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Thread: CWM Hovercraft - school project!

  1. #1
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    Default CWM Hovercraft - school project!

    Hi, I'm new, and I've been recommended here by Griffon
    I'm 17, from the Wirral, and for my A2 design and technology project this year I'm building an 8x4ft dual engined craft for use on land and water. The hull will be made from fibreglass.

    The base is a sheet of marine plywood (1" thick). It'll be a jet ski seating arrangement, and I'm hoping for it to carry 3 people - but the smallest hovercraft i can find is around 10x6ft...

    The lift engine is a 125cc 15hp@9500rpm Vespa engine; and thrust comes from a Fuji Robin EC34 330cc 32hp@6500rpm with a 4:1 induction.

    I'm looking into skirt materials, and I'm thinking of making a bag skirt out of polyurethane material - I've also been considering making a catamaran setup, having 2 skirts, with 2 equal fans powering it - until you steer, when one will lose a little bit of its air supply to make the craft lean to the inside of the turn.

    My research so far involves making lots of model hovercraft, one remote control, and going to visit New Brighton RNLI hovercraft - a Griffon 470TD.

    I'm starting to collect parts for it now, and the deadline is mid March.
    The whole project is being funded by business sponsorship and donations from inividuals, and after only a few weeks I've got quite a sum to make the most of.
    I've got a team of motorbike engineers from a local college, and they're the source of the Vespa engine. They're also looking at getting the fabrication students involved to make engine mounts, and maybe propellers and impellers.

    So I'd like any ideas or advice put forward please - or criticisms of my design so far.
    Thanks!
    Calum
    cwmhovercraft@calday.wirral.sch.uk

  2. #2
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    Default Re: CWM Hovercraft - school project!

    Hi Calum

    Its a great project, sounds just like the one that got me started ... years ago. You need to aim for a cushion pressure of 10-15 lbs/sq ft. ie each square foot of craft area is lifting 10 - 15lbs of total weight. If its 8 feet by 4 feet thats 32 sq ft so your total lifting capability is 320lbs (480 lbs max) Three people alone will weigh more than that let alone your engines, hull, fans, engine/fan frame etc. Start by adding up the total estimated weight of everything including your payload and dividing by 10 - that will give you your area, then sketch out a craft slightly less than twice as long as it is wide that gives you your area. Don't forget to allow for any area lost by rounding off the corners.

    We don't use ply as thick as 1 inch, it's too heavy. Use 1/4 inch or less but build it in a box structure to get your strength that way.

    You haven't got enough power to for a craft that will take 3 people over hump on water, at a push it might carry 3 on a flat land surface like flat tarmac.

    Hopefully someone from the NW Branch will be along shortly to offer help.

    Keith

  3. #3
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    Default Re: CWM Hovercraft - school project!

    it's a heck of a task you're undertaking - keep it simple and keep the weight down 1" ply is far too heavy - a sandwich of thin ply between polystyrene sheet will be stiff and lightweight or a wooden frame with thin ply skin - weight will be your biggest challenge

    keep the skirt simple - any deflation of a skirt could cause problems even though the big griffons retract the skirt they have the knowhow and horsepower to be able to do that

    if you can get up to the university of central lancashire (UCLAN) in preston i'd be glad to show you my racing hovercraft to give you ideas

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    Default Re: CWM Hovercraft - school project!

    The other alternative to ply would be foam sheet as a core material, then use fibreglass cloth either side. Tricast 4 or tricast 5 is good and easy to work with.

    I would say a hull of 8foot x 4 foot is at least 50% too small. My racing hull is 10 x 6 and I wouldn't want to use it in open water with 3 people on board, as there would be very little saftey margin in the bouyancy.

    Go back to the drawing board as Keith advised and estimate total weight of the finished craft then add the 3 passengers thats around another 225 KG. From there you can start to work out how much lift pressure the craft will generate.

    I have two hovercraft & the smallest is 10' X 6' and that's a single seater (unfinished at the moment). The other is 16' X 7' and can carry 5 averaged size people. Most small hovercraft are at least 6 feet wide, so that would be a good place to start.

    Good luck with your project.

  5. #5
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    Default Re: CWM Hovercraft - school project!

    how much does the height of the skirt affect the lift? I've been thinking in volume, and was looking at having the skirt height around 30-45cm.
    So using your calculations, the minimum surface area of the craft would have to be 9x5ft to carry 450kg? It doesn't NEED to carry 3 people, it just seemed like the right size at a squeeze - I'm not too bothered if it only ends up carrying one.
    I've already got my sheet of ply, but its in layers - it can easily be peeled apart, and I'll look into the foam sandwich and fibreglass. I'm designing the hull on SolidWorks, and I'm looking at local companies to have it made professionally from a fibreglass company.
    And I can get up to preston; keep the offer open and I'll look into it

    I also need a backup source for the impeller in case the college can't make it, so I'm looking at air conditioning fans - the graphs only show up to around 1500rpm though, can anybody give me an idea on what sort to look for?

    Thanks for the help so far!

  6. #6
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    Default Re: CWM Hovercraft - school project!

    Realistically if you want to carry 3 people it needs to be 6-7 ft minimum wide by about 12-13ft long.
    If you're carrying 2 people it should be 6ft wide by 10-11ft long.

    Hoverheight should be 6-9 inches.

  7. #7
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    Default Re: CWM Hovercraft - school project!

    Wouldn't it be easier to build from commercially available plans, then alter the hull design above the skirt.

    At least you will end up with a craft that works and still be able to add your own design ideas to the less critical areas.

    "Just a thought"

  8. #8
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    Default Re: CWM Hovercraft - school project!

    another thought is to ditch the Vespa and go for a simpler integrated craft, with a nice big footprint 3m x 1.8m

    look at existing plans that you know work and adapt them to your own design

    good luck

    tony

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