Page 26 - Theory and Design of Air Cushion Craft

ACV and SES development in the UK 11

this reason, the construction and operation costs were high relative to other marine
vehicles. Although it was only originally intended for a six month trials programme,
it eventually proved to be an excellent research tool for over four years. This small
craft (weighing 4 tons) demonstrated the basic principles of riding on a cushion of air
to be sound. A series of development modifications associated with alternative power
plant and plan-form shapes in succeeding years increased the speed boundary from 25
knots to as high as 60 knots. More significant than the increased speed in calm con-
ditions was the development of long flexible skirts which enabled the craft to operate
successfully in 4-5 feet waves, whereas in its original form it was only capable of oper-
ating in wave heights of no more than 1.5 feet.
The invention of flexible skirts by C. H. Latimer Needham in 1958, which he sold
to Saunders-Roe in 1961, and later the segmented skirt by Dennis Bliss of Hovercraft
Development Limited (HDL) represented a break-through in hovercraft technology
from experimental investigation to engineering practice. The cushion depth could be
increased several hundred times, allowing practical operation of hovercraft on rough
water and unprepared ground. In addition, skirt shifting systems, controllable pitch
air propellers, jetted rudders and puff ports began to be used for improving the
manoeuvrability, course stability and obstacle capability of hovercraft.

Concept development: from the early 60s to the early 70s

The results of research trials with SR.N1 indicated that a truly competitive commercial
hover ferry would probably need to be 125 to 150 tons in weight and some four times the
length and breadth of the SR.N1 manned model, in order to cope with 4 to 6 feet seas.
A jump from 4 to 125 tons represented such a major engineering step that it was decided by
Saunders-Roe to approach this in three stages over a 7 years programme.[207] The first
stage was implemented with the 27 ton SR.N2, which was used to develop the swivelling
pylon mounted propeller control system, and the integrated lift/propulsion concept. The
second step was to stretch the SR.N2 design to become SR.N3, and obtain the largest
craft capable of being operated with the 3600 horsepower of the SR.N2. The intended
final stage was to use the experience gained with the developed machinery and systems
to produce a 125 ton SR.N4 (Fig. 1.11). Westland Aircraft Limited, who had taken
over Saunders-Roe Limited in 1959, backed this long range programme, and in 1960
the SR.N2 was jointly funded by N.R.D.C. and Westland. SR.N2, capable of carrying
70 passengers at 60 knots, was launched in January 1962 and was used on trial pas-
senger services in the Solent and the Bristol Channel. Additionally it was taken to
Canada for trials and made an historic crossing of the Lachine Rapids on the St.
Lawrence river just below Montreal. The SR.N3 was originally intended as a 150 seat
craft, but it was eventually ordered by the British Government for military evaluation
trials. These continued for many years, culminating in explosion trials for shock resis-
tance of air cushions against underwater mines [21, 213] (see Fig. 1.12). These trials
were the start of a new application for ACV and SES, mine countermeasures, which
continues in many countries today, particularly Norway and the USA.
During the 1961-3 period a number of other British companies developed research
and experimental craft with a view to commercialization later on. Vickers built the
VA. 1 to 3 series, Denny Brothers built two sidewall craft, Britten-Norman built the