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Hovercraft Hull Design

Hull Design

Particularly on light hovercraft the hull designs have gone through many changes, the use of a bag skirt meant there was plenty of room for a boat-like hull with inspectable buoyancy, chambers often filled with ping-pong balls or plastic bottles. The hull, usually made of plywood [cheap but not good in the long term], could be made to float well and was easy to construct, and by this time planing surfaces which give a self-righting effect in the event of a plough-in had become a safety requirement of the HCGB.

Initially segments were fitted to the same cross-sectional bag skirt design of hull but usually with a more rounded bow shape. The lift air was pumped directly into the plenum through a duct, usually positioned on the front of the craft and in front of the driver: but unlike bag-skirted craft the hull needed to be correctly balanced in order to lift up squarely, otherwise the craft would have a tendency to sit tail down with all the lift air escaping from under the front fingers. This situation was alleviated by the fitting of some internal ducting to the rear fingers, or a small bag that jacked up the rear of the hovercraft allowing the plenum air to flow backwards. There were still drawbacks in the system such as the lift fan being vulnerable in the event of the craft ploughing-in at speed and water hydraulicking up the lift duct; also from stones flicked up and into the fan by the front fingers.

The next step in hull evolution, which had many spin-off ramifications, was the full-flow hull system in which all of the lift air was fed into a chamber running around the entire periphery with a dedicated hole located just under the top of the top skirt attachment of each segment. The advantage in lift air efficiency is best quantified on two similar sized craft, one of which will adequately use 20 horsepower for lift when fitted with a segmented skirt, whereas a similar craft with a loop segment skirt can use considerably more horsepower. The reason takes us right back to Sir Christopher's original peripheral air jet system. As the outer face of the skirt is angled inwards at approximately 45 degrees the air is jetted down each segment from the feed hole in the hull, creating an air curtain extending beyond the edge of the skirt, thereby reducing the skirt friction, but probably more importantly, as the air is being jetted inwards under the craft it significantly reduces the air loss in comparison with the other system. To the casual observer the main difference is the reduction in spray from craft fitted with this type of skirt.

This type of hull which also acts as a duct, or air box, means that the location of the lift in the system is less critical and therefore it lends itself to a single engine / single fan [integrated] concept which enables small fun craft to be made more cheaply; in fact so successful was this concept that for many years this design was the basis of the two lower European racing formulae.