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This article was first published in 2008.
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Hovercraft as we know them
today started life as an experimental design to reduce the drag that was placed
on boats and ships as they ploughed through the water. The first recorded design
for an air cushion vehicle was put forward by Swedish designer and philosopher,
Emmanual Swedenborg in 1716.
The craft resembled an
upturned dinghy with a cockpit in the centre. Apertures on either side of this
allowed the operator to raise or lower a pair of oar-like air scoops, which on
the downward strokes would force compressed air beneath the hull, thus raising
it above the surface. The project was short-lived and it was never built, for
Swedenborg soon realised that to operate such a machine required a source of
power far greater that that which could be supplied by a single human
occupant.
In the mid 1870s, Sir John
Thornycroft built a number of model craft to check the 'air cushion' effects,
and even filed patents involving air-lubricated hulls. From this time both
American and European engineers continued work on the problems of designing a
practical craft. But not until the early 20th century was a hovercraft
practically possible, because only the internal combustion engine had the very
high power to weight ratio suitable for hover flight.
In the mid 1950s brilliant
British radio engineer Christopher Cockerell (later Sir Christopher), and French
engineer Jean Bertin, worked along similar lines of research, although they used
different approaches to the problem of maintaining the air cushion. Cockerell,
while running a small boatyard on the Norfolk Broads in the early 1950's, began
by exploring the use of air lubrication to reduce hydrodynamic drag, employing
first a punt, then a 20 knot ex-Naval launch as a test craft. The limitation of
this approach quickly became apparent, and before long he was fired with a far
more ambitious idea, one in which a thin layer of lubricating air gave way to a
deep air cushion which would raise the craft above the surface, enabling it not
only to clear small waves, but also to make the transition from water to land
and back again.
To check his own theory,
Cockerell used little more than a couple of tins, a blower and a pair of scales.
By inserting a cat food tin into a coffee tin, and blowing a jet of air through
the gap between the walls of the inner and outer tins, he demonstrated the
possibility of a machine that could one day travel on a cushion of air. Aimed at
the scales, the blower's jet pressure alone was 1lb (0.45kg). The jet coming out
of the tin assembly when brought near the scales gave 3lb (1.36kg).
Unlike earlier plenum-type air
cushion vehicles, in which air was simply forced by a fan into a larger chamber
beneath the vehicle and allowed to escape, Cockerell's concept was that air
would be taken in through a large fan and separated, a percentage being directed
to the cushion via a system of slots around the underside perimeter of the
craft, with the nozzle aimed downwards and inwards to form a continuous air
curtain. This would create a jet of high pressure air that would move under the
hull and be retained to a certain extent by the jet line that formed the curtain
effect, the balance of the air being ejected through ports at the rear of the
craft for propulsion. It was this idea - an airflow system - that was the key to
progress.
Working Model
Christopher Cockerell had a
neighbouring boat builder produce a working model hovercraft. His idea worked
very well in model form, but as was later found, considerable power was required
to maintain the cushion, and also the hard structure clearance was not ideal.
It was then the duty of anyone
who thought an invention had any military value to contact the Service
ministries and give them first refusal. Accordingly, the model hovercraft flew
over many Whitehall carpets in front of various government experts. It was then
promptly taken out of its inventor's hands and put on the secret list where it
languished for over a year, no one having any idea what to do with it. News
gradually filtered in from abroad of similar projects, particularly that of a
Swiss engineer, and Britain seemed to be in danger of losing the lead it held.
A member of the Ministry of
Supply staff, Mr R A Shaw, then took a step which was to have important
consequences. He authorised a small contract to the aircraft company Saunders
Roe, to check the design of the vehicle.
The Saunders Roe report was
favourable and Cockerell gained permission to approach the National Research and
Development Corporation (NRDC) to see if they might be prepared to give the
hovercraft backing if the project could be freed from the secret list. This
Corporation was financed by loans from the British Board of Trade to develop and
exploit (where the public interest is involved) British inventions to the
manufacturing stage.
Experimental Craft
Cockerell took a film of the
model performing various manoeuvres to show the NRDC in April. The next morning
they made their offer - they were prepared to put up £1,000 immediately for
securing Cockerell's patent rights. In a later article, Cockerell recounted,
"hovercraft very, very, very nearly didn't happen" - at the crucial NRDC board
meeting it was only the Chairman's casting vote that secured NRDC backing for
the hovercraft project.
