion vehicle raises itself by a self-generated bubble of pressurized air that reacts against the water surface, creating an aerostatics balance of forces to generate lift. There is another type of vehicle that depends upon motion to develop lift, and blends the aerodynamics of an aircraft with the hydrodynamics of a ship. This vehicle is called a hydrofoil.
The hydrofoil is a cross between the ship it looks like and the airplane which it is built like. It's raised above the water by small, wing-like foils. Although the foils move through water, they operate on the same principle used by an aerofoil. As you recall, Bernoulli's principle is that the faster a fluid or gas moves, the lower the pressure it exerts upon objects along which it flows. The curved upper surface of the hydrofoil causes water to flow at higher speeds above it than beneath it, causing a difference of pressure between upper and lower surfaces. The water streaming over the curved upper surface has to move faster than that flowing beneath, leading to a reduction of pressure on the upper surface and increased pressure on the lower surface. The foils are connected to the hull by struts and at a given speed the lift generated by the foils raises the hull bodily out of the water.
Not surprisingly, many of the pioneers in aviation were at the forefront in the development of the hydrofoil. The first successful hydrofoil was designed and built by the Italian helicopter and airship leader, Enrico Forlanini, in 1905. His craft was equipped with foils set like the rungs of a ladder, and could achieve 38 knots.
Forlanini's first client was Alexander Graham Bell, who became so enthused after a ride on Lake Maggiore that he purchased a license to build and develop the Forlanini ladder-foil system in North America. One of his craft established a world water speed record of 70 mph in 1918.
Two familiar obstacles delayed progress for several years: lack of a suitable lightweight metal, and lack of government financing. Both were clearly demonstrated in 1920, when Bell's hydrofoils were tested by the British navy. The structures could not withstand the heavy seas, fell apart, and government interest was withdrawn.
Another German engineer and aviation enthusiast, Hanns von Schertal, is considered the father of the modern hydrofoil. In 1936 he proved that he had overcome propulsion and stability obstacles with a 230 mile flight in bad weather, which led to contracts with the German navy. In 1940 a 17 ton minelayer achieved an ocean velocity of 47 knots - a speed record that stood for 25 years. Von Schertal's hydrofoils were employed exclusively by the military during the second World War.
It was not until 1953 that commercial value of the hydrofoil was dramatically demonstrated. Ferries that steamed across Lake Maggiore, connecting Switzerland and Italy, took nearly 3 hours to cover the 30 miles. Cars driving around the lake arrived at the destination in one and a half hours. The 10 ton, 28 passenger von Schertal hydrofoil took just 48 minutes.
Unlike the international response to Cockerell's hovercraft landing in Dover some six years later, the hydrofoil showing was largely ignored. But Carlo Rodriguez, head of Sicily's largest shipyard, was duly impressed. In 1956, with hydrofoils built to von Schertal's specifications and carrying 75 passengers, Rodriguez launched hourly service between Sicily and Italy. In four years over a million people had traveled by hydrofoil - the flying ship had proved itself a safe, speedy, profitable vehicle.
There are basically two types of hydrofoils:
surface piercing and submerged. Surface piercing foils are usually
shaped like a "V", with the upper part riding above the surface and the
lower part below. The major advantage of surface piercing foils is that
they are inherently stabilizing. The forces restoring normal trim are
provided by the area of the foil that becomes submerged. When the craft
begins to roll to one side, the immersion of increased foil area
automatically causes additional lift to be generated. This counters the
roll and restores the craft back to an even keel. The same stabilizing
effect occurs when the vehicle pitches downward. The forward foil at the
bow becomes more immersed, more lift is generated, and the bow raises
back to its normal height. When speed is increased so is lift, and the
craft raises further out of the water - which causes the wetted area of
the foil to be reduced, and lift is reduced as well.
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