View Full Version : Fibreglass Duct Creation

bitman - n/a
26-Feb-07, 08:30 AM
Hello from New Zealand

I am in the early stages of building a hovercraft.

I have a little of experience in hovercraft principles etc as my father used to own a cyclone, then a New Zealand built hovercraft called a cruisecat I believe it was.

I am planning on designing and building it from scratch.

Current plan is to stretch fibreglass over a ply skeleton with ply sheets being used where extra strength is needed.

I plan to use a 1.2m diameter multiwing fan driven by a subaru ea81/82(this motor fan combination is very common in New Zealand built craft).

Currently I am planning the craft to be 2m wide and 3 or perhaps 3.5m long (hardbody)

My first part of the project is the fan duct. I am thinking to wrap polystyrene sheets (styrofoam you may call it) around plywood discs as a duct form then 'machine' the polystyrene core into shape then glass oven it with epoxy and woven cloth.

Are my above idea's wise, what can other forum members suggest regarding my plans for hull/ duc construction.



Keith Oakley - n/a
26-Feb-07, 09:10 AM

Sounds like a good plan - you haven't mentioned lift. Are you planning an integrated design as per Cyclone using part of 1.2m fan for lift? In which case if you have under say 6-8 blades in the fan I may have concerns about the 'solidity' of the fan being able to generate the pressure you need. You may do better with a separate lift fan. I'd go for more length since this offers you lower cushion pressure. Lower pressure and longer hull will help it go over hump on water at lower power, reducing noise.

My own 25 year old subaru cruiser is 4 seat, 16feet (4.9m) long and 7feet (2.1m) wide but has a separate 25 bhp lift engine. Having developed a severe allergy to epoxy I can't use the stuff so make sure you use adequate ventilation when laying up. By 'stretching fibreglass over ply skeleton' I presume you mean using flat sheets of fibeglass curved over ply ribs? Just be careful the whole thing doesn't get too heavy. Most craft get their stuctural stiffness from use of monocoque box construction with fibreglass panels around 3mm thick rather than a 'chassis' approach.

Good luck


bitman - n/a
26-Feb-07, 04:51 PM
Sorry, Yes my oversight.

I am planning the craft to have be integrated lift/thrust with an adjustable splitter plate. In alot of ways the engine and fan arranghement will be very similar to the superwedge235.

Why a separate life engine Keith? alot of people have built the superwedge in New Zealand with the subaru 1.2m multiwing combo and seem to have plenty of lift?

I am not the expert though.



Keith Oakley - n/a
26-Feb-07, 08:00 PM
Integrated design (using one fan for lift and thrust) is fine but you have to make compromises between the fans role as a lift fan (more blades = better pressure) and a thrust fan (generally more efficient with fewer blades (eg some Uk racing craft using 3 blades for thrust only). The Superwedge 235 is a Southern Hemisphere design so I'm not the expert on that but looking at pics on the web (http://www.hovercraft.geek.nz/index.html) it looks a fairly conventional integrated design. Lazza's craft has a 950mm dia fan with 9 blades so no problem with lift pressure. Increasing it to 1200mm dia all I'd observe is keep the blade count up and you'll be fine.

Why not get in touch with Lazza in NZ?

Ian Brooks - n/a
26-Feb-07, 08:40 PM
There are two key reasons for considering a separate lift and thrust fan, performance and noise.

In a crowded island like the UK, noise is the defining issue affecting cruising hovercraft, causing the loss of several cruising sites over the years. We have a noise limit of 87dB(a) at 25m. It turns out that there is a relationship between noise and power, and it is likely to be difficult to get an integrated craft over about 35hp to meet the 87 dB limit - which is not to say that it can't be done, but it won't be easy. The good news is that the 1.2m fan is going in the right direction - bigger is better so far as noise is concerned, although the opposite is true so far as lift is concerned

Regarding performance, I just ran a few calcs for two craft of about the size you are considering, and compared the twin-fan single-engine configuation with the single-fan single-engine configuration, the numbers look like this:


"First lift" is the point at which you get the first signs that the craft is coming up; "design lift" is the point at which you have a fully developed cushion and enough lift air flow to operate on a reasonable surface.

What you can see is that the twin fan craft provides better thrust and better lift, but what is less obvious is the improved low-speed handling - notice that the single fan craft is not lifting until 2500rpm and is developing 135lbf of thrust before it shows the first signs of lift, whereas the twin fan lifts at 1550rpm and develops only 40lbf thrust. This translates to more docile handling and a more enjoyable craft...

You can design a simple twin fan drive from the subaru- the thrust fan drives from the back of the engine, the lift from the front - take a look at the SevTec designs for inspiration.

Anyway, it is possible to create a good craft either way, but I just thought I'd just give you a few things to think about as you begin the design process!!!



bitman - n/a
27-Feb-07, 03:42 AM
Hi, my experience with non integrated craft is nill, Tell me... Are the two motors operated by a common throttle? If not is this not difficult due to 'brake' (if you forgive the expresion) by throttling down thrust alone and droppiing the cushion. Pressummably a second set of controls involved?



bitman - n/a
27-Feb-07, 03:59 AM
Sorry I misread your post Ian, Yes my fathers old hovercraft was a subaru powered; 1 engine, 2 fans config (lift on front and thrust on rear)

Are 2 fan 2 engine crafts worth considering for a well performing craft?

Thanks Again

Ian Brooks - n/a
27-Feb-07, 07:41 PM
As it happens, the craft I have built is a two-engine two-fan type - they do have some disadvantages and some advantages.

The advantage is that you can control the lift independantly of the thrust; the disadvantage is having two engines, which is less reliable than one.

In terms of efficiency & noise, they are similar to the twin fan single engine type. In the end, it comes down to personal choice... you trade ease of operation with the single engine, for separate control of lift/thrust with the twin engine.


team black - n/a
27-Feb-07, 07:57 PM
As it happens, the craft I have built is a two-engine two-fan type -

So- given up on the prop have you http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif