The Princess Margaret and her sister ship the Princess Anne ply the channel between Dover and Calais, floating on a cushion of air.

The 330 tonne vessel cost £1.75m in 1968 - the equivalent of around £16m today. It and its sister ship are the only ones in commercial service carrying cars and are the largest hovercraft in the world.

Powered by four engines developing 38,000lbs of thrust, the ships are capable of speeds of over 50 knots, and can cross the channel in 35 minutes.

Conventional ferries take twice as long to do the journey, but hovercraft are expensive to run and maintain and high development costs have discouraged new designs.

Even the advent of the channel tunnel in May 1994 did not kill off the hovercraft, but though they will still be carrying cars come the millennium they are now expected to be phased out in between three and five years.

A great British innovation

The use of an air cushion to speed ships was considered as early as the 18th century, and Sir John Thornycroft got as far as patenting an early design in the mid 1870s.

Engines that produced enough power to make hovercrafts practical were not available until this century, however, and it was not until 1959 that an experimental hovercraft, designed by Christopher Cockerell (later Sir Christopher) first took to the seas.