History of the Hovercraft

History of the Hovercraft

Authored and compiled by:

Christopher Fitzgerald

Chairman, World Hovercraft Organization; President, Neoteric Hovercraft, Inc.

Introduction

Water: The Ancient Highway

The growth of civilization occurred within view of – and in many ways because of – our seas and rivers. Since the beginning of human history, we have been shaped by our ability to carry goods and people across water – our most ancient highway.

Without a means of water transportation, ancient mariners could not have explored the world or traded goods. Civilizations that mastered ship building and sailing inevitably prospered as centers of trade, culture and power, and the earliest cities were located on seashores or rivers. The superiority of water transport over ground transport was so apparent to even the earliest civilizations that canal building was one of mankind's earliest engineering achievements.

In 1775 Adam Smith, the first economist, recognized the importance of water transportation in his revolutionary book An Inquiry into the Nature and Causes of the Wealth of Nations. In his analysis of why some nations are more prosperous than others, Smith examined the advantages of water over ground transportation - one ship with six or eight men could carry as much as 50 wagons attended by hundreds of men and 400 horses - and concluded that communication across water has always been the least expensive form of transportation. Water travel requires less manpower than overland travel and can accommodate far greater loads than wagons, animals or more recent ground transport vehicles.

Breaking the Water Barrier

Throughout history, mankind has been intent upon finding ways to transport larger loads and to increase the speed of load movement. From their inception, ground and air transport vehicles have dramatically and continuously increased their speed. Such is not the case with vehicles that travel across water, because they have to contend with the strong resistance of water – the water barrier.

One factor that creates the water barrier is water density. The density of water is 815 times the density of air. As a ship increases speed, the resistance of the water increases exponentially, causing huge increases in power to achieve only small gains in speed.

One method of describing transport efficiency is the movement of a specific load over a specific distance in a specific time. Speed equals distance divided by time; therefore, transport efficiency is the movement of a specific load multiplied by the speed at which it can be moved.

When a load is moved via water, the various resistances increase as the velocity times itself, and the energy needed to affect an increase in speed rises more than the energy cubed, or energy multiplied by itself three times (the exact power is 3.5.) This is a huge number.

Another way to think of this problem is to consider the lift-to-drag ratio. The load has to float or be lifted by the water; this results in drag (resistance) when movement commences. A boat has a lift-to-drag ratio about ten times lower than a steel wheel rolling on a steel rail. The only way to improve the lift-to-drag ration of a boat is to lift the boat's hull and load completely out of the water, which reduces wave production and surface parasitic drag.

In a quest to break the water barrier, to improve the lift-to-drag ratio and decrease the resistance of water, many vehicles have been invented, especially during the last three centuries. It is an old idea to pump air under a ship's hull in order to reduce resistance, but the obvious and simple approaches to this idea do not work; the entire hull has to be lifted off the surface. The majority of the modern inventions are based on the idea of lifting the water displacement hull, or lifting the load-carrying device out of the water. These include hydroplanes, hydrofoils and air cushion vehicles. (The hovercraft is one type of air cushion vehicle.) Among them, the air cushion vehicle has the best lift-to-drag ratio of any device that travels across water when speeds exceed 35 mph.

1700 – 1900: The Genesis of Air Cushion Vehicles

When it comes to flying machines, ideas easily date back to ancient Greece. This is not the case with air cushion vehicles. The first recorded design for such a vehicle was in 1716 by Emanuel Swedenborg, a Swedish designer, philosopher and theologian. Swedenborg's design appeared in the fourth edition of Sweden's first scientific journal, Daedulus Hyperboreus, and is the first detailed technical description of a flying machine of any type.

Swedenborg's man-powered air cushion platform, basically a circular aircraft, resembled an upside-down boat with a cockpit in the center or a "flying saucer." His manually operated device required the would-be pilot to use oar-like scoops to push air under the vehicle on each downward stroke in order to raise the hull out of the water. A working model of the design was never built, because Swedenborg soon realized that a human could not sustain the energy needed to power the oars. His concept required a source of energy far greater than any available at that time. As with many other forms of transportation, significant progress had to wait until a lightweight motor was developed in the nineteenth century.

In 1865, William Fronde of the British Admiralty sent a letter to B. J. Tideman, who was the Chief Constructor of the Royal Netherlands Navy, proposing the principle of air lubrication. The letter is on display at the David Taylor Model Basin in Washington D.C. and also appears on page 109 of J. Scott Russell's book, The Modern System of Naval Architecture, 1865, Vol. I.

