Hovercraft - the History and Development
The word "hovercraft" was probably coined by a newspaper, or even by Christopher Cockerell himself, to try to capture the essence of the vehicle. The more accurate modern description "air-cushioned vehicle" is more similar to the German Luftkissenboot.
As with any new idea it is fuelled by need, finance, and curiosity, and motivated often by the need to improve an existing product; the word development covers all this and more.
In view of Vosper Thorneycroft's involvement in military hovercraft in the 1960's, it is probably ironic that they were making use of an idea patented by Mr.Thorneycroft, a boat builder in the 1870's who suggested that by pumping air under a boat of a certain design it would help to lift the hull, reducing draft and thereby the drag of the vessel would be reduced. There is a patent to this concept taken out in 1910.
Later in the 1920's a Swede developed an improvement on the ice sledge by propelling it with an air propeller in such a way as to get ram airlift on his platform. Further development was not continued.
The hovercraft as we know it today has largely been credited to Christopher Cockerell, a lateral thinker of his day who, after developing crucial components for use on British radar during the war, retired to a boatyard in Norfolk, where in the interest of making a faster powerboat that would plane at lower speeds came up with a similar idea to that of Thorneycroft; what goes around comes around!!! He took the idea further by the skilful use of vectored air to make the whole vehicle hover, thereby removing the drag from any part of the hull which might have been in contact with the surface, be it land or water.
It is worth mentioning at this point that I do not think Cockerell ever intended his craft to be anything more than an amphibious marine vehicle. Where insurance underwriters and others get the idea that a hovercraft is some sort of low flying aeroplane says much about their lack of understanding of the concept.
At a similar time in the late 1950's a Doctor William Bertelsen in the USA was looking at alternative forms of transport in order to reach his patients when the roads were impassable. He continued to develop various hovering vehicles and roadways for hovercraft over the next 50 years. He is probably best known for the use of fans mounted on gimbals, but as with all things it is not necessarily the product, but the marketing of it, that makes the difference to its success or failure. In the case of Cockerell with his military contacts he was able to head up a British military development group, which with some time, reluctant government funding finally produced SRN1 which successfully crossed the Channel from England to France on the eleventh of June 1959, fifty years to the day after Bleriot did the same crossing in his aeroplane.
As with all things if there is the possibility of a military use money is found to further development from which civilian side-products are often developed.
The Russians built a fleet of very large military craft for use in the upper Baltic, particularly useful over the ice and snow, while the Americans were later to see the military advantages of hovercraft under the Reagan administration: when they did they invested heavily in both amphibious landing craft and rigid sidewall designs as the basis of their hundred miles an hour navy. But as with most things political, the military investment is usually in proportion to the perceived threat, and as the USSR was coming apart at the seams the investment was reduced. But the US Navy now has a fleet of vastly powerful air cushion landing craft, which, with their mother-ships, are now an integral part of the US Marine structure.
Some very successful craft that evolved from the military development work were used on civilian ferry designs, culminating in the large cross-Channel craft.
The small or light [a British government definition of under one tonne] hovercraft of today were originally built, some of them by people in the commercial industry, but mainly by people inspired by the concept. Some of the early craft can be seen at the Hovercraft Museum at Lee-on-Solent. As one would expect many of the designs were scaled down versions of the larger commercial types of the day.
Largely by virtue of the commercial industry being based around Southampton Water, the largest grouping of like-minded people was in that area and so the Hover [later Hovercraft] Club of Great Britain and the Hovercraft Society were formed.
At the Browndown hovercraft event in 1976, which took place on the 10th anniversary of the first Browndown event, Christopher Cockerell told me of the reluctance of his team to the idea of fitting "a ring of Mackintosh" around the SRN1 after its cross-Channel trip. In retrospect the comment is quite interesting because the use of a flexible seal or skirt was possibly the single most important step forward in making the hovercraft a practical transport vehicle, and secondly highlights the difficulty of working with a team of aircraft engineers trying to produce a marine product for military use.
As the British Isles are a relatively small area making interaction between individuals relatively easy, it is not surprising to me that craft produced by the home-builder or small company rapidly evolved and have continued to do so long after the larger military/ commercial designs appeared to consolidate their basic idea's. A lot of time was taken up in the early development of the larger commercial craft in the improvement and refinement of the skirt. Companies such as Hovercraft Development Ltd [HDL] were specifically involved in making skirts more efficient, which gave the craft improved handling. This work made use of and added to the skirt patents, which were held by the British government.
Some development on propulsion systems was also noticeable. Again, in my view, commercially the breakthrough came when the industry stopped trying to make hovering aircraft with aero-engines etc, and started to make air-cushioned boats, both amphibious and non-amphibious [rigid-sidewall]. The rigid-sidewall craft were made using Glass Reinforced Plastic [GRP] technology for use where a shallow draught boat was required, or where there was a lot of water debris, in harbours etc. As these craft were normally diesel-powered driving both lift fans and water props, it was a natural progression to build diesel-powered amphibious hovercraft - e.g. the AP 188.
