Page 17 - Theory and Design of Air Cushion Craft

2 Introduction to hovercraft

Fast marine craft

Primary support Vessel classification Vessel subclassification

Stepped planning hull
Captured air bubble craft
Hydrokeel

Fig. 1.1 Classification of high-performance marine vehicles.

just far enough away from the water surface to reduce the surface interference, water
drag and wavemaking, while at the same time close enough to trap the pressurized air
between the ground and the lifted body. Under these circumstances the pressure gen-
erated is many times greater than the increased pressure under a free aerofoil, while
the drag of the lifted body is much reduced compared to a planing surface.
The idea to take advantage of an air cushion to reduce the water drag of a marine
craft has actually been established for over one hundred years. [210] [211] In Great
Britain, Sir John I. Thornycroft worked on the idea to create a thin layer of air over
the wetted surface of a ship, and was awarded a UK patent in 1877. He developed
a number of captured air bubble hull forms with cavities and steps in the bottom
and model tested them as alternatives to conventional displacement torpedo boats,
which his company built for the British Navy at the time. No full scale vessels were
built to translate the idea into practice, though the model testing did give favourable
results.
A patent for air lubrication to a more conventional hull form was awarded to Gustav de
Laval, a Swedish engineer, in 1882. A ship was built based on the proposals,