www.Poundswick .org.uk

The Poundswick Hovercraft


 
In 1967 a new school society called the Technical Activities Group was set up under the guidance of Mr. K. Moorby (Metalwork) and Mr. M. F. Russell (Physics) to cater for pupils who wanted to learn more about how the knowledge of the scientist is combined with the applications of the technologist.

The group's first (very ambitious) project was to build a two-seat hovercraft which was entered into a national competition sponsored by the Daily Express and B.P. It was 18 feet long, weighed over 1000lb and was powered by three 197cc Villiers two-stroke engines. It was all home-built, including the propellers which were hand-carved by a particularly talented pupil.

In April 1970 the following article appeared in the School Council Publication Project Technology. It describes the hovercraft in some technical detail and I have reproduced the text and photographs in full.

Poundswick High School's
Hovercraft

In this candid and unpretentious account K. Moorby, Head of Technical Studies
and M. Russell, Head of Physics, describe how one school developed
a small hovercraft. The considerable success which their hovercraft achieved
in competition with established designs supports the view that at present,
in this field, design theory is struggling hard to keep up with practice.

 
The opportunity of developing a hovercraft as a project for the upper school Technical Activities Group was enthusiastically welcomed by the boys. The team consisted of twelve boys under the supervision of Mr. K. Moorby and Mr. M. Russell.
The terms of reference were simple - to build a craft capable of travelling at 30 m.p.h. over land and water and capable of carrying two adults, yet safe enough for operation by the boys themselves. The total available funds amounted to 50 and knowledge was limited to the basic principles of hovercraft operation.

Since the ages of the pupils involved in this project were, at the time of building, only 13 -15 it was felt that they could not be expected to appreciate the more difficult mathematics of air flow calculations and the design of fans and propellers. In consequence, our mathematical treatment was limited to four approaches. These were:

1. Consideration of craft area in relation to the expected cushion pressure and the overall weight of the craft.

2. Consideration of the top speed of propellers of different sizes operating at various rev/min.

3. Simple moment calculations which assisted in the balancing of the craft.

4. Calculations to estimate the buoyancy of the craft.

The design was planned in stages which permitted modification at various stages of the construction, in the light of experimental trials. This technique appealed to our younger boys and provided a suitable link between experimental science and technology.

Our initial design was based on the assumption that an adequate lift can be obtained at a pressure of 10lb/sq.ft. and that the width of the craft had to be no greater than 6ft. 3in. so that it could be removed from the building. This restricted the shape of the hull to an unconventional 6ft. 3in by 14ft. Weight was kept to an absolute minimum except where safety demanded additional strength.

Hull
This is built from thin ply and 1in. square strip wood. Strength is obtained by using a sandwich technique with plywood and polystyrene, making a base one inch thick. This quite easily supports the direct load of two adults. Two lateral members run the length of the hull at 3ft. 6in. centres. The depth of these is only twelve inches but the deflection observed when loaded above the calculated maximum is very small. The bows are designed to give an uplift when travelling through water.

Lift Unit
This uses a multibladed fan twenty-four inches in diameter driven by a Villiers 197cc engine. This drives the fan at upwards of 4000 r.p.m. and very rigid mountings are essential. The whole unit is made independent of the hull so that accurate positioning can be undertaken on an experimental basis. This method has proved that there is only one suitable position for the lift unit which gives an even distribution of air under the craft. A further advantage of this "bolt-in" unit is the ease with which maintenance and modifications can be carried out if required.

Thrust engines and mountings
This is again of robust construction capable of supporting the the weight and resisting the torque of two 197cc Villiers engines. This unit is constructed from one-inch diameter tubing welded together in the shape of two "A" frames, cross-braced and bolted between the lateral hull members.

Propellers
These are hand-carved from hardwood and are thirty inches in diameter. They are carefully balanced and secured to a steel boss by four high-tensile steel bolts which are wired together for additional safety.


David Howe (standing) and
John Sharp prepare for lift-off.

Cockpit
This provides accommodation for two adults sitting side-by-side. As the money available for this project was extremely limited, the comfort afforded to the operators is negligible.

Controls
There is a central control lever for all three throttle cables, a rudder control lever and a main switch for the fuel supply. All engines have electric self-starts and ignition warning lights are fitted which serve to give a visual indication in the event of engine failure.

