What is a hovercraft?

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What is a Hovercraft

 

When you mention the name Hovercraft most people think of a helicopter. If someone see's one from the river bank, the term "airboat" comes to their mind almost instantly. I say "almost" because most people are not quite sure what they are looking at. If that hovercraft were to leave the water and glide up the beach or boat ramp and then glide onto it's own trailer, well, now you have a crowd. Most of them can't believe what they just saw. Few people know of the personal sized hovercraft that are used worldwide for recreation, racing and rescue.

 

The modern sport hovercraft has evolved into a lightweight, high performance machine. Hovercraft can glide across the surface, almost any surface, totally supported on a cushion of air. An engine driven fan mounted on the craft supplies the air cushion. Air pressure is retained beneath the hovercraft by a flexible fabric skirt around the perimeter that gives about 200 mm of ground clearance. A propeller or fan also provides thrust for an average speed of 40 to 100 km/h. Traditional craft used twin engines, one mounted up front to supply lift and another in the rear for thrust. More recently integrated craft have become popular. These use a single engine/fan setup for lift and thrust. Rudders in the airflow give the driver precise control once the techniques are mastered. The hull of a hovercraft, underneath the skirt, is similar to a boat. In fact, just like a boat, a hovercraft will float with the engines off. Hovercraft can travel over land, snow, water or ice without pause or modification- a true amphibious vehicle.

 

Hovercrafts are not difficult to drive but they do have a few quirks that make them unlike any other vehicle. First off there is little or no friction with the surface. Hovercraft will instantly go downwind or downhill unless the driver is alert. Steering can also be quite tricky and turns must be anticipated, as hovercraft tend to slide due to momentum. Without a firm grip on the surface, they can spin and do a 360 degree turn while traveling in a straight line. Hovercraft are not difficult to control, just different from any other powered vehicle. Most people can get the hang of it after a few minutes of practice in an open area.

 

This slippery platform of air lubrication gives the hovercraft it's unique amphibious capability and allows them to travel in shallow water or muddy areas that would cause other vehicles to get stuck. The light-footed hovercraft is also very easy on the environment. The hovercraft can be driven over an egg without breaking the shell for the load is spread over a large area. And, since the hovercraft rides on top of the water, as opposed to in the water, it creates almost no wake and no petrol and oil pollution.

 

As you might imagine there are no "brakes" of the familiar variety. Hovers stop by settling to the ground or water on skids under the craft. Now that's not always very effective, but a hovercraft can also spin 180 degrees and use the craft's thrust for braking. With a little practice in an open field anyone can become a proficient hovercraft driver in a very short time.

 

With it's ability to traverse land, water, mud, snow, ice or any mixture of these elements the hovercraft has proven itself to be an efficient, rapid response, rescue vehicle.

 

How it all began

Christopher CockerelI's experiments with coffee tins and a vacuum cleaner in the early 50's led to the first manned Hovercraft 'flight' in 1959. The first Hovercraft was the SRNI and its limitations soon became apparent. It could not negotiate waves of more than 500mm high or land obstacles more than 240mm high.

To combat these limitations the flexible skirt was developed and from then on Hovercraft technology made rapid progress.

Hovercraft have now become much larger, more efficient and are in widespread use all over the world. Christopher Cockerell was knighted for his achievement in 1969.

Like all other 'interesting' inventions, the Hovercraft principle attracted many 'pioneer' amateur builders.

These pioneers eagerly experimented on a small scale with the new technology, eventually forming themselves into an organised group, thus in 1966 the Hoverclub of Great Britain came into being (now Known as the Hovercraft Club of Great Britain)

The first Hovercraft rally was held in Nottinghamshire, where craft were lucky if they could operate at speeds of 15km/h over smooth grass. Since that time light Hovercraft have evolved into very sophisticated machines capable of speeds in excess of 110km/h.

A set of safety requirements were formulated for the design and construction of light hovercraft and the responsibility for monitoring the development of light hovercraft was passed to the Hovercraft Club of GB by the CAA.

Car and motorcycle engines were used for power units in those early craft and centrifugal fans and propellers were adapted for thrust units and hulls came in all shapes, sizes and materials.

Most craft in those early days used two engines; one for lift and one for propulsion.  Propellers  were eventually discarded in favour of axial fans and segmented skirts replaced bag skirts for racing Hovercraft

For simplicity, lightness and efficiency, most of smaller modern hovercraft are of an integrated design powered by just one engine driving a single fan just as in Cockerell's' SRN1 back in 1959.

How it Works

The hovercraft is quite simply a vehicle which is wholly supported on a cushion of air supplied by a powered fan mounted on the craft.

Around 25% of the airflow produced by the thrust fan is deflected by a horizontal splitter plate mounted in the air duct to provide the lift air. This lift air is ducted through the hull and fed underneath the craft to provide the cushion pressure and is retained by the skirt.

Thrust and forward movement of the craft is achieved by the remaining 75% of the propulsion air which is directed through the duct to the rear of the craft.

The skirt is usually made from a thin plastic coated fabric and can be in the form of a tubular bag or separate narrow segments.