In the mid-1870s, the British engineer Sir John Thornycroft built a number of ground effect machine test models based on his theory that an air cushion system would reduce the drag of water on boats and ships. His theory was that if a vessel's hull were designed with a concave bottom in which air could be contained between the hull and the water, it would create significantly less resistance. He filed a number of patents involving air-lubricated hulls through 1877. The internal combustion engine had not yet been invented, however, so the technology required to power his inventions still did not exist. In addition, no one had yet discovered a practical solution to the problem of how to keep a cushion of air trapped so it could not escape below a vessel.

In 1876, John B. Ward of San Francisco, California USA, suggested an aluminum platform with rotary fans to drive air down and backwards, but wheels would push the device along. He received US Patents 185465 and 195860 for his "aerial machines."

The first patent for air lubrication in Great Britain was issued to another Swedish engineer, Gustaf de Laval, in 1882 but because the method for retaining the cushion of air was not yet resolved, de Laval was not successful with his experiments. British Patent 5841 details a ship built with de Laval's ideas. Information on this ship can be found on pages 33-34 in the book Speed and Power of Ships by Admiral D.W. Taylor, published in 1933.

In 1888, James Walker of Texas was granted US Patent 624271 in which channels along the underside of boats contained air that would be captured in the adjacent channel as it tried to escape. US Patent 608757, obtained in 1897 by Culbertson, includes an idea that led to the first suggestion for sidewall air cushion vehicles.

Air lubrication has been applied to many industrial processes and applications, including railways. The concept of a "sliding railway," a train that rode on small hoverskirted pads using water under pressure, was first proposed in 1868 by the French engineer Monsieur Louis Girard. A working example was operated in 1886 for 900 miles in the LeJouchere Park. After Girard was killed in the Franco-German war, one of his assistant engineers, M. Barre, improved upon Girard's ideas and constructed a sliding railway at London's Crystal Palace in 1891. The London News hailed the invention as "a marvelous invention … a singularly original contrivance for enabling trains to run by means of waterpower at speed hitherto undreamed of … something which may eclipse the electric motors."

1900 – 1950: The Evolution of Air Cushion Vehicles

Experiments with air cushion vehicles began in earnest after a suitable power source, the engine, became a reality, and after imaginations were fostered by the development of the airplane. As the airplane evolved as a viable vehicle after the renowned Wright Brothers flight in 1903, more attention was paid to the fact that additional lift was created if an airplane flew close to land or water, creating a "funnel effect" or cushion of air. This became known as ground effect.

Realizing that pressurized air reacts against the surface of water and enables a vessel to skim over the water rather than through it, naval architects patented several designs intended to solve the problem of water resistance, or hydrodynamic drag. Onboard fans would force compressed air into a chamber beneath, lubricating the hull with air from stem to stern, which would raise it slightly above the water.

World War I brought the development of the airplane as a military weapon which, in turn, fostered technological interest, and scientists and innovators began exploring the ground effect/air cushion effect in earnest.

Various forms of air cushion craft began to evolve after the first working example was demonstrated in 1916. At that time, Dagobert Muller von Thomamhul, an Austrian engineer, designed and built an air cushion torpedo boat for the Austrian Navy, which used fans to pump air beneath the hull to form a lubricating air cushion. Further development was abandoned when World War I destroyed the Austrian Navy and the empire.

During this same period of time, there were a number of prolific inventors of air-lubricated boats. F.W. Schweder obtained British Patent 4131 in 1906 in which improvements upon De Laval's ideas were proposed. In 1907 Joseph Clark received US Patent 989834 for an air vehicle. Charles Theryc of France proposed yet another rail concept between 1902 and 1915, for which he received British Patent 5569. These trains rode on air and many patents were issued that dealt with air edge seals. Two examples are US Patent 1152451 and British Patent 9011 of 1915. Another French inventor, M.A. Gambin, submitted British Patent Application 188648 in 1921 for a sidewall-type air cushion vehicle.

James Porter, a British engineer, received a series of patents dating from 1908, including British Patent 21216 and US Patent 1016359. In 1913, Porter suggested a machine with ideas very similar to annular jet air cushion air supply systems, and received British Patent 975 in 1914, which shows an annular duct quite similar to those of present day hovercraft.

Also in 1908, Charles Worthington, an American, suggested a vehicle supported on air but riding in a conduit. A similar proposal was made in 1913 by A.F. Eells, also an American. Other early air cushion vehicle inventors in the United States included F.G. Trask of North Dakota, who patented a sliding railway in 1922; V.F. Casey of Minneapolis, Minnesota; and Douglas Kent Warner of Sarasota, Florida.

In 1925, Casey received U.S. Patent 1621625 for the first air cushion recirculation concept. His design featured a flat-bottomed vessel with a series of longitudinal air channels open on the underside by which cushion air could be returned.

