View Full Version : Solid Skirt

loud'n'proud - n/a
28-Apr-08, 12:47 AM
Hey all,

I'm new here but have some questions when it comes to designing hovercrafts..

Has anybody seen or looked into using a stiff skirt rather then a rubber flexable skirt?

a stiff skirt that has a decent about of flex in it, like stiffer light weight rubber or something.

Will hold you off the water(would be made to be extra boyant)/land making it quicker and easier to presurise the plenum to take off.

Has anyone heard of anyone doing this sort of thing?

profqwerty - n/a
28-Apr-08, 08:19 AM
That packaging Dell send out with new computers (sorry I've been around it too much http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif ) might be good. It's sort of closed cell plastic foam (possibly PE...? it has a waxy feel).

In a sheet it's stiff and can support itself easily, but folds if enough pressure is put on it then springs back to its old shape.

Dunno if that helps though http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif


Iceman - n/a
28-Apr-08, 03:14 PM
light weight rubber

Is there such a thing?!

I think a stiff skirt is ok if you are operating over a consistently flat and smooth surface but for anything with a slight lump or bump will cause a sudden increase in drag and cause very peculiar handling characteristics. For 'normal' operating environments the skirt needs to be compliant so that any obstacles and/or uneven terrain can be absorbed by the skirt and not be transmitted through a stiff skirt into the hull.

Think of an F1 car with stiff suspension driving through a field!

As for quicker lifting off times, most craft are up and moving in a few seconds so I'm not sure there is any need for trying to improve this?

sixpackpert - n/a
28-Apr-08, 05:03 PM
There was a craft at Whittlesey last year with a solid skirt in FS, I will try and find a picture of it later. It was a French or Belgian craft from memory.

Went like a rocket but looked uncomfortable as hell!! http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

loud'n'proud - n/a
29-Apr-08, 12:17 AM
kinda hard to describe.. and cant say too much since its all hush hush lol (i work for an R&D company in Australia)

but the idea of a light weight yet rather stiff skirt has been bounced around abit and we are looking into the idea.

on prototyping with our stiff skirt against the rubber skirt, the stiff skirt floated alot better,

with rubber if air escaped out one corner, the air would lift that corner up more and then all the air would escape from 1 point rather then evenly around. on the stiff skirt we found that 4% of the skirt would touch the ground (slightly over balenced to 1 point) and the entire rest would be completely off the ground as the skirt holds its shape.

our full size hovercraft will be 10m x 4m so its going to have the weight to plow through alot of things and the skirt will be long enough and have enough flex to go over rather large bumps. the maths works out that the hull will be 80cm above the deck

Ian Brooks - n/a
29-Apr-08, 07:23 AM

Skirt design has evolved since the '60s to become more compliant (softer) for two main reasons:

1 - wave drag. A firm skirt interracts with wave systems to cause very high levels of skirt drag. This is particularly important when taking off from a standing start in water ('going over hump'), where the hump wave interaction ('scooping') holds the craft back and prevents it from taking off.

2 - choppy water. When operating in choppy water, wave drag increases therefor reducing the maximum speed. A compliant skirt is used to reduce this effect by allowing the waves to roll through the cushion with minimum drag, and a stiff skirt could make it worse.

In an ideal world, the skirt would move up & down freely, to enable it to accomodate lumps, bumps and waves, but would not move laterally at all. This gives rise to large bulbous nose seen on the SRN series of craft (prevent plough-in) and the large side bags seen on SevTec craft.