There are two types of hovercraft, those which use separate engines for lift and thrust and those which use one engine for both lift and thrust, the latter are known as integrated craft.

For an integrated craft the engine and fan are usually mounted at the rear of the craft, to balance out the driver sitting in the front.

Hovercraft fitted with two engines usually have one at the front which drives a fan to produce the lift air and a much larger engine at the rear to drive one or more propulsion fans.

One major advantage of the twin engine configuration is that it allows the craft to be on full cushion at all speeds, which provides much improved maneuverability

Both types of craft use rudders in the propulsion airflow for directional control. The driver operates the rudder from the driving position by a deck mounted joystick or a cycle handlebar

The engines are controlled by lever type throttles mounted on the joystick or handlebars.

Sporting Hovercraft

The sporting hovercraft has evolved over more than 20 years. From the lumbering tank-like creations capable of only 15km/h, if you were lucky, to speeds of over 110km/h in to-day's highly developed craft.

The hull can be constructed from plywood or fiberglass or maybe a combination of both. Some racing hovercraft have even been constructed from thin aluminium sheet or polystyrene.

The skirt is usually made from a flexible waterproof material with the purpose of retaining a cushion of air which supports the hovercraft. In general today there are both bag and segmented skirts. Skirt material must be light and able to withstand adverse conditions such as stones, sand, salt water and strong side forces.

The forward propulsion system may consist of either one or two thrust fans contained in specially shaped fiberglass ducts. The fans are engine driven via a toothed belt or gearbox, steering is achieved by a rudder in the air stream from each of the ducts. Many different engines have been tried over the years. the main requirement being an engine which can achieve high power to weight ratio.

This has resulted in a new breed of engines modified for use in hovercraft, these being similar to those currently used in Microlight aircraft. Many such engines can develop up to 7Obhp yet only weigh about 30kg. In recent years a number of hovercraft have been powered by motorcycle engines, most utilise the Yamaha which has proved to he very reliable low cost power unit.

The size of a hovercraft is governed by the payload weight and the required cushion pressure, which is usually around 7 pounds per square foot. Single seater craft tend to be about 3m long by 1.8m wide and in the case of a racing craft, can weigh as little as 50kg.

Formula 1                Engine capacity greater than 500cc

Formula 2                Engine capacity between 250cc and 500cc

Formula 3                Engine Capacity up to 250cc

Formula Junior                Engine capacity up to 250cc

Formula 25                Maximum engine capacity of up to 25hp

 

Cruising Hovercraft

Cruising is the term used to define any type of recreational use for light hovercraft. Generally speaking, cruising involves the use of a Hovercraft in an uncontrolled environment, such as operating on a river or close in shore on the sea.

Present day cruising craft are generally much larger than racing craft and incorporate additional buoyancy and freeboard to render them safe for use in open waters.

Modern lightweight car engines are extensively used to power cruising craft to ensure reliability and quiet operation in all situations.

It is possible to construct a hovercraft yourself using a preformed fiberglass hull available from a hovercraft supplier. All components required to build your craft are usually available from the same source

Perhaps the most interesting advantage of a cruising craft over a racing craft is that it allows the owner to enjoy his hobby at any time he chooses and at a more leisurely pace

The craft must be registered with the Hovercraft Club

The operator should hold a Hovercraft Operating license

Formula Junior

Formula Junior was introduced in 1979 to cater for the increasing number of youngsters eager to take part in the sport and to provide scope for school owned hovercraft to be driven by younger pupils. Schools in particular have supported Formula Junior because this is usually the least expensive hovercraft to build for the first time

Before a new Formula Junior driver can take his or her place on the racing grid for the first time, they are required to pass a simple handling test and answer relevant questions about hovercraft.

The engine capacity limit is designed to comply with the Formula 3 requirement. to allow craft to be driven in at least two formulae at a race meeting. In this way several pupils of varying ages can drive a school hovercraft or, a father and son or daughter are able to participate in racing by sharing the same hovercraft.

The popularity of Formula Junior racing has gradually increased over the years with more and more craft appearing on the starting grid. The age limit of Formula junior makes it unique and rather special. It is an ever changing formula with the older competitors moving on to the National Racing formulae and new younger competitors joining the grid for the first time.

There is a friendly and helpful atmosphere in Formula Junior circles and a most competitive yet, responsible spirit in the racing. Many of today's leading National drivers acquired their skills and racing confidence in Formula Junior.

Tbe handling test is designed to demonstrate that the driver knows how to use his craft's controls effectively by driving around markers placed a short distance apart in an open area. The examiner, who is an experienced driver, will be looking for the correct use of throttle and other controls and the effective use of the drivers body weight to maneuver the craft. The questions are related to safety and to demonstrate the drivers understanding of the hovercraft Racing Competition Regulations.

Formula Junior is an excellent introduction to the exciting sport of hovercraft racing and fulfils the need of many youngsters for speed and competition in a safe and controlled environment.

Formula Junior racing is for drivers aged between 11 and 16 years driving hovercraft with an engine capacity of not more than 250cc.

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