Anthony Disbrow PER.6 5/20/04 Various Types of Hovercrafts and Designs This here is what a lot of people think of when they hear the word hovercraft. At Illustration : hovercrafts://www.starbase8.de/starwars/toys/k- Vehicles/LANDSPEX.JPG least, this is what most people that do not know what a hovercraft is think of when they hear the word hovercraft. While technically this is a hovercraft, it is one that only exists in science fiction. This does not necessarily mean that hovercrafts do not exist, it just means that they do not look or operate like the one shown here. There is probably some sort of energy or magnetic field that keeps this particular hovercraft floating, however we have not yet mastered the technology for this, either that or it is not perfected, or the government is trying to hide something. A lot of people also think that hovercrafts are a futuristic form of transportation. While hovercrafts are becoming more popular, they have actually been around for a long time. The hovercraft was actually invented in 1952 by a British scientist by the name of Sir Christopher Cockerell Even though a vehicle held up by energetic fields sounds pretty cool, normal hovercrafts are held up by a cushion of air. Basically, there is a skirt around the perimeter of the hovercraft that is pressurized, and as air leaks out from under the skirt, it creates an air cushion that allows the hovercraft to glide on many surfaces. It almost has the same principle as an air hockey table, only a hovercraft is much bigger and louder. Now this sounds simple enough, a big platform sitting on a pressurized skirt. As
a matter of fact, this almost sounds like an inflatable raft; well this is only partly true. If you flipped a raft upside down, and filled the sitting area with air so that it would be constantly pressurized, with a constant airflow coming out from under it, it would be a hovercraft. The main difference between just a raft and a hovercraft is that a hovercraft can operate on more than just one terrain. A raft floats on water because it is less dense than the water. When you put a raft on land, there is nothing but air to float on, and a raft is way more dense than air. However a hovercraft solves that by making enough air pressure under itself, that it sits on that air instead of on the ground. So when a hovercraft goes out onto the water, it is not floating on the water, it is still floating on air. Although there are several types of hovercrafts, they all operate in the manner described above. The reason that there are more than one type of hovercraft, is that not just one hovercraft can complete every task that people need done. Smaller and cheaper hovercrafts are used for just playing around in. Others are for multiple people to go fishing, hunting or camping. In Europe, hovercrafts are used a major form of transportation for people and vehicles because of their variety of terrain. The military also uses hovercrafts to transport troops over land and sea, rather than having them change vehicles when it comes time to change terrain. Some people even go to the extreme and build hovercrafts for racing purposes. So since there are so many different uses for hovercrafts, there have to be different varieties of hovercrafts to fulfill those uses. Besides there being different types of hovercrafts for different uses, there are a couple of categories of hovercrafts. The most basic form of hovercraft is one that is one hundred percent man/woman operated, powered, driven, etc. So basically there are no gas or electric motors to power this hovercraft, just person-power. Having no outside
power does not necessarily mean that the person in charge has to keep afloat by puffing through a straw or a hose. With these types of hovercrafts, mechanical advantage is on your side. Most people use some sort of setup with a bicycle hooked Illustration http://www.hovercraft.com up to a fan. So basically, the person on the hovercraft is powering all of the fans, blades, etc. and not a motor of some kind. Other than that the principle of how it works is exactly the same. Now in the case of the kid above on the left, he is faced with three choices. Either A, stop pedaling to take a rest and swim at the same time, or put some flotation in the craft so that is will rely on its buoyancy to float and not the cushion of air. Most people would call this cheating because with flotation it is not actually hovering. However, once the rider starts to pedal, it returns to the use of the cushion of air. Not only are there different methods for powering a hovercraft, hovercrafts can be made out of different materials depending on what they are being designed for. The most common materials used to make a hovercraft are wood, foam, or metal. Normally a hovercraft has a little bit of all of these materials on it, but depending on the use of the hovercraft. It also depends on how much one is willing to spend on a hovercraft. A wood design will run around 1700 dollars, while a foam one will only cost between 300 and 500 dollars. There is not really a huge difference between the two designs other than the price. Both designs weigh about the same, and go about the same speed, they even look very similar. The hovercraft on the left on the previous page is one made of foam,