Warner, the head of Warner Research Laboratories at Tamiama Trail, Sarasota, Florida, carried out considerable research and development on air-cushioned boats in the 1920s and he held many patents; examples are US Patents 1819216, 2277620 and 2365676. To simplify his designs, which apparently experienced wave-pumping problems, his machines incorporated the ram wing concept. In 1929, Warner won boat races in Connecticut by the use of the trapped air cushion or captured air bubble principle on his sidehull craft. Warner's craft was the genesis of the surface effect ship (SES) of today.

A.U. Alcock, an electrical engineer in Perth, Australia, built a working model air cushion vehicle, which was demonstrated to the press and government officials in 1912. Alcock called his invention "Floating Traction," for which he received Australian Patent 14309. He later demonstrated other models at the Cricklewood Ice Rink in 1939.

In 1927, K.E. Tsiolkovski, a noted Russian scientist, developed what today might be called the hovertrain. T.J. Kaario of Finland built and tested a ground effect machine in 1935, and received Finnish Patents 18630 and 26122. Other inventors of air-lubricated boats during this period of history include J.C. Hansen-Euehammer of Denmark, Henry Clay of London, Great Britain and C.J. Lake of the United States. There were more than 100 patents on this subject filed before 1962.

Soon after heavier-than-air flight began, it was discovered that flying close to the surface, within the width (cord) of the wing, requires less energy to remain in the air. This became known as the ground effect phenomenon. Ground effect is a function of the width of the wing; to take advantage of the ground effect, any vehicle must fly above the ground at an altitude less than the distance between the leading and trailing edge of the wing.

The German Dornier DO-X twelve-engined flying boat proved the reality of the air cushion ground effect in 1929 by crossing the Atlantic Ocean entirely in ground effect at low altitude close to the water. As a result, the aircraft's fuel consumption decreased. During World War II, aircraft were flown to make use of air cushion ground effect in order to extend reconnaissance flight range.

American aviator Charles Lindbergh is reported to have flown in ground effect in order to conserve fuel during his historic transatlantic flight in 1927. The challenge of flying along the wave tops no doubt also served to stave off boredom during his long journey!

These and other beginnings formed the foundation for the various forms of air cushion supported vehicles that later appeared on the modern scene. Not until the 1950s, however, was a solution found for the problem that had thwarted all previous attempts: how to retain the cushion of air beneath the vessel.

1950 – 1964: The Birth of the Air Cushion Vehicle/Hovercraft Industry

The successful use of the air cushion effect in World War II aircraft inspired British, American, Russian and Swiss engineers to seriously explore innovative ways to apply it. The many experimental models that emerged prior to the 1950s were developed as flying boats rather than true air cushion vehicles, and they were known as ram wings as well as ground effect machines. The terms air cushion vehicle and hovercraft were not used until the late 1950s.

Serious practical development of today's hovercraft began in the mid-1950s in Great Britain, when Christopher Cockerell, generally accepted as the inventor of the hovercraft, began to explore the use of air lubrication to reduce hydrodynamic drag. Cockerell was a brilliant radio engineer who was retired from the army and operated a boatyard on the Norfolk Broads. During his lifetime, Cockerell was granted more than 70 patents for his inventions, many of them dealing with hovercraft, and he was knighted for his achievements.

Sir Christopher Cockerell's theory was that instead of using the plenum chamber – an open-bottomed empty box such as Thornycroft had devised – if air could instead be pumped into a narrow tunnel around the perimeter of the underneath side of the craft, it would flow toward the center, creating a more effective air cushion. This peripheraljet would allow the air pressure to build enough to equal the weight of the craft and, since the air would be trapped, the pressure would elevate the craft off the surface upon which it traveled.

Cockerell tested his theory with a test model constructed of two empty cans, an industrial air blower and a pair of kitchen scales. By inserting a cat food can into a coffee can, and blowing air through the gap between the two cans, he showed that it was possible to increase the hoverheight and to construct a vehicle that could travel on a cushion of air.

Originally, Cockerell had imitated previous designs that used fans to force air down from the deck into the chamber below, which meant that air had to be continually pumped back in to replace the air that had escaped. He then devised a new system: he made the hull of the craft concave and angled air jets from the circumference in toward the center of the craft to create a continuous air current. This effectively solved the problems of retaining the air beneath the craft, kept the air pressure stable and raised the hoverheight.

In 1955, Cockerell built a working model and was issued British Patent 854211 for a vehicle that was "neither an airplane, nor a boat, nor a wheeled land craft." Cockerell described his invention as "a very expensive motorcar tire with a permanent puncture." He named it the hovercraft, which he registered as a commercial name, so it was not available for general use until later when he generously handed the name over to public domain.