Page 62 - Theory and Design of Air Cushion Craft

46 Introduction to hovercraft

strengthening and road repairs is obviated, and with the latter the payment of com-
pensatory costs to farmers is also avoided. Unfortunately the high work capacity of
such systems, and the limited number of movements required, make this a niche
market.
At the other end of the scale are hover pallets which operate on the air lubrication
principle at relatively low pressures, such as are available from normal industrial sup-
ply air lines. Current types can carry containerised loads of up to 5 tons in weight and
several are in service with shipping companies and other industrial organizations.
Their high manoeuvrability and simplicity of operation have led to economics in
manpower, time and a more efficient utilization of storage space. This application has
a wider market than large transporters, but is more easily considered an extension of
industrial mechanical handling systems than a new standalone business. The most
successful ventures in this area have been just that - extensions of existing industrial
handling companies.
The main subjects of this book, the ACV and SES, are both vehicles which have sig-
nificant potential for further improved economy and performance. In common with
most other forms of transportation, development of the vehicle is closely linked with
technological developments in the power units which are used to drive them. In the
case of the ACV and SES the story began with aircraft engines and gas turbines, and
has now moved on to high speed diesel engines. The development of new lightweight
water cooled diesels is encouraging ACV design at larger payloads, though perhaps
not yet with the potential to provide a replacement for the SRN.4. Marine gas tur-
bines with increased efficiency and lower maintenance demands, driven by the
demand for powering catamarans, open the opportunity to develop larger SES which
will begin to release the full potential of the technology. At speeds above 60 knots, it
may be the propulsor which is the limiting factor once again rather than the power
plant, until further improvements in waterjet technology are available.
ACV and SES at small to medium size have been developed by standalone busi-
nesses. Larger ACV and SES have to use the leverage of existing shipyards or marine
construction companies if they are to achieve necessary economy in construction. A
number of shipyards now have experience in large scale aluminium and GRP con-
struction which may form a suitable basis.
The rate of development of the hovercraft principle has been relatively rapid. The
widespread adoption of this principle may take many more years, but it has never-
theless started encouragingly. However, its future growth will depend on the continu-
ation of research and development efforts throughout the world.

1.9 SES and ACV design

The reader should now have a fair idea of ACV and SES historical development.
Before looking at the design process itself, we will now spend some time exploring the
theory behind air cushions, and their interaction with the water surface over which
these craft normally operate.
We begin with an explanation of the air cushion itself. The basic idea behind these
vehicles is that all (ACV) or a large part (SES, 60 to 90%) of the weight is supported