Illustration http://www.hovercraft.com and the one on the right is made out of Illustration http://www.hovercraft.com wood. So the only real difference from the two designs is durability. A wood hovercraft can withstand more than a foam one because wood is stronger and more resistant to certain chemicals and elements. A third material used to construct hovercrafts is metal, usually aluminum because it is light. This material is usually used on larger designs because it is more durable. An example of this would be in a racing hovercraft that has to withstand the shock of jumps and other hovercrafts. Aluminum hovercrafts weigh about 200-300 pounds more than a wood or foam one of the same size, but with the extra durability, they can go upward of 60 miles per-hour without flipping over. An obvious difference in two designs could be the size. Most of the hovercrafts discussed above are small vessels that only hold 1-4 people. There are hovercrafts the size of houses that are used in Europe as transportation, and the military also uses very large hovercrafts for transporting troops and equipment. It is safe to say that these types of hovercrafts are a lot more bulky, and are made out of a little more than just foam wood or aluminum. These crafts can almost be classified as floating super structures. It is not clear what this
hovercraft to the left is used for, but it is either used for transportation or some sort of rescue vessel. The vessel on the to the right is one used by the U.S. Navy, and as you can see, it is much more impressive than the single person designs. Just to add some perspective, the skirt on this naval hovercraft is probably 10-15 feet off the water, with a fan of about 10-15 feet in diameter. Now to go into the specifics on how a hovercraft is made. In a lot of ways a hovercraft is like a boat. As a matter of fact, when the skirt is not inflated, most hovercrafts act as boats. The reason for this is that the hull is often made like a boat and packed with flotation so that it will float on water while at rest. The main reason for this design is that if your motors happen to die while on the water, you will not sink. People that use hovercrafts for various sporting activities such as fishing and hunting, use this feature to be more quiet, instead of having to keep the engines roaring to stay afloat. A concept closely related to flotation is lift. The lift on a hovercraft varies from design to design based on the intended purpose of the hovercraft. Now the lift on a hovercraft is slightly different than the lift on an airplane. An airplane generates lift by gaining speed to create pressure under the wings, and a vacuum above it. All of these factors result in the airplane being lifted into the air. The difference in lift between the two vessels is that the lift of a hovercraft is not as dependent on speed as an airplane. Usually a hovercraft will have a lift motor that pressurizes the chamber under the skirt. The more pressure that is built up under the skirt, the higher it hovers because it is creating a larger cushion of air to sit on.
The lift factor becomes a major factor in the design of the hovercraft. In the case of the single person design, a lawnmower or leaf blower is usually used to generate pressure because not much needs to be lifted. In the case of the naval hovercraft shown above, a lot more than a lawnmower is needed. A lot of these big vehicles use complex high power fans, while smaller (but still huge) versions can use 300+ h.p. car engines for lift. So the obvious factor with the lift design is the size and weight of the craft. Another major factor in how the lift system is designed is the balance of the hovercraft. A lot of the smaller design have the lift motor in the front because that is where the driver sits (far away from the propulsion propeller). By adding lift to the front rather than the rearm the hovercraft sits flat. If the person were to sit up front, and put the lift in the back, the front end would eat the dirt, or get buried in water, which would easily put a dent in one's enjoyment that day. The bigger the craft gets, the easier it is to balance. If you think of a small canoe, you can probably guess that it is a little tipsy, or you found that out in a not so happy way. Now if you were to get into a ski-boat, you will find that there is a lot more room to walk around without tipping the boat over. The same idea applies to hovercrafts. If you have a hovercraft that is small and narrow, it becomes extremely crucial as to where you put your lift and rider. Referring to the big naval hovercraft once again, it is extremely big, and the balance is not as touchy as with a small hovercraft. A group of people or even a car could move around the outside of the vehicle without noticing a shift in balance. So why is balance important if you cannot tip over a hovercraft? The answer is simple. Looking back on how a hovercraft floats, a constant cushion of air is needed. If one side is heavier than another, it will be harder for the air to escape the skirt from that side, and it will all go out the other side. If you are lucky, this will just cause the