Which is not to say that your new concept cannot solve these issues and open up a whole new avenue of craft development - good luck! I would recommend Theory and Design of Air Cushion Craft by Yun and Bliault as one of the few up to date texts on the subject



loud'n'proud - n/a
29-Apr-08, 07:30 AM
this is what we have learnt yes, though i will track the info down..

our main reason for building a hovercraft is not to redesign the wheel... we have areas developing new propellors and fans and this is a way to test out the different designs and also because who doesn't want a hovercraft to mess about on.. i mean seriously lol...

we have an single engine E2 3 seater we are learning from atm to find the faults and design errors.

whilst we are not redesigning it, out inventor owner has a few ideas that he would like us to look at and maybe change, like different skirt designs and plenum designs.

i mean we are building this from scratch so we might as well see if we can improve as we go... if not then so be it but we are exploring all angles

Keith Oakley - n/a
29-Apr-08, 07:50 AM
As David and Ian have said it will depend very much on the surface you're trying to operate over. On a totally flat surface such as ice, calm water, tarmac etc it would be fine but on undulating ground the skirt would alternately ground, causing drag, or be high in the air causing air loss which ultimately would need more lift power to overcome. A particular example would be a transition from water up a steep slipway.

Over water if the skirt is hard enough it could be wave piercing which could have advantages but not if the skirt is presented sideways on to the wave where it could cause high drag. Thus travelling sideways could cause the skirt to be snatched by a wave and with the crafts momentum higher up it could cause a rollover. Similarly the wide straight bow on most craft would present high drag to waves and a far more pointed shape is needed for a wave piercer - eg that seen on the UH18

loud'n'proud - n/a
29-Apr-08, 08:03 AM
the craft is not for crossing the English channel as well.. we dont have one...

it will be used in open seas but only coast running never on huge swell.

profqwerty - n/a
29-Apr-08, 08:11 AM
But the coast running would tend to have more small waves (wavelets...?) as opposed to large swell.

If they are small small, then they'd just splash onto the stiff skirt, but a bit bigger and they'll start to ascert a significant force against the skirt's movement.

What you could do - hinge the skirt into 2 bits, so the bottom 1/3 or w/e can easily fold upwards, so you maintain the structural strength of the upper bit to make it float, and have the very flexy bottom for waves etc, but larger obstacle can make the top bit flex too.

Right I've copyrighted that idea http://hovercraft.org.uk/images/icons/smiley_icons/icon_twisted.gif

Good luck though http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif !

Keith Oakley - n/a
29-Apr-08, 08:41 AM
Thats a great description of a bag and finger skirt as used on most of the larger craft (eg > 1 ton), and on Rachel Giffords 201 F35

profqwerty - n/a
29-Apr-08, 08:42 AM
Yep - I realised just as I posted it http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif http://hovercraft.org.uk/images/icons/smiley_icons/icon_rolleyes.gif

loud'n'proud - n/a
29-Apr-08, 11:42 PM
lol yeah ive gone through all the general skirt types

http://www.australianhovercraft.com/design_skirts.htm (www.australianhovercraft.com/design_skirts.htm)

and we have been looking into having a similar idea, it would be more like the segmented skirt then the bad and finger skirt.

As for the waves we wouldnt be going out in anything more then 1/2m swell and as the hull will be 800mm off the deck most of the wave will be able to force the skirt up and slide under whilst the rest will flow around it.

as for that book.. its on order

team black - n/a
30-Apr-08, 07:35 AM
800mm is a lot of hoverheight for a 4m wide craft, then general guide is for a maximum of 1/8 the width ie. about 500mm

loud'n'proud - n/a
30-Apr-08, 07:42 AM
well we are actually shrinking the design down to 2.5m max so it can be towed legally without an escort so the skirt height will change anyway.

Keith Oakley - n/a
30-Apr-08, 07:55 AM
Usual solution to the 2.5m wide towing limit is to fold the sides. Normal length to width ratio is a little under 2:1 so a 10m long craft would be 5-6m wide.

loud'n'proud - n/a
30-Apr-08, 11:51 PM
fold the sides? the hull is fabricated as 1 piece, unless you design it to fold but then your just making it more complicated then it needs to be

yes the overall length has been reduced to 8m aswell

jon_curtis - n/a
2-May-08, 04:56 PM
the width you can tow depends on the towing vehicle, 2.3m is the max with a vehicle up to 3500kg, 2.55m with a vehicle over 3500kg.

so if this is something you expecting to pull with a car, the limit is 2.3m

Keith Oakley - n/a
2-May-08, 05:37 PM
maybe the roads are wider in oz!

loud'n'proud - n/a
4-May-08, 11:45 PM
lol the max width for any road going 'thing' trailer, car, truck is 2.5m, over that you need wide load tags and an escort.

