HOVERCRAFT

& THE HOVERCRAFT MUSEUM

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A few photographs from the show.

Little and large - the Isle of Wight shopping trip leaves Lee on Solent ..... with company...

Full sized craft, light hovercraft, models and thousands of enthusiasts....

the sea was too rough for many of the smaller craft to operate comfortably although it didn't stop some pilots from having a go,

The shoppers return......

and AP1-88 'Freedom 90' turns a bit too slowly during the fly past and kicks up spray....

before making a spectacular entry to the ramp - it takes skill to operate a large craft in a strong crosswind.....

while the grand old lady - the twin prop SRN6 -  showed that despite being the oldest working hovercraft, she is very much alive and well at the Museum.

Small craft displayed in the arena - here is the ex Heathrow airport rescue craft...

while all the other full sized craft were open for inspection.

 

 

 

Some pictures taken at the Hovercraft Museum in October 2003 - well worth a visit but phone first for details.

See web link at the bottom of this page.

FARVIS

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The first full size hovercraft - SRN1  - a model at the Hovercraft Museum

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A model of an SRN2- this craft operated a ferry service in the Bristol Channel  in the early 1960's. The SRN3 was a much bigger version of this craft.

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Model of an SRN6

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Two examples of the 'Winchester' class SRN6 - a longer version of the 'Warden' Class Military SRN5 The Canadian Coastguard use the SRN6 as a rescue vehicle as it can operate on water, land, mud and ice.

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An SRN4 - these 'stretched' craft are huge!

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Military hovercraft models.

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A military hovercraft  'CC5' as used by the Marines

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Hovercraft 'Swift' a car carrying SRN4 at the Museum prior to demolition - and a Radio Control model of an SRN6

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Hovercraft SRN4 'Princess Margaret' a cross-channel car ferry, framed by the bow doors of 'Swift'

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Something you won't see very often - the flight deck of a big hovercraft - in this case 'Swift'.

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The SRN4 was huge!

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A Rolls Royce 'Gnome' engine used to power SRN5 & 6 craft - a lot of power in a small package.

This engine is also used in many helicopters. It used a lot of jet fuel and modern craft are powered by diesel engines.

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How about this for an early skirtless 'one man' hovercraft ........ speed in excess of 30 mph - hover height about 2"

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HOVERCRAFT

BASIC PRINCIPLES

Hovering reduces drag and friction between the bottom of the hovercraft and the ground or water.  Air is blown into a chamber (plenum chamber ) under the craft by the lift fan and this air is held in place with a flexible skirt that acts as a seal.  The air lifts the craft off the ground and acts as a lubricant so that a craft can travel faster with a smaller engine. Leakage from the base of the skirt forms a  small air gap so that there is very little friction between the craft and the ground,  although in practice the edge of the skirt is usually in contact with the surface.    A boat has to push water aside to move, but a hovercraft skims over the surface and doesn't have to  waste energy in moving huge volumes of water.   A hovercraft exerts the same downward force on the surface as a seagull standing on one foot.

At low speed the craft will travel in a cloud of spray as it literally blows a hole in the water, but as it speeds up, it climbs out of the depression - known as 'going over hump' and then begins to skim.  Once skimming the power required both for the air cushion (lift) and propulsion ( thrust ) is much reduced and the craft can go  faster for a given engine output. Generally hovercraft work better as they get bigger as surface area squares in comparison to the circumference of the hull where the air leaks out - thus the bigger you get, the more efficient it is.  Although it may seem that the lift engine uses a lot of power, in comparison with the advantages of speed and reduced friction, it doesn't.

The skirt acts as a seal to trap the air and hold it under the craft but  also enables the 'hard structure' of the craft to hover at greater height and thus avoid obstructions - being flexible the skirt deforms to match the shape of the ground. Additionally although hovercraft are 'bumpy', the skirt absorbs much of the wave movement and they are generally smoother than a ship. 

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A  model hovercraft.

