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.
The author does not allow comments to this entry