This rather simplifies the case for the commercial development, but against the same period of 20 to 30 years the light hovercraft had gone from craft that would barely move to ones that could race in tight formation over land and water courses in groups up to 25 at the same time at speeds in excess of 50 miles an hour. On the light leisure/commercial side, craft are being used as viable forms of transport on a day-to-day basis, as well as for just plain fun. At the same time development on small craft often being built with limited resources has been innovative in squeezing the best results out of the often limited materials available. When they were then used competitively or in groups it was inevitable that the better concepts prevailed.
The original skirt designs were many and varied: examples ranged from brushes to folding semi-flexible material - even plywood was used; but the first skirt that we would recognise, was just flexible material attached to the outer edge of a craft, the lower edge of the material stopped from blowing away by being attached to the hull with ropes or chains. The air trying to escape from under the hull blew up the skirt into a "C" shape.
Naturally a complete bag with a top and bottom attached to the hull quickly evolved, but the method of inflation was done in two different ways; in one, tip air from the lift fan was fed directly into the bag with the majority of the air from the fan going directly into the plenum [the space beneath the craft]; this gave a very stiff bag but with problems if the bag became damaged, very similar to an air-lubricated inflatable dinghy. This system is still very popular in North America where it is perfectly adequate when used on long expanses of smooth water. The alternative is a bag into which all the air from the lift fan is fed, and the lubrication air is let into the plenum through holes cut in the skirt or through a gap in the hull. These two types are known as "no-flow" and "full-flow" bag respectively.
The bag skirt design was used initially on both the larger commercial craft and on some smaller designs, but particularly on the larger craft it was too bouncy and the drag on anything other than a smooth surface was unacceptable. The next evolution on the larger craft was and is the H.D.L. "loop and finger" or "bag and finger" type skirt - a series of convoluted pieces of open-ended material "fingers" attached to the bottom of the "C" or bag type skirt which became the standard design for the larger rough water use amphibious Hovercraft.
In contrast to the American craft the majority of early British racing craft used the full flow bag skirt, the stiffness of which could be adjusted to the driver's requirements by virtue of the outlet hole size. Experimentation with loop and segment designs was tried but lacked the necessary stability for high-speed use; experiments were also tried with straight segments and although having less drag than the bag skirt, stability was a problem. In racing terms this meant that the bag skirt was slower in a straight line but more stable in the corners.
The skirt that evolved was in effect, a segment cut through both the loop and finger Profile skirt to include one complete segment and the same width part of the loop: this is the basis of the segmented or finger skirt which, with various modifications, is the most commonly used skirt on all European craft up to maybe 20 feet in length.
Before leaving skirt design I should mention the design of Monsieur Bertone of France, whose conical "jupe" (skirt) was used on the ill-fated Sedam cross channel craft. To use this design [for which the British Government had no patent] a series of large individual material cones are arranged across the bottom of the craft and individually fed with air. A peripheral spray skirt is used obscuring the jupes from the casual observer. I can think of one craft where this spray curtain looks similar to segments. In many respects the previous comments with regard to skirt design are the visual changes, and as with a lot of things hovercraft, what you see is only part of the story.
Particularly on light hovercraft the hull designs have gone through many changes, the use of a bag skirt meant there was plenty of room for a boat-like hull with inspectable buoyancy, chambers often filled with ping-pong balls or plastic bottles. The hull, usually made of plywood [cheap but not good in the long term], could be made to float well and was easy to construct, and by this time planing surfaces which give a self-righting effect in the event of a plough-in had become a safety requirement of the HCGB.
Initially segments were fitted to the same cross-sectional bag skirt design of hull but usually with a more rounded bow shape. The lift air was pumped directly into the plenum through a duct, usually positioned on the front of the craft and in front of the driver: but unlike bag-skirted craft the hull needed to be correctly balanced in order to lift up squarely, otherwise the craft would have a tendency to sit tail down with all the lift air escaping from under the front fingers. This situation was alleviated by the fitting of some internal ducting to the rear fingers, or a small bag that jacked up the rear of the hovercraft allowing the plenum air to flow backwards. There were still drawbacks in the system such as the lift fan being vulnerable in the event of the craft ploughing-in at speed and water hydraulicking up the lift duct; also from stones flicked up and into the fan by the front fingers.
The next step in hull evolution, which had many spin-off ramifications, was the full-flow hull system in which all of the lift air was fed into a chamber running around the entire periphery with a dedicated hole located just under the top of the top skirt attachment of each segment. The advantage in lift air efficiency is best quantified on two similar sized craft, one of which will adequately use 20 horsepower for lift when fitted with a segmented skirt, whereas a similar craft with a loop segment skirt can use considerably more horsepower. The reason takes us right back to Sir Christopher's original peripheral air jet system. As the outer face of the skirt is angled inwards at approximately 45 degrees the air is jetted down each segment from the feed hole in the hull, creating an air curtain extending beyond the edge of the skirt, thereby reducing the skirt friction, but probably more importantly, as the air is being jetted inwards under the craft it significantly reduces the air loss in comparison with the other system. To the casual observer the main difference is the reduction in spray from craft fitted with this type of skirt.
