Hovercraft Skirt

Skirt Design

The original skirt designs were many and varied: examples ranged from brushes to folding semi-flexible material - even plywood was used; but the first skirt that we would recognise, was just flexible material attached to the outer edge of a craft, the lower edge of the material stopped from blowing away by being attached to the hull with ropes or chains. The air trying to escape from under the hull blew up the skirt into a "C" shape.

Naturally a complete bag with a top and bottom attached to the hull quickly evolved, but the method of inflation was done in two different ways; in one, tip air from the lift fan was fed directly into the bag with the majority of the air from the fan going directly into the plenum [the space beneath the craft]; this gave a very stiff bag but with problems if the bag became damaged, very similar to an air-lubricated inflatable dinghy. This system is still very popular in North America where it is perfectly adequate when used on long expanses of smooth water. The alternative is a bag into which all the air from the lift fan is fed, and the lubrication air is let into the plenum through holes cut in the skirt or through a gap in the hull. These two types are known as "no-flow" and "full-flow" bag respectively.

The bag skirt design was used initially on both the larger commercial craft and on some smaller designs, but particularly on the larger craft it was too bouncy and the drag on anything other than a smooth surface was unacceptable. The next evolution on the larger craft was and is the H.D.L. "loop and finger" or "bag and finger" type skirt - a series of convoluted pieces of open-ended material "fingers" attached to the bottom of the "C" or bag type skirt which became the standard design for the larger rough water use amphibious Hovercraft.

In contrast to the American craft the majority of early British racing craft used the full flow bag skirt, the stiffness of which could be adjusted to the driver's requirements by virtue of the outlet hole size. Experimentation with loop and segment designs was tried but lacked the necessary stability for high-speed use; experiments were also tried with straight segments and although having less drag than the bag skirt, stability was a problem. In racing terms this meant that the bag skirt was slower in a straight line but more stable in the corners.

The skirt that evolved was in effect, a segment cut through both the loop and finger Profile skirt to include one complete segment and the same width part of the loop: this is the basis of the segmented or finger skirt which, with various modifications, is the most commonly used skirt on all European craft up to maybe 20 feet in length.

Before leaving skirt design I should mention the design of Monsieur Bertone of France, whose conical "jupe" (skirt) was used on the ill-fated Sedam cross channel craft. To use this design [for which the British Government had no patent] a series of large individual material cones are arranged across the bottom of the craft and individually fed with air. A peripheral spray skirt is used obscuring the jupes from the casual observer. I can think of one craft where this spray curtain looks similar to segments. In many respects the previous comments with regard to skirt design are the visual changes, and as with a lot of things hovercraft, what you see is only part of the story.