Page 28 - Theory and Design of Air Cushion Craft

ACV and SES development in the UK 13

reduction. A number of accidents occurred to hovercraft in service at this stage due
partly to the lack of handling experience and understanding of the capabilities and
limitations of transverse/course stability of ACVs at high speed. ACVs operate with
significant sideslip, and have very different handling characteristics to other marine
craft due to unique phenomena such as 'plough-in'. Operation over land or ice has
no real parallel with other vehicles, so experience had to be built from zero.
Understanding the causes of these accidental events and revising craft design or han-
dling procedures to prevent recurrence was essential to continued technical progress.
Reference 4 recorded damage from accidents which happened in the period between
1963-1978, as shown in Table 1.3. This shows that 82% of such accidents were in the
time interval between 1967-1974, i.e. the concept development stage of hovercraft.
The table details all accidents except those in the former Soviet Union, for which data
are not available. A large number of accidents also happened to smaller (utility or
recreation) hovercraft, but only a small number of these were used commercially so
that the details are not accessible. A selection of the key accidents recorded in this
period are illustrated in Table 1.4.
To return to developments in the 60s, BHC's experience gained with SR.N2 and
SR.N3, together with the improving skirt technology developed through SR.N1,
SR.N5 and SR.N6, indicated that the original proposed design of the SR.N4 needed
to be revised. Project studies commenced in 1964, and the SR.N4 emerged with a new
shape, structural design, engines and skirt arrangements at an all up weight of 165
tons. The SR.N4 commenced trials in February 1968, and made its first channel cross-
ing from England to France on the 11th June, about 9 years after the historic SR.N1
crossing. SR.N4 was the first truly open-water passenger/car ACV ferry capable of all-
year-round services over sea routes where wave heights of 8 to 12 feet can be encoun-
tered. It has achieved speeds in excess of 90 knots and, operated out of specially
designed terminals at Dover and Calais, can normally deliver passengers and cars
across the channel faster than services through the new Channel tunnel, 25 years after
the craft first entered service.
The SR.N4 Mk2 (Fig. 1.11), in its basic form weighing 165 tons, can accommodate
254 seated passengers and 30 cars. SR.N4 is powered by four Rolls-Royce 'Marine
Proteus' gas turbine engines of 3400 shp, each driving a variable pitch propeller
mounted on a pylon (see Fig. 6.7). Interconnected with the propellers are four cen-
trifugal fans for delivering cushion air. The craft is operated by a three man crew and
is controlled by varying the propeller blade angles and by swivelling the pylons to
change the direction of thrust. Some 5 years after its introduction the SR.N4 was

Table 1.3 Accidents and Incidents to Hovercraft in Western countries from 1963-1978
Incident Damaged Sinking Total
Overturning 41 2 43
Damage due to strong wind, rough sea, grounding 31 3 34
Collision 19 1 20
Fire and explosion 5 8 13
Damage due to technical faults 18 - 18
Ice damage 21 1 22
Other damage 5 - 5
Total 140 15 155