With the eventual object of
forming a British Hovercraft industry, NRDC ordered an experimental craft, the
SR.N1, from Saunders Roe (Aviation) in the autumn, after having eventually got
the craft off the secret list. Hovercraft development for civilian use could now
go ahead, as its value for military purposes had not yet been proven.
Saunders Roe threw themselves
into the project with great enthusiasm, with the result that the craft was
completed two months ahead of schedule, only eight months after starting work.
The Cockerell-designed research vessel Saunders Roe Nautical One (SR.N1)
appeared in May at East Cowes, Isle of Wight - the first flight taking place on
11th June.
The press were present in
force and watched with astonishment as the model craft was demonstrated to them
on a lawn and over a small 'obstacle course'.
It was then the turn of the
full-sized craft to demonstrate its capabilities, and this was carried out on
the concrete slipway. It was obvious that this was not going to be enough
though, so the craft was then towed out in to the East Cowes yacht basin by a
launch - a nervous time for its engineers as the craft had not taken to water
before!
All went well though and that
craft performed flawlessly, having photographs taken against the Queen Mary
which was passing at the time. A few weeks later it performed in a Combined
Operations exercise and won commendations from the very services that had
spurned it only a year before.
This first skirtless craft
could operate only in calm seas up to 1½ feet (46cm) in height and negotiate
obstacles of 6 to 9 inches (15 – 23cm).
First Channel Crossing
Having been shipped to France
by tender, the world's first all metal hovercraft crossed the English Channel
between Calais and Dover in 2 hours, 3 minutes on the 25th July with Captain
Peter Lamb piloting, Mr John Chaplin as navigator and the inventor, Mr. (later
Sir) Christopher Cockerell in his own words as 'moveable ballast' on board.
Extracts from the N1's 1959
log:
The following morning at 3.00am there was a flat calm and
after a brief look at the sea outside the harbour the Chief Test Pilot decided
to make an attempt with as little delay as possible. With Mr C S Cockerell and
Mr J Chaplin aboard, the craft cleared the harbour entrance at 4.55am just ahead
of the RAF Rescue Launch. The wind, such as it was, outside the harbour was
about 5kt from the north east. With the craft above hump speed the pilot decided
to make as much northing as possible, for the true track was 294 degrees and any
increase of wind, which was to be expected with sunrise, could then be used as a
quarter to tail wind in the latter stages.
When the craft was well clear of the Whistle Buoy,
marking the approach entrance to Calais, the pilot decreased rpm to 2,700 with
the craft still above hump speed (estimated 18-20kt). At this time the loom of
the South Foreland light was just visible, but fast disappearing with the
approach of daylight. For the next five miles, navigation was made by dead
reckoning, with the RAF Rescue Launch, which had previously agreed to maintain a
true track from Calais to Dover, well away on the port side.
At approximately 5.30am the
white cliffs of Dover, tinged with red in the morning sunrise, were first
visible. Up to this time the SR.N1 appeared to have made extremely good progress
at a constant setting on 2,700 rpm. However a slight swell was now apparent
which retarded the progress of the craft and on several occasions dropped her
back below hump speed. When this happened the pilot was forced to accelerate by
increasing rpm and altering to a westerly heading to ride above the hump, before
resuming the true course with minimum engine power.
In mid-Channel the wind
appeared to increase. This was especially noticeable by the drift of the craft
when well above the hump. This factor coupled with the larger swell, which
appeared to be travelling in the same direction as the wind, produced at times
the disconcerting fact of nosing-in with the port propulsion duct. It was at
this point that a small boat, over which the SR.N1 had the right of way,
appeared on a constant bearing. When it was obvious that a collision would be
imminent if both craft pursued their present courses, the pilot was forced to
alter heading in to wind to starboard. At the same time the small boat, which
either had not been keeping a good look-out or had been completely misled by the
drift of the SR.N1, altered course to port. With muttered oaths to mariners, the
pilot was forced to put the SR.N1 statically on the water and allow this small
boat to manoeuvre out of the awkward in which it found itself.