hovercraft to turn slightly to the heavier side. If you are unlucky (which most people usually are), no air will escape from the heavy side, creating a huge amount of drag, rendering the hovercraft immobile. Once again, on the smaller hovercrafts, this is not hard to do, while on the bigger models, it takes a lot more weight to put it off balance. One thing that always looks impressive on a hovercraft is the propulsion system. There is nothing cooler than a huge fan spinning at dangerously high velocities. This area of design is also very touchy. If a fan is slightly unbalanced at 3000+ R.P.M. then you are just asking for trouble. You may also notice that there is a circular thing going around the outside of the fan. This is usually made of metal, plastic, or fiberglass, and is called a shroud. Most people assume that it is for protection from the blades. While it does serve as protection from the blades, it also does something else just as important. What it does is it directs the air coming from the propeller. Without this shroud, the air will just shoot off anywhere. By directing the airflow, more pressure is created resulting in more thrust. Also when it comes time to steer the hovercraft, having a strong airflow over the rudders helps a lot. Once again, comparing the big hovercrafts to the little ones, the little ones usually use some sort of small motor to spin the blade. The power of the motor depends mainly on the size and weight of the hovercraft. The average one person hovercraft will normally use at least a ten horsepower motor with one propeller. This usually creates enough thrust to push a hovercraft at anywhere between 20 and 60 miles per hour. The bigger hovercrafts mostly use a two propeller system, and each one has over 300 horsepower. The way the propellers are set up is also interesting. The fans spin opposite directions like o airplanes and boats. The reason they do this is a torque issue. Whenever something is being spun, torque is involved, and it often results in things
becoming unbalanced. By having the blades spin in opposite directions, it cancels out the torque, allowing for more balance than a single bladed design. The torque issue is only a minor issue in the realm of issues with a hovercraft. The main reason why larger hovercrafts have two propellers is that they need the extra power. A single-person hovercraft can operate just fine with a single fan, but on the larger ones, a lot more power is needed. One other thing with a two-prop design is steering. A single propeller design uses rudders behind the fan to change the direction of airflow, thus changing the direction of the hovercraft. With two propellers, not only are rudders used, but the speed of each propeller can be adjusted. This allows for a greater airflow on one side of the hovercraft, also resulting in the hovercraft turning. With the larger hovercrafts this is necessary because a single propeller with rudders does not work very well with steering a wide hovercraft. With two propellers there is much more mechanical advantage on each side, resulting in much more efficient steering. Finally the skirt is another very important element. For personal hovercrafts, a skirt is generally very small, and can be made out of anything from a shower curtain to certified skirt material. On larger hovercrafts, they use more than just a shower curtain. Normally the higher-grade skirts are made of some sort of high durability rubber and plastic mixture stitched together with kevlar thread. Not all skirts last forever. On all hovercrafts the skirt has to be either repaired or replaced after so much usage because a lot of times there is a lot of stress on the skirt, especially from the large hovercrafts. To conclude, all hovercrafts work on the same principle of floating on a cushion of air. Also, all hovercrafts share a lot of the same issues such as balance, and lift pressure. Where the differences are is how the hovercraft is intended on being used, and how much money is willing to be spent on the construction of the hovercraft. Personal
hovercrafts tend to need less advanced lift and thrust systems, but depend more on balance due to the fact that it is easier to put a smaller vessel off-balance. On the larger designs, the lift and thrust become a bigger issue because more mass is being dealt with, while balance is less of an issue because a larger surface area is being dealt with. Sources 1. "Hovercraft History" Link999 1997-2003 http://www.links999.net/science/hovercraft/hovercraft_history.html 2. "Hovercrafts" Mark Wells http://www.jessamine.k12.ky.us/ejms/teched/9899stds/mwells/ 3. "Universal Hovercraft" Universal Hovercraft 1999-2003 http://www.hovercraft.com 4. "East Coast Marine" Ship Brokers http://shipsusa.com/index.cfm 5. "Hovercraft" Wikipedia, the free encyclopedia. http://en.wikipedia.org/wiki/hovercraft 6. "History of the Hovercraft" Gizmo Highway Technology Guide