In this unit we will learn about the differences and similarities of air and water. We will also learn what led to great improvements in speed "over" water.

Air and water are both fluids. A fluid flows easily and assumes (takes on) the shape of its container. Wind and ocean, for example, move with ease and are in constant motion. If you pour milk (a liquid fluid) into a bowl, the milk takes the shape of the bowl. You cannot see air, but you can see a container, like a balloon, filling with air. The air obviously takes the shape of the balloon.

One major difference between air and water is density. Density is a way of expressing how much matter is in a given amount of air or water. Water molecules (matter) are packed much more tightly together than air molecules - 815 times more! Therefore, it is much more difficult for a ship to "push" through water than for an airplane to fly through the air. Add to this the fact that the faster a ship travels, the greater the resistance. Even adding large amounts of power gives only 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 ship.

Various hull designs were tried and tested, but none of them were completely successful in avoiding the motion of the waves or increasing speeds.

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. This was a good idea, but a human could not continue to "row" fast or long enough to keep the air bubble going. This idea had to wait until a lightweight motor was developed.

Later, when an engine was developed, several designs were tried. The basic idea was to raise the craft above the water. This was done with fans that forced compressed air into a chamber beneath the craft allowing it to rise slightly above the water. Although one successful craft was reported in Germany in 1916, it was destroyed in the First World War. Further attempts were made over the next 40 years, but lack of funds (money) slowed progress.

International interest was again raised on July 19, 1959, near Dover, England when a full-scaled craft was seen literally floating over the water and up onto the land and back out over the water. This strange craft had just crossed the English Channel from France exactly 50 years after an airplane had flown the same route, for the first time. 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. This craft employed an air jet to circulate the air. Its English inventor, Christopher Cockerell called it a "hovercraft".

Cockerell's hovercraft was a start, but it still did not rise high enough above the water. If the waves were higher than a foot the ride was too rough for any practical use. But, another English inventor, C.H. Latimer-Needham, had read of Cockerell's experiments and decided to try a different design. Using Cockerell's air jet idea, Latimer added a "skirt". 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. Jets of air entered between the two walls of the skirt, which inflated and discharged the air into the cushion, causing the craft to rise.

By 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! This was much faster than most surface ships. Just as important, the craft was operating at twice its original weight with no increase in lift power. Within 10 years newer hovercraft, 50 times heavier and able to travel above 60 knots carried 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 hovercraft design vehicles able to carry or pull heavy loads.

On land, "super trains" ride on air cushions, on special tracks, at speeds up to 300 mph with less air and noise pollution of regular trains.

Today, a small hovercraft 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 pushes against the water. There is another type of vehicle that depends upon motion to develop lift. It 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 it is built like. It's raised above the water by small, wing-like foils. These foils (like aircraft wings) work on Bernoulli's principle. The water streaming over the curved upper surface has to move faster than that flowing beneath. This causes low pressure above and high pressure below the foil. At a given speed the lift generated by the foils raises the hull of the ship bodily out of the water.

From the beginning famous names have been involved with hydrofoils. Enrico Forlanini (builder of helicopters and airships), in 1905, designed and built the first successful hydrofoil, using foils set like the rungs of a ladder. His craft could travel up to 38 knots. Later, Alexander Graham Bell used Forlanini's ladder-foil system in North America and in 1918 established a world water speed record of 70 mph.

Two problems existed that slowed the progress of hydrofoils. The metal structure of the craft could not withstand heavy seas and government funding, for research, was not available.

Hanns von Schertal, is considered the father of the modern hydrofoil. With new designs he was able to overcome propulsion and stability problems. After a 230 mile flight in bad weather the German navy, in 1940, gave him a contract to build a 17 ton minelayer which achieved an ocean velocity (speed) of 47 knots - a speed record that stood for 25 years.

It was not until 1953 that the commercial value of the hydrofoil was recognized. On Lake Maggiore, connecting Switzerland and Italy, the 10 ton, 28 passenger von Schertal hydrofoil took just 48 minutes to cross the lake. The regular ferries took 3 hours. In 1956 Carlo Rodriquez, head of Sicily's largest shipyard, built several hydrofoils using von Schertal's design. These hydrofoils carried 75 passengers between Sicily and Italy. In four years more than a million people had traveled by hydrofoil. The flying ship had proved itself a safe, speedy, profitable vehicle.

There are two types of hydrofoils: surface piercing and submerged. The surfacing piercing type is very stable in calm water. As the craft turns or pitches forward the hydrofoil automatically brings the ship back up-right. The surface piercing model does not do well in rough seas because the foil tends to follow the up and down movement of the waves, which can result in unwanted jolts (bumps). For this reason, surface piercing hydrofoils are the design preferred by the hobbyist or sports enthusiast on lakes and in calm bays (usually a smaller craft).

The fully submerged foil was designed to limit the "jolting" action of the waves. By adding a complex system of motion and depth sensing devices that in turn manipulate (move) flaps and elevators on the foil the hydrofoil craft is able to "skim" above the waves for a smooth ride. The submerged foil design is very similar to a jet airliner and is therefore used mostly for military or large commercial craft.

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