Hovercraft and hydrofoils

People have been building boats now for thousands of years, but engineers are still finding better ways to carry us over the water One of the things that slows boats down is the choppy waves brushing underneath them so, if you want to go faster, you need to go higher too. How can you make a boat race over the waves? One way is to use a hydrofoil: a kind of underwater wing that makes a boat fly, very slightly, like a plane. Another option is to use a giant fan and ride your boat on a cushion of air. Boats that work this way are called hovercraft (or, in the military, as LCAC, Landing Craft Air Cushion vehicles). Let’s take a closer look at how they work!

Hovercraft

Hovercraft are among the world’s most versatile boats. Because they are amphibious (they can travel equally well over land or water), they can ride right up onto the shore. They can also carry massive amounts of cargo. A US military hovercraft called the Landing Craft Air Cushioned (LCAC) can carry a 70-ton (64-metric ton) tank at speeds up to 74 km/h (46 mph) and can land on roughly three quarters of the world’s coastline. Lighter hovercraft can reach speeds of 130km/h (80mph) or more.

How a hovercraft works

In a hovercraft, a giant centrally mounted fan creates a massive down-draft of air that pushes the hull upward anything from a few centimeters/inches to a couple of meters (5-6 ft). A cushion of air is trapped underneath the craft by a flexible rubber skirt that can bend around obstacles on water or land. Smaller secondary fans mounted on top, and driven either by the same diesel engine (or separate engines), create a backward force that pushes the hovercraft forward. Rudders behind the fans swivel this backward draft of air from side to side to provide steering.

Small “fingers” of rubber attached to the bottom of the skirt improve the seal between the skirt and the waves beneath it. This maintains the cushion of air, keeping the hovercraft above the water and making the ride smoother for passengers.

A typical hovercraft has two or more fans. The main fan in the center blows air downward to push the craft upward, above the water. Two or more other fans at the back blow air backward to make the craft go forward. This is an example of action-and-reaction (Newton’s third law of motion) at work!

A side-wall hovercraft has two rigid sides that extend under the water and so needs a skirt only at the front and back. Although it cannot travel on land, it can use propellers or water-jet engines, which makes it much quieter than a traditional hovercraft.

How much can a hovercraft carry?

The actual amount of lift depends on how much weight the craft is carrying. A fan of a given power will create a certain amount of pressure under the craft. Now since:

pressure = force / area

it follows that a bigger hovercraft (one with a bigger overall area) can carry more weight than a smaller hovercraft with a fan the same size.

Looking closer at a hovercraft

These pictures show some key features of a hovercraft in close-up:

Left: Close-up of a hovercraft skirt making a tight seal with the water beneath.

Middle: Vertical rudders behind the fans steer the hovercraft by directing air to the side.

Right: The fans are driven from engines in the side by giant axles.

Hydrofoils

Walking through water takes much more effort than walking through air and this explains why ships travel much more slowly than automobiles and aircraft. Water is almost 1000 times more dense than air, so most of the energy produced by a boat is taken up overcoming drag (water resistance). Hydrofoils travel much more quickly than ordinary boats not by pushing through the water but by raising the hull (main body) of the boat upward so it can glide above the waves.

Photo: This US navy hydrofoil has one foil at the front and two at the back. Note how the entire hull lifts clear of the water as the boat picks up speed. Photo of the USS Taurus (PHM-3) patrol missile ship by Mark S. Kettenhofen, courtesy of Defense Imagery.

What are hydrofoils?

Hydrofoils are among the fastest boats on the water, with top speeds of around 100-110 km/h (60-70 mph). The most powerful hydrofoils have three different engines, two diesel engines for pushing the boat through water at low speeds and a powerful gas turbine engine to lift it onto its hydrofoil and power it along at top speed. Hydrofoils have been widely used as high-speed ferries and as fast military patrol boats.

How does a hydrofoil work?

A hydrofoil is like a cross between a boat and an airplane. It has three wings on stilts called “foils” just beneath the water level. As the boat begins to pick up speed, water accelerates over the curved top surface of the wings and is then forced downward behind them. Since the wings push water down, Newton’s third law of motion tells us the water must push the wings up. That’s what creates an upward force called lift, strong enough to raise the entire boat above the waves. Sharks have a pectoral fin on the sides of their bodies that produces lift in the same way.

Photo: You can clearly see the wing-shaped hydrofoil under the surface of the sea in this shot of the missile hydrofoil USS Hercules (PHM-2). Photo by Mark S. Kettenhofen courtesy of Defense Imagery.

What are jetfoils?

The fastest hydrofoils are pushed forward not by propellers but by massive jets of water forced backward at high speed. A gas-turbine engine pumps out up to 180 tons (164 metric tons) of water per minute, roughly the same as 75 fire engines working together. A typical boat of this sort, the Boeing Jetfoil, speeds along on three inverted T-shape foils. Each foil is fitted with sensors wired to an on-board computer. This constantly adjusts flaps on the foils to maintain lift and keep the ride smooth.