This type of hull which also acts as a duct, or air box, means that the location of the lift in the system is less critical and therefore it lends itself to a single engine / single fan [integrated] concept which enables small fun craft to be made more cheaply; in fact so successful was this concept that for many years this design was the basis of the two lower European racing formulae.
LIFT & THRUST
I have indirectly mentioned lift systems; there are a few basic guidelines to consider. The maximum height at which the craft hovers from the surface is approximately one eighth of its width (any more than this will affect the stability of the craft) and the larger the footprint of the craft relative to its weight the better, a design weight of between 10 and 15 lb per square foot will enable the use of an axial fan.
The axial fan is very good at moving a quantity of air but not so good at generating pressure, therefore the airflow efficiency will drop away very quickly with only a small increase in back pressure. If a craft is undersized or overloaded and the skirt pressure is too high the skirt stiffness will cause excessive drag and wear as well as making it difficult getting the craft over hump [up on plane]. A bigger fan or more a powerful engine will not overcome an inherently bad design.
It should be mentioned that as craft get bigger e.g. 25 feet or more in length, a higher skirt pressure in combination with a skirt geometry giving a more flexible performance e.g. a loop and segment type, will usually use the centrifugal fan, because:
A. The overall craft footprint is bigger and the depression in the water when the craft is at sub hump speeds is proportionately shallower than that of smaller craft.
B. The increase in craft size brings with it its own stability.
C. The increase in skirt pressure will of course be at the cost of increased friction, spray, noise, and fuel consumption, but have the advantage of a greater payload for a given size of craft.
The choice of a ducted axial fan will depend on a combination of factors; the h.p., available flow required etc., but as a basic guideline for any axial fan the smaller it is and the more blades it has the more pressure it can produce - and the more noise! The reverse is true of both lift and thrust fans, the bigger diameter they are with the least number of blades, the more efficient is the air flow and again for a given h.p., that reduces noise.
Of course this is an over simplification, but it helps to explain why as integrated thrust / lift fans get bigger the lift efficiency will drop away, requiring either very fast rotating fans or a lot more blades: both inefficient and noisy solutions.
As with all things, simplicity of design cost etc., will play a part in the designer's options but I would suggest that any designer doing their job properly should start with what they perceive as the most efficient package.
The readily available range of powerful two-stroke engines have meant that even the crudest designs will work, but an efficient design will enable less suitable [from the power to weight of respective] engines to be considered e.g. automotive four-stroke petrol or diesel. The compromise has to suit the use to which a craft is being used; for example operators using two-stroke engines on commercial operations do so because for a given sized hovercraft the payload is increased in comparison with a similar craft using heavier engines. On the other hand for people building craft of their own design or from a kit, the use of a suitable second-hand automotive engine can considerably reduce the building cost.
With regard to thrust systems I have mentioned ducted fans: why ducted? Primarily for safety reasons when fitted with a 50 mm square metal mesh guarding the front of the unit and rudders, elevators and possibly vectors at the rear, the thrust system should be capable of containing any broken blades, belts etc., as well as preventing any human ingress. It is true to say that for a given diameter the ducted propulsion system is more efficient in thrust terms than an open-bladed unit, but it is noisier than, for example, a broad-bladed, slow running paddle propeller. Noise is energy, therefore the more power the unit has to absorb for a given diameter, the more noise it will generate and the amount of thrust per horse power will reduce. It might be necessary to use more blades to absorb extra power, as with the lift system the use of more blades will increase pressure and reduce thrust. But within certain limits some pressure is not a bad thing as it makes control surfaces [rudders etc] more effective. In conclusion, the compromise if available, is to fit the minimum number of efficient profile blades travelling at an adequate speed to absorb the power available.
After forty plus years of almost continuous development the ability to build from scratch a successful hovercraft still relies on common sense, to the extent that a successful hovercraft is the sum of its parts. A good hull, skirt or power unit will only be effective if they are connected to other components that can make use of each other's good qualities. Air density, wave characteristics etc., are constant, as are the basic physics with which we are trying to work: over the years we have bent the rules often by the use of complex skirt designs or by the use of lightweight engines etc., but there is always a trade-off - a price to pay, which in other operating conditions might make the craft unacceptable. So as my final observation, I would say, make a list of priorities for your hovercraft - define its operating conditions, payload, performance, convenience and cost.
Hopefully my comments will enable you, the reader, to make a balanced decision with regard to the compromises necessary to own the ideal hovercraft to satisfy your particular operating criteria.
Happy, Safe, Successful, Hovering.
Bill Baker Vehicles Ltd, 2001.
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