Note how the craft is skimming over the water and not pushing through it.

A conventional boat would require much more power to run at hovercraft speeds and would be less stable. Another important factor is the wash - the model shown  weighs about 1.5 KGs is travelling at over 20 miles per hour - and there are just a few ripples.  At this speed a boat would create a huge wake. This is a major environmental benefit for full sized craft. Although not able to operate in extreme conditions they are very reliable. The hovercraft is one of the safest forms of passenger travel ever made.

Large craft have a lift fan dedicated to blowing air into the chamber under the hull and also into the skirt, and one or more fans or propellers for thrust.  In some cases full size craft also use a single set of fans for lift and thrust ( like the CC5 above ) and use ducted air for propulsion jets.   Others use one engine to drive lift and thrust systems through gearboxes. Bigger craft have multiple engines and drive systems.

Steering is achieved using rudders, swivelling propellers with variable and reverse pitch blades, moveable outlet ducts which literally blow the craft sideways or backwards and also by altering the balance of the craft and moving the skirt to adjust the centre of gravity and the centre of lift.

Another factor that has been essential in the success of hovercraft is the ability to come out of the water to load and unload.   Being amphibious, there is no need for complex docking and mooring required by a ship.  The craft can operate from any flat area and passengers and cars merely drive on and off using built in ramps.  Whereas a ship needs jetties and moving ramps to deal with tidal conditions, a hovercraft just drives ashore, drops the ramp and loads the cargo.  This means that you can get a lot more work out of a hovercraft - and a concrete pad is much cheaper to build and maintain than a harbour!

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A 'chip tray' sized radio control 'integrated' model hovercraft.

Small craft use a different system that enables them to hover and move with only one engine. The fan is placed at the back and pushes the craft along, but  some of the air  - usually about 1/3 of the total - is diverted into the hull to make it hover.  These craft must move forward to hover and can't operate in a static position on water, but a skilled driver can balance surface friction (drag ) against drive (thrust) on land and inflate the skirt without moving forward. This is known as an 'integrated' craft.   The model above works well and is very small. Most smaller 'one man' craft are of this design. Small hovercraft are steered by a rudder and the driver leans the craft like a bike to increase the drag on one side and help it turn.

Driving a hovercraft is like nothing else.   The nearest you can imagine is a tea tray on an ice rink.  The craft will travel as easily sideways as forwards and will slide away on any slope or in a wind.   You  have to use the rudders and thrust of the engine to point the craft in the correct direction and then to push it where you want to go using the engines while taking any ground conditions, slope and wind speed into account.  Turning is done by combining power,  steering and balance. 

Stopping can be done by gradually reducing power or in an emergency a small craft can be stopped by spinning round  and opening up the engines while travelling backwards.  Bigger craft have reverse thrust, small ones don't!  'Dumping' the lift is not an option as the craft will stop dead when it hits the ground and do a lot of damage to the passengers and cargo.  It might also rip the skirt off and smash in the hull! Almost all craft  are designed to survive a loss of lift on water, but it is not a pleasant experience and is like hitting a wall at full speed. In an emergency, such as an unexpected sandbank or gully, the driver has to open up the lift engine to give more cushion air to ride over the obstruction.  This is against all natural instinct which is to shut down and stop!  This makes for some interesting moments until the controls are mastered!

Craft are all designed to float and they can stop on water and take off again.  They have landing pads to alight on hard ground without risk of hull damage. However the skirt will wear rapidly on dry surfaces and operating on concrete or tarmac is not suitable for long periods. All craft have buoyancy tanks to keep them afloat and can operate as an air propelled boat if the lift system fails - although they won't be very fast!

The design of the craft makes them extremely stable and despite a rather strange way of driving, they are easy to operate and very safe. They are also great fun to drive.

FARVIS

c Farvis Ltd  2009

Illustrations and text may be copied for educational purposes ( and homework ) only.

Visit the Hovercraft Museum Website   www.hovercraft-museum.org

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