After this incident the
SR.N1 took an appreciable time to get above the hump speed: in all probability
due to the variable wind and moderate swell. Both Mr Chaplin and Mr Cockerell
tried moving their positions with little success. After proceeding for two miles
below hump speed, the craft crossed the swell of a large ship proceeding north
through the Straits of Dover. An appreciable amount of water was shipped on the
port side from this swell and both observers moved to starboard to compensate
the trim of the craft and avoid getting washed overboard.
After refuelling, the craft
was clear of the swell in the lee of St Margaret's Bay, and it proceeded at the
best speed it had obtained throughout the whole passage up and through the
entrance to Dover Harbour. It was estimated that the craft probably achieved
30kt in the last mad dash. The craft proceeded through the harbour and beached
adjacent to the Clock Tower some 2 hours and 3 minutes after getting underway at
Calais.
Breakthrough
Although the SR.N1 was the
first hovercraft to cross the English Channel successfully, it was plagued with
slow performance and the inability to traverse even very small waves easily,
with a hoverheight of only 9 inches (23cm). At first it had seemed as if the
peripheral jet would provide sufficient clearance height to allow a medium size
craft to negotiate coastal waters, at least, without employing more than one
half or one quarter of the power required by a conventional aircraft or
helicopter of similar capacity.
But in practice, the clearance
height was only one twentieth or one thirtieth of its beam. This meant that
craft 40ft (12 metres) wide and 80ft (24 metres) long would have a clearance
between the base of their hardstructure and the surface beneath of only 1 to 2
ft (30 – 60cm). Had this situation continued, the air-cushion vehicle would not
have advanced beyond the stage of an interesting aerodynamic phenomenon, but
with very limited practical application.
Further work by another
inventor, C H Latimer-Needham, on the flexible skirt produced the breakthrough
required to enable the craft to maintain a deep enough air cushion for the
negotiation of waves and obstacles. After reading about Cockerell's experiments,
he thought about the size of the waves that these craft would likely encounter
in the English Channel and the Atlantic, and was convinced that this clearly
called for some form of flexible skirt to contain the air cushion and enable
vessels to traverse significantly rougher surfaces. On contact with the
obstacle, the skirt would tend to collapse, but by reducing the peripheral
diameter at the base, either by built-in taper or curvature, there would be a
downward component of force tending to keep the skirt extended.
In October 1961,
Latimer-Needham sold his skirt patents to Westland, the parent company of
Saunders Roe Ltd, which had built the SR.N1. These earliest Westland skirts were
simply extensions of the inner and outer edges of the peripheral air ducts at
the base of the hardstructure, made in two sheets of rubberised fabric and
feeding air in to the cushion through the gap that separated the skirts at the
hemline. Air from the lift fan simply entered between the two walls of the
skirt, which then inflated, and was discharged in to the cushion at its base. As
the skirt concept was developed, easily replaceable 'fingers' or loops of
material were fastened at the hemline to reduce water drag and take the
wear.
The introduction of the skirt
was a vital engineering breakthrough. It meant that the total depth of the air
cushion beneath the solid structure was now equal to the depth of the skirt,
plus the daylight clearance or hovergap between the skirt hemline and the
ground. Engineers at Westland soon ascertained that, for a given power, the
obstacle clearance height was ten times greater. Apart from being
subjected to very considerable wear and tear, particularly at high speed over
water, it was felt the skirt would offer few operational problems. It would
deflect on coming in to contact with waves, rocks and jetties, and since
afterwards it would return promptly to its normal inflated shape, air leakage
would be minimal.
In 1962 the SR.N1 was fitted
with a Rolls Royce Viper jet engine for forward propulsion, and now made 50
knots with ease. Previously it had a piston-engined maximum of 35 knots. With a
4-foot (1.2 metre) skirt fitted around the perimeter of the craft, the craft
could cope with 6 to 7ft (~1.9 metres) waves, cross marshland with gullies up to
4 feet (1.2 metres) deep and traverse obstacles up to 3ft 6in (1 metre) high.
Moreover, the craft was now
operating at twice its original weight, with no increase in lift power. With
this new configuration, hovercraft developers around the world took note and
high performance craft started to appear.
The craft was gradually
modified over time, both in hull shape and by the addition of a propulsion air
intake 'shed' as well as cushion air bleed control ducts. Being of such
important historic interest, the SR.N1 is now in the care of the Science Museum
in London.
Next: commercial hovercraft services
start
*Hovercraft Museum
Trust – material used with permission
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