I used to design semi trailers and custom earth moving equipment for years, we know the ADR's back-to-front.

kach22i - n/a
7-May-08, 05:45 PM
I've spent some time tinkering around with the same concept.

If the foam is soft enough to deform reasonably it also tears easily and does not wear well at all.

The inclination is to protect the delicate foam by either coating it with rubber or a thin skirt fabric. Which gives you the worst of both material characteristics, and not complementary (in my opinion).

One step I took was to try using a very heavy skirt fabric and learn how this new type of skirt might behave. I'll admit that it's a stretch to claim that a lightweight foam and skin together is exactly equal to a heavy skirt material, but I think if you take 1+1=2 or 2x1=2 the math is about the same.

A heavy skirt is a disaster on a small craft, I would think scaling it up would be just more of the same X2. A foam skirt would "act" heavy, if you don't believe me pinch some foam and time the reaction time.

As a sidebar, the US military (Bell Aerospace of Buffalo NY) in the early 1970's came up with several air cushion landing systems, one involved moving air though a cellular soft foam. Perhaps it's this combination which holds some if any promise.


General info..................

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1979001 (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790017874_1979017874.pdf) 7874_1979017874.pdf

More info (porous high loss filler) - see pages 31 now called 44 of 189, and page 6 now called 21..............

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/1982000 (http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19820004192_1982004192.pdf) 4192_1982004192.pdf

kach22i - n/a
7-May-08, 06:08 PM
but the idea of a light weight yet rather stiff skirt has been bounced around abit and we are looking into the idea.

I bet it BOUNCES. http://hovercraft.org.uk/images/icons/smiley_icons/icon_lol.gif

There were/are several inner-tube high pressure (stiff) inflated rubber raft like skirts which have been tried on hovercraft. They have been high drag on water and high drag on anything but perfectly flat land.

loud'n'proud - n/a
7-May-08, 11:51 PM
we have our own coating that will protect the foam from wear so that isn't a problem. and still have all the same flexable traits as uncoated.

On weight our skirt actually weighs less then a rubber one of the same size.

anyway if you study some forms of maths 1+1=1 lol...

we are very much interested in any ones comments here its all part of the research so keep them coming!

profqwerty - n/a
8-May-08, 08:55 AM
What about a giant block of sponge, maybe with a single layer of something durable on the bottom. Put lots of holes through it and pump air through them. There'll be a thin layer of air underneath the block keeping the craft afloat. Probbaly a lot of friction though.

loud'n'proud - n/a
8-May-08, 11:44 PM
na, wouldnt work over any rough terrain.. would just bottom out.

kach22i - n/a
9-May-08, 06:05 PM
we have our own coating that will protect the foam from wear so that isn't a problem. and still have all the same flexable traits as uncoated.

Are the "traits" anywhere near as "reactive" as a low pressure shirt though?

I have spent at least one hundred hours doing Internet searches and playing with different foams over the past seven years. I had given up on the idea until this thread.

If this technology can be made to work, I'll be first in line to attempt to put in on my hovercraft (if I can afford it).

Any idea on cost?

It cannot be cheap.

A few years ago I priced out buying a couple of rolls of soft closed cell foam from a supplier in Chicago. It was going to cost me over $500 for 1-1/2" thick stuff and I would have to keep it rolled. Custom foam places were talking about $30,000 set up fees before even looking at the project for a solid piece tube in 30 inch diameter.

I'm in love with the foam skirt concept the more I read about heave acceleration and dampening. However "mass" is the ultimate critical issue, even before abrasion or reactive abilities.

Misc. Images:

http://www.wing.com/construction.htm (www.wing.com/construction.htm)


http://www.freepatentsonline.com/7201865.html (www.freepatentsonline.com/7201865.html)


Another Patent with large image:

http://www.freepatentsonline.com/6810827.html (www.freepatentsonline.com/6810827.html)

Foam is in the tube above only:

http://www.aks.r52.ru/eng/mars.phtml (www.aks.r52.ru/eng/mars.phtml)


I think the "hybrid_collar_detail (D-fender) is from "Wings Inflatables, not sure though.

loud'n'proud - n/a
12-May-08, 03:09 AM
we'll let you know in a month or 2 when we've built one http://hovercraft.org.uk/images/icons/smiley_icons/icon_wink.gif

kach22i - n/a
12-May-08, 03:00 PM
Microwave sensors to enable cables to pull up on skirts in anticipation of a wave might work well with the heavier mass of soft sponge rubber foam. This is just an off-shoot of one of the many ideas I've played with.

More discussion here:

http://www.hoverclubofamerica.org/forum/index.php?showtopic= (www.hoverclubofamerica.org/forum/index.php?showtopic=1365) 1365


My sorry attempt at foam at the waterline ended in disappointment, I wish you better luck.

http://www.hoverclubofamerica.org/forum/index.php?showtopic= (www.hoverclubofamerica.org/forum/index.php?showtopic=412&st=75) 412&st=75


http://www.hoverclubofamerica.org/forum/index.php?showtopic= (www.hoverclubofamerica.org/forum/index.php?showtopic=412&st=135) 412&st=135


Current Project (now torn apart and ready for 5th rebirth):

http://www.hoverclubofamerica.org/forum/index.php?showtopic= (www.hoverclubofamerica.org/forum/index.php?showtopic=1243&hl=yellow+skirt) 1243&hl=yellow+skirt

Cheers, George/kach22i

Keith Oakley - n/a
12-May-08, 07:16 PM
Saw a solid skirt craft in action at Freiburg Germany last weekend. It's the F2/FS craft of experienced French racer Alain Larribe which has a large bag skirt which appears to be full of foam of some description giving it an effective hoverheight under the bag (where there is a 1 inch high mini Bertin skirt a la Vortex bags) of around 1 inch.

Obviously its very stable at speed over water, which is probably the main idea, but on the land to water transitions it makes an almighty splash as it falls onto the water and the skirt doesn't curve to match the change in angle of the surface of the land/water.

But hey it works and in those situations travelling at 30mph plus momentum will carry it through. I did however see at least one situation where on land it required the assistance of a marshall to get it over a very tiny hill (probably 2-3 inches high). Due to a tight turn the craft had lost momentum and despite around 100 bhp and grass under 1 inch high it was stuck where there was a small change in ground surface angle.

It will be in Holland in a few weeks and at the worlds in Sweden so would be solid skirt enthusists should maybe make a pilgrimage there.

kach22i - n/a
12-May-08, 09:39 PM
It just hit me.

I talked to the gentleman which runs the museum and wrote a book about NJ flight (I own the book now). The first hovercraft skirt was rigid/solid.

Thread on HCA about Charles Fletcher:

http://www.hoverclubofamerica.org/forum/index.php?showtopic= (http://www.hoverclubofamerica.org/forum/index.php?showtopic=1461) 1461



profqwerty - n/a
12-May-08, 09:54 PM
The first hovercraft skirt was rigid/solid.

But surely then there's a reason for the move to flexible skirts...

kach22i - n/a
12-May-08, 09:56 PM
Not in a perfectly flat world there is. http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

1clue - n/a
12-May-08, 11:33 PM
Yes, there was at least one very good reason for moving to soft skirts.

Obstacle clearance is the distance between a flat surface and the nearest stiff part of the hull.

The old hovers had no flexible skirt, which means that the air gap was the only thing giving obstacle clearance. In order to get 8 inches of clearance on a peripheral jet machine, you had to supply enough air that there would be an 8 inch visible air gap under the hull. That air movement translates directly into power required.

So figure it takes 10 hp to get you an average 1/4 inch air gap on your modern hover, and let's figure it has 12 inches hover height. To do the same thing with a 12 inch air gap would require 480 hp, assuming that there are no other considerations. Usually, in my understanding, there are other considerations such as duct diameter which cause greater losses.

I think there may also be other considerations which might mitigate the power requirement. But you're still dumping an incredible amount of power into lift, and that translates directly to money spent.

Flexible skirts are used to decrease the air volume requirement and still give obstacle clearance. The softer the skirt, the better it conforms to the surface and the less power you theoretically use.

Keith Oakley - n/a
13-May-08, 07:13 AM
Good point. Just to reinforce that in 1959 I saw the SRN1 demonstrated at the Farnborough Airshow zooming up and down the flat runway with a 12 inch peripheral jet airgap hoverheight and no skirt. It used pretty well all its 450bhp to lift a pilot and 3 soldiers. Just 18 years later my own craft Spirit of Snodland was hovering at 10 inches skirt height (3/8inch airgap?) carrying 4 adults around a scottish loch and coping with some steep land/water transitions with a lift engine of just 12 bhp.

profqwerty - n/a
13-May-08, 09:27 AM
But then at some point it'd be better off just using a larger fan and using it as a helicopter of sorts...

1clue - n/a
13-May-08, 03:02 PM
But then at some point it'd be better off just using a larger fan and using it as a helicopter of sorts...

Well, yes. Especially since you can get a one-man helicopter with only 66 hp. Some of them use a Rotax 582 aircraft engine, and that's how much power it produces. For all I know, there are smaller ones.

loud'n'proud - n/a
13-May-08, 11:45 PM
even though a helicopter is a completely different fan/propeller set up...

may only be a smaller hp motor running it but the propeller shape is different and a much much larger surface area.

those 1 man one weight little more then the motor and the man, no hovercraft will ever weight that little.

we have looked into doing that as a direct down thrust but it won't work.

plus, a hovercraft lift fans should not be driving flat out to get the lift, they should be running abit above idle and producing just the right ammount.

1clue - n/a
14-May-08, 02:37 AM
My point was twofold:

First, that there is a limit as to what's practical on a hovercraft lift system, and that the skirt causes a hovercraft to cease being impractical for all uses and start to be practical for many uses.

Second, use the right technology for the purpose. If your goal is to have a hovercraft, then build one. In the case of land traversal, I don't think a hard or stiff skirt is practical at all. In a water case, I could see it being useful on the sidewalls in some uses. If your idea is to fly outside ground effect, then a helicopter makes sense, or an airplane. It's a good idea to keep in mind where your current project is in relation to other technologies. 480 hp to lift a few hundred pounds off the ground is ridiculous, in my opinion. Current fuel prices would make it prohibitive if nothing else.

The reason for a skirt is to increase the distance between the ground and the hard structure on the hovercraft without increasing power. A stiff skirt negates that gain, depending on the stiffness. I know there was comment earlier that it wouldn't be a hard skirt, but stiffer than normal. That has possible application, and some of the limits were mentioned before I started posting on this thread.

This comes down to an exercise in skirt material. Stiffness has certain advantages and so does flexibility. Those interested need to weigh the advantages and disadvantages appropriately and decide what their application needs.

loud'n'proud - n/a
14-May-08, 02:46 AM
beauty is our fuel is still only $1.39/L and thats a record high so we can still afford to enjoy our toys....

we only plan to use a total of 45hp i believe to lift our 1t+1t payload craft.. lol as for thrust... we have 500kw of power lol.

we are not interested in solid solid skirts, it will have plenty of flex.

due to new ideas and being a r&d company its hard to openly discuss topics like this without saying too much to keep it confidential.

kach22i - n/a
14-May-08, 01:13 PM
I don't think a hard or stiff skirt is practical at all. In a water case, I could see it being useful on the sidewalls in some uses.

Going back to the foam skirt and my bow experiment for a moment. I think Ken you have struck on the same conclusion I did. Which is any hope of a stiffer than normal foam skirt being useful might be traveling on ice first (water and land distant second choices) in an "Bag & Drape" (Sevtec style) configuration with the foam in the sides only.

I still might get around to playing with that myself, but power and weight are issues on my craft as well as on any other hovercraft.

loud'n'proud - n/a
14-May-08, 11:54 PM
well design has begun and parts orders,

We have props coming in from the states, 4 engines already here.

I think you guys will be interested in the design when its done, there are a few things I cant talk about that i know you will like http://hovercraft.org.uk/images/icons/smiley_icons/icon_cool.gif http://hovercraft.org.uk/images/icons/smiley_icons/icon_twisted.gif

kach22i - n/a
15-May-08, 02:15 PM
Props from the US?

If I were to guess, that means you are using a US based design, perhaps a Sevtec - for all the reasons I've already promoted.

This will be very interesting.

I looked on-line for pictures of the French hovercraft at the Germany rally of a couple of weeks ago, but found nothing.

If anyone finds photos, please post them in this thread or provide a link to the original source.

Saw a solid skirt craft in action at Freiburg Germany last weekend. It's the F2/FS craft of experienced French racer Alain Larribe which has a large bag skirt which appears to be full of foam of some description giving it an effective hoverheight under the bag (where there is a 1 inch high mini Bertin skirt a la Vortex bags) of around 1 inch.

Keith Oakley - n/a
15-May-08, 02:48 PM
Here it is in Sweden last year - there are other photos of 217 on this site.

http://anjou-aeroglisseur.com/retro/index_retro.php?year=200 (anjou-aeroglisseur.com/retro/index_retro.php?year=2007&event=5&id=5&image=DSC_0839.jpg) 7&event=5&id=5&image=DSC_0839.jpg

kach22i - n/a
15-May-08, 05:24 PM
Thanks, I found more images in Formula-S, nothing in Formula-2 though.

link (white on white craft - AIRGOAL):

http://anjou-aeroglisseur.com/retro/index_retro.php?year=200 (http://anjou-aeroglisseur.com/retro/index_retro.php?year=2007&event=3&id=4) 7&event=3&id=4

loud'n'proud - n/a
20-May-08, 05:30 AM
I'll give you a hint



Nick Long - n/a
20-May-08, 08:31 AM
Just a thought, but have you checked these propellers against the various construction regulations that apply around the world? You might end up with something that could only be used in certain situations.


kach22i - n/a
20-May-08, 02:55 PM
I'll give you a hint

http://www.whirlwindpropellers.com/index.shtml (www.whirlwindpropellers.com/index.shtml)


I discovered those about a year ago. Everyone on the HCA thought they were nice, but a little pricy.

Scroll to middle of page:

http://www.hoverclubofamerica.org/forum/index.php?showtopic= (http://www.hoverclubofamerica.org/forum/index.php?showtopic=1244&hl=propeller&st=75) 1244&hl=propeller&st=75

They could go on a Sevtec or a UH (Universal Hovercaft), or any other thrust only type craft.

They can even power something 10x's larger. http://hovercraft.org.uk/images/icons/smiley_icons/icon_cool.gif

You have to grin a little when saying; I have a "Stump Puller" prop.

loud'n'proud - n/a
21-May-08, 01:29 AM
YES Well this is one of the perks when money is no object lol

kach22i - n/a
22-May-08, 06:04 PM
I don't think you could find a description of this sort (mentioning hovercraft) in the USA.

Foam Dock Pads UK

http://www.saralbs.co.uk/dockpads.html (www.saralbs.co.uk/dockpads.html)


<table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote&#58;</td></tr><tr><td class="quote">
Standard pads have a rear frame of seasoned timber or galvanised steel bonded with a polyether foam then covered with a 900gm/m2 woven polyester based pvc. This is then covered with a heavy-duty 1500gm/m2 nylon based neoprene black hovercraft-skirt material.

Has anyone done a double layer skirt before?

The inside one would cover the foam connected to the hull for buoyancy and be lets say 1M dia., the second layer would be 2M (or twice as large) and have pressurized air flow between the two layers.

I'm not going to patent it, just asking if it's been done before.

I'm still pondering the advantages - if any.



The trend seems to be that air bladders are replacing foam though (in some applications).