Skirt
This is a "C" type skirt of heavy-duty rubberised canvas giving a lift of about four-and-a-half inches when fully loaded. The shape of the skirt is restricted by the use of strong rubber straps secured to the underside of the hull.


Sketch showing section through lift duct.

Safety
Sheet steel is fitted between the cockpit and the lift unit. There is adequate guarding of the two propellers. The guards and the thrust engine mountings are capable of supporting the craft in an upturned position in the unlikely event of the craft rolling.

THE CONSTRUCTION OF THE CRAFT
Work was done outside the corriculum, usually during lunchtime and after school. It soon became apparent that a craft of this size could not be built in the existing workshops. Thus it was that a strange object began to take the shape of a hovercraft behind the closed curtains of the school stage. There, under the hot stage lighting, the boys solved one difficulty after another.

PRELIMINARY TRIALS AND EXPERIMENTS
Outdoor trials were held frequently throughout the construction of the craft and contributed greatly to its successful completion. These trials were observed with much amusement, and in some cases scepticism, by many interested pupils. From these trials we were able to determine the exact position of the lift unit, thrust unit and cockpit. The balancing of the craft had been so carefully maintained that it would operate to a limited degree without a skirt of any description.

Experiments were carried out using manometers to measure air cushion pressures under various conditions. Stroboscope techniques were used to determine fan and propeller speeds. In addition an investigation of the water absorbed by various buoyancy materials was carried out in order to determine the most suitable material on a cost versus effciciency basis.

THE FIRST PUBLIC APPEARANCE
Preliminary trials over, the final finishing touches were quickly undertaken and the craft was ready for its first public run. One of the senior boys who had worked on the craft was given the honour of demonstrating its prowess.

With a deep roar, the lift engine easily lifted the gross weight of half a ton and then, very smoothly, the power of the two 197cc thrust engines sped the craft across the school fields, to the accompanying applause of the many friends and parents gathered to watch the maiden run.

Suddenly disaster loomed up in the shape of a goal post directly in the path of the oncoming craft. Safety drill had been carried out in simulated conditions and it was with confidence that we waited for the craft to slide to a halt, the lift engine having been cut. It was not to be. The craft kept on a collision course and the inevitable happened. With a crash, the work of months wrapped itself round a very strong 4in. x 3in. upright and pieces of hovercraft flew off in all directions.

A rush to the scene confirmed the worst. The goalpost had carved into the bows for a depth of 3ft. 6in. and had reached the cockpit. The occupants had escaped with minor shock. With a wry smile the driver said "It's not so bad sir, we'll soon fix it". And they did. Back to the school it was carried, repair work commenced almost immediately. What drives twelve boys, two members of staff and several parents to work until midnight rebuilding a severely damaged hull? Such was their enthusiasm that by the next day it was clear that the craft would be ready to compete in the Daily Express Air Rider competition at Harlow only forty-eight hours after the crash.

SHOWS AND DEMONSTRATIONS
As an unofficial entry in the Daily Express Air Rider competition it was a considerable surprise to find that the craft was, in every respect, superior in performance to the Air Riders present, beating the winner's time for the course by a clear ten minutes. The care and effort expended by the team had been adequately compensated. We knew our machine had fulfilled all our expectations.

After this success the reliability and the appearance of the craft were improved and we have given many demonstrations at various schools and shows. The pilots became very proficient and could manoeuvre the craft easily and safely.

The craft was recently demonstrated before the Manchester Regional Group of Project Technology, under rather unusual conditions - 8pm on a November night involved obvious difficulties. Headlights were fitted to the craft and goalposts indicated by marker beacons. This is possibly one of the few occasions that a light hovercraft has performed during the hours of darkness.

FUTURE DEVELOPMENT
It is proposed to carry out research into improved lift, through a modified skirt and an increase in lift engine capacity. In addition it should be possible to improve our propeller design and the craft still requires intensive tests over water courses.

Since some of the boys involved in the construction are now 17 years old, it is probable that we shall carry out more involved experiments to test the correlation of theory with practice in the design of light hovercraft. This is a subject offering much scope and, judging from the arguments in amateur hovercraft circles, the answers are by no means clear!

 
If any Old Poundswickians who worked on the hovercraft would like to add their recollections of the project to this page, please get in touch.