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Skirts and other improvements

Testing quickly demonstrated that the idea of using a single engine to provide air for both the lift curtain and forward flight required too many trade-offs. A Blackburn Marboré turbojet for forward thrust and two large vertical rudders for directional control were added, producing the SR.N1 Mk II. A further upgrade with the Armstrong Siddeley Viper produced the Mk III. Further modifications, especially the addition of pointed nose and stern areas, produced the Mk IV.

Although the SR.N1 was successful as a testbed, the design hovered too close to the surface to be practical; at 9 inches (23 cm) even small waves would hit the bow. The solution was offered by Cecil Latimer-Needham, following a suggestion made by his business partner Arthur Ord-Hume. In 1958, he suggested the use of two rings of rubber to produce a double-walled extension of the vents in the lower fuselage. When air was blown into the space between the sheets it exited the bottom of the skirt in the same way it formerly exited the bottom of the fuselage, re-creating the same momentum curtain, but this time at some distance from the bottom of the craft.

Latimer-Needham and Cockerell devised a 4-foot (1.2 m) high skirt design, which was fitted to the SR.N1 to produce the Mk V, displaying hugely improved performance, with the ability to climb over obstacles almost as high as the skirt. In October 1961, Latimer-Needham sold his skirt patents to Westland, who had recently taken over Saunders Roe's interest in the hovercraft. Experiments with the skirt design demonstrated a problem; it was originally expected that pressure applied to the outside of the skirt would bend it inward, and the now-displaced airflow would cause it to pop back out. What actually happened is that the slight narrowing of the distance between the walls resulted in less airflow, which in turn led to more air loss under that section of the skirt. The fuselage above this area would drop due to the loss of lift at that point, and this led to further pressure on the skirt.

After considerable experimentation, Denys Bliss at Hovercraft Development Ltd. found the solution to this problem. Instead of using two separate rubber sheets to form the skirt, a single sheet of rubber was bent into a U shape to provide both sides, with slots cut into the bottom of the U forming the annular vent. When deforming pressure was applied to the outside of this design, air pressure in the rest of the skirt forced the inner wall to move in as well, keeping the channel open. Although there was some deformation of the curtain, the airflow within the skirt was maintained and the lift remained relatively steady. Over time, this design evolved into individual extensions over the bottom of the slots in the skirt, known as "fingers".