Page 18 - Theory and Design of Air Cushion Craft

Hovercraft beginnings 3
but Laval's experiments were not successful. The air lubrication created a
turbulent mixture of air bubbles and water around the hull, rather than a consistent
layer of air to isolate the hull surface, and so drag was not reduced.
Air lubrication has been pursued at various times since these early experiments by
engineers and scientists. In practice it has been found that it is very difficult to create
a consistent drag reducing air film on the wetted surface of a normal displacement
hull. On the contrary sometimes an additional turbulent layer is added, increasing the
water friction drag. A more substantial 'captured air bubble' is needed.
In 1925, D. K. Warner used the captured air bubble principle to win a boat race in
Connecticut, USA. He used a sidewall craft with planing bow and stern seals. A little
later, the Finnish engineer Toivio Kaario developed and built prototypes of both the
plenum chamber craft and the first ram wing craft (Fig. 1.2).
To investigate thin film air lubrication, some experiments were carried out in the
towing tank of MARIC in Shanghai, China by the author and his colleagues in 1968,
but the tests verified the earlier results of Laval and others. Based on these results they
confirmed that a significant air gap was necessary to separate the ship hull fully from
the water surface. This needed a concave or tunnel hull form.
In the mid 1950s in the UK, Christopher Cockerell developed the idea for high
pressure air jet curtains to provide a much greater air gap. This invention provided
sufficient potential for a prospective new vehicle technology that the British and later
the US government committed large funds to develop ACV and SES. China and the
USSR also supported major programmes with similar goals over the same period.
Air cushion supported vehicles could only be successfully developed using suitable
light materials for the hull and engines. Initial prototypes used much experience from
aircraft design and manufacture to achieve the necessary power to weight ratio.
Experience from amphibious aeroplanes or flying boats was particularly valuable
since normal aircraft materials are not generally designed to resist corrosion when

Fig. 1.2 Finnish ACV constructed by Toivio Kaario in 1935.