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Small Hovercraft Hull Material

In the early days of personal hovercraft manufacture fiber glass was used extensively to build hulls in just one piece. For one thing, the material is very light and so the power to weight ratio is well taken care of. Of course, bigger craft have by necessity hulls made of stronger and more rigid materials, such as aluminium or composite polyester construction, but the particular design characteristics of personal air cushioned craft for leisure use means that certain plastic scan be used. The huge disadvantage of GRP is that it split when hit with a hammer, or when it comes across an obstacle in it's path. It's expensive to repair and takes time.

Many manufacturers are turning to new materials that are in tune with the small hovercraft requirements, namely, lightness, robust and maintenance free. High density expanded plastic delivers all of the above and more - see diy hovercraft kits. As an added bonus, it can be supplied in any color that's requested - the dye is simply added to the plastic granules before the hull is cast, so there's no paint to upkeep and worry about. Hence, it is maintenance free. Most personal vehicles are less than four metres long, so it's possible to mold the hull in one piece, which saves a lot of time in the manufacturing process. Holding bays for engines and other important fitments can be designed right in, without having to worry about fabricating extra pieces later.



One manufacturer of leisure hovercraft present demonstrations where he repeatedly bashes one of the company's hulls with a ball pein hammer. Of course, the ball of the hammer dents the plastic and if were mine I wouldn't be too pleased, but the point is there is no splitting and no extensive damage beyond the point of impact. No expensive repairs are needed or even necessary, which represents a big advantage for prospective buyers and family men concerned about the safety of their kids, for example.

The concept of monocoque construction lends itself beautifully to hovercraft manufacture. All the holes for steering linkage and engine housing are simply factored in and the steel mold is machined ready for production. Plastic molds are expensive, and are intended for runs of many finished parts, not just a one off, unless it's a prototype. Even cupboards and other accessible areas will be present in the finished article. As the plastic expanded material has the consistency of wood, fitments can be fixed using special marine screws.

The Factors Affecting Hovercraft Speed

In principle, the forward speed of a hovercraft is largely determined by the power of the forward propulsion engine and the craft's aerodynamics, but this is too simplistic. In theory, on a perfectly flat surface it should be possible to attain incredible speeds, but there are factors that make this quite undesirable and positively dangerous. Racing craft have a special design which reduces the danger.

Imagine a hovercraft moving over a flat surface faster and faster, with only the air resistance to slow it's movement. The air in front of the vehicle doesn't part immediately and just get out of the way, but a bow wave, or turbulence occurs as the air is pressurized. This pressure wave is always looking for an outlet so that atmospheric pressure can be quickly achieved. Normally, this outlet is found by the air moving round and over the hull. If the skirt at the front is weak, or is faulty and air pressure pushes it in, then the air pressure under the bow will increase dramatically.

Like any mechanical system, instability increases with no apparent effect until a tipping point is reached. After this point, the system state changes very rapidly. In the case of our over-speeding hovercraft, the bow will lift rapidly and flip over. Obviously, this is a disastrous situation threatening the lives of everyone on board. There is little time to escape as tipping points are by definition very fast. Designers need to specify a maximum speed for their hovercraft which should be around 50% of the speed which would introduce instability. These considerations are important when looking a place to purchase a hovercraft.

In practice, this is achieved by specifying an engine size that is not capable of pushing an ACV beyond it's unstable speed, which greatly reduces the danger. However, other factors could also come into play, such as moving up a slight gradient with a frontal wind. In this case the craft would not be horizontal, but leaning backwards, and the forward wind would provide further air pressure to the bow.

Racing hovercraft have a particularly flat hull design with a stiff skirt which allows them to travel at speeds of up to 75km per hour without too much trouble. The surface are of the hull underside is also large compared to other craft used for leisure, rescue or survey purposes, which also helps to keep the center of gravity low enough to resist flipping over. That said, part of the excitement of watching air cushioned craft racing is the fact that they can, and often do flip over.