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calumwm
19-Oct-10, 08:49 AM
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

Keith Oakley
19-Oct-10, 11:33 AM
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

broad5186
19-Oct-10, 09:38 PM
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

Al
20-Oct-10, 06:57 AM
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:eek:.

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.

calumwm
20-Oct-10, 11:12 AM
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 :D

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!

gavinparson
20-Oct-10, 03:35 PM
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.

Al
20-Oct-10, 07:02 PM
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":)

broad5186
20-Oct-10, 09:06 PM
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

drew1712
20-Oct-10, 10:46 PM
You could use the sheet of ply for the trailer.

gavinparson
21-Oct-10, 09:42 AM
If you try to re-invent the wheel, it probably won't work, you'll be disappointed and lose interest. Best to build something that WILL work and you'll have hours of fun.

broad5186
21-Oct-10, 11:44 AM
another thing to bear in mind Calum is even when we have hovercraft that work they still take some tweaking and understanding to get them to work properly, it's taken me 5-6 years of development to get my hovercraft working as it is

i agree with Gavin - stick to what works, don't change things just for the sake of changing them or being different

just my advice

calumwm
26-Jan-11, 08:16 PM
Hi again,
Just to update you all, I'm keeping the deck at 8x4ft, and the skirt surface area will expand to 10x6ft when inflated.
The Vespa engine has been scrapped, I've come across a Robin EC34 330cc engine - and got a 1.1m fan from K&M, plus I've had the engine mount and duct welded this weekend - we'll be assembling all of it tomorrow :D

A third of the air will be diverted through a duct at the back, directly into the skirt, using a PVC connecting piece. I'm also looking into variable splitter plates - what mechanism is used to control them from the cockpit? The plate isn't necessary, I just think it could be very useful.

I've also been looking into bodywork, fibreglass has proved too expensive, so I came up with the idea of using wooden ribs and struts with stretched fabric (probably PVC again) to give it a good look and slight rigidity? Has this been tried before? All space inside it would be filled with 2 part PU foam to help with buoyancy, so I'll also have to make it waterproof.

I'll also be picking up the skirt material tomorrow too - again, PVC, ~600gsm and 10x1.5m metres of it. As it will be a bag skirt, I'll get the whole thing shaped and connected to the deck in one piece.


Also, I've heard there is a yellow hovercraft on the Wirral, and it was seen near Hilbre Island last night? If you're on here, or know who it might be, get in touch please! :)

Calum

calumwm
29-Mar-11, 09:32 AM
The hovercraft is very nearly finished, I never got the skirt material but that WILL be done tomorrow!
The engine pylon, duct and fan are all in one unit waiting to be bolted to the deck and the steering is waiting for the same. Not long to go now!

Jon Pert
29-Mar-11, 10:26 AM
Is the fan guard infront or behind the fan?? Difficult to tell in the picture.

broad5186
29-Mar-11, 11:50 AM
good job so far - have you weighed it ?

tony

calumwm
29-Mar-11, 11:51 AM
there's a steel cage on the rear of the fan, but they'll be attached to the front too - covering up the engine and exposed fan area. Also looking into nylon net, as that's what the RNLI hovercraft used :)

calumwm
29-Mar-11, 12:02 PM
I haven't had a chance to weigh it, and I don't think I will - we haven't got the equipment in school. My best guess is probably at around 80kg-ish so far.

Also.. to save weight we're looking at starting the engine with a jump pack instead of keeping a battery on board, thoughts?

Jon Pert
29-Mar-11, 12:23 PM
as that's what the RNLI hovercraft used :)

Are you sure? Which craft was that? All the RNLI ones I have seen are either BBV or Griffon, with steel fan guards.

The Sevtec guys use net only as prop/fan guards, can't remember what the material is but it is very strong as you can imagine.

Al
29-Mar-11, 01:27 PM
My surveyor has steel mesh around the outside of the guard and fishing net in front of the guard to stop objects being sucked into the prop. I would be surprised if the netting could stop an exploding fan. I may slow it down a bit, but I wouldn't want to be around to find out.

broad5186
29-Mar-11, 09:04 PM
have a battery on a trolley and a plug to start it but have a small motorcycle or go kart engine battery on board just so you can start it if you accidentally pull out the lanyard

tony

calumwm
30-Mar-11, 11:31 AM
Are you sure? Which craft was that? All the RNLI ones I have seen are either BBV or Griffon, with steel fan guards.

The Sevtec guys use net only as prop/fan guards, can't remember what the material is but it is very strong as you can imagine.

It was a Griffon 470TD, I've had a look over some pictures and remembered there were also no rear guards whatsoever, and a nylon net over the front that can be removed - the duct is fibreglass, but I don't think it would survive a large metal blade going through it...

6110

Jon Pert
30-Mar-11, 11:43 AM
Most craft do not have rear guards. In fact the Hovpod is really the only one I can think of that does! I would imagine that a rear guard would be a bit of a noise generator.

On racing craft the flow straighteners and rudders are deemed sufficient to stop people falling into the prop/fan.

Al
30-Mar-11, 12:29 PM
I find people tend to keep away from the rear of my hover when it is running.