View Full Version : loopy skirts

jon_curtis - n/a
3-Feb-07, 06:18 PM
i have been thinking about doing a loop and contact strip skirt, maybe, but dont really know how they work, the attached sketch, how far off is it?

if say the segmented bits were fabricated out of nylon webbing or something like that?

jar2 - n/a
3-Feb-07, 07:02 PM
Hi Jon,

Your pic is a combination of a full flow bag skirt and a contact strip. The cushion feed slots in the bag are guaranteed to cause major skirt damage the first time you hover over a spiky stick! If you are building what you say then stick to the simple bag - it works! Adding a contact strip will make the contact line stiffer - that can't be a good thing on rougher surfaces IMO (Keith Smallwood will no doubt be along shortly to explain why I'm wrong http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif ).

Ian Brooks - n/a
3-Feb-07, 10:10 PM
I think that one of the reasons why the SevTec skirt works is because of the flexibility, which reduces wave drag especially when going over hump. Yun and Bliault talk about flexible skirts being the way to go, and the Sev skirt seems to follow the spirit of thier suggestions.

I have similar worries to John - I'm not sure how the contact strip will affect the flexibility?

The other thing that I wonder is... why would the contact strip remain pointing straight down?? It has a pressure difference across it, and no real transverse stiffness (along the straight portions), so it'll just flop over, unless it is attached in some clever way that exploits the tension in the bag material to pull it straight. Except at the front, where a semi-circular profile will hold it upright. But of course not on a SevTec skirt!

Which is not to say that the SecTec skirt cannot be improved on, theres a fair chance it can be. I am watching with interest to see mow mine performs, compared to the segment skirt on the Osprey.

So far have noticed that it sticks to the mud more - although, that may have been the particularly sticky 'drying' mud on the Severn - where I have seen a segment skirt well and truly stuck too, so it's too early to tell.

The other thing I havenoticed is that it does get over hump very easily - which I have ahunch is to do with the flexibility and lack of parts that can "scoop" the water (such as contact strips???)

The skirt is pretty quick to make - I would suggest you make it to plan first, see how it goes and then make a modified 'experimental' version later?



jon_curtis - n/a
3-Feb-07, 10:15 PM
hi ian

was thinking about trying the standard one first, but do fancy trying some kinda loop and strip or loop and fingure also.

i should just get it flying first http://hovercraft.org.uk/images/icons/smiley_icons/icon_smile.gif

hover t - n/a
4-Feb-07, 12:08 AM
Hi Jon'

I am no expert but dont you think a loop type skirt on a sev may affect the brake on the front of the craft, If the bag lost preasure because of a open skirt it could plough in.

I have never seen john`s sev plough in under any conditions and the brake works pretty well to http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

I have only traveled in the sev a few times (usually when my osprey 5 has a problem) http://hovercraft.org.uk/images/icons/smiley_icons/icon_confused.gif so most of my observations are taken from traveling along side him on our longish cruses on salt and fresh water.

I have got to say it is a pretty good hover http://hovercraft.org.uk/images/icons/smiley_icons/icon_cool.gif

all the best


Ian Brooks - n/a
4-Feb-07, 12:09 PM
The front partition would need to be retained as separate from the main cushion, and some provision made to fill it - which could just be spill from under the partition skirt, which ought to work but might result in slow inflation, so a separate fixed feed would be required at slightly higher than cushion pressure.

But having said all that, I'm not sure why you would want to have the bag open to the main cushion like that? What might make sense, would be to modify the skirt for small segments at the contact point, a bit like a contact strip but shaped like the bottom of a segment skirt so that they "stand up" under the cushion pressure, but you would need dozens of them! They would also have the conflicting requirement of collapsing flat under wave loads, otherwise you will be back to conventional hump performance, requiring lots of power to get over hump.

The skirt design is one of the key factors (along with light weight & efficient airdrives) that allows the SevTec designs to operate on such low installed power, and Barry Palmer has spent 20-odd years on it... it would be interesting to know what he has tried over the years, and how the configuration came about. It is an interesting design; the front curtain looks primitive, but the rear-sloping curtain is inherently resistant to tuck-under, and when the curtain does dip into the water, the resulting "wetting drag" & pitch-down is less than with a typical bag or segment. Once the nose does pitch down, the front compartment pressure rises, providing an opposing pitch-up moment, which is absent in many other designs. Taken together, these factors provide for the impressive plough-resistance of the skirt. It may look primitive, but in reality I don't think it is.

There are negatives to the design - one possibly being higher levels of skirt drag over the ground, but I am still waiting to see about that, and in any case these are not racers... for the same reason, my MPV does not have low-profile types!


Vortex - n/a
4-Feb-07, 10:13 PM
Hi there,

Boss here, e mail me or give me a call details are on www.vortex.org.uk

or better still go to the AGM and have it all explained 'LIVE' provided the bar isn't open!!!


kach22i - n/a
5-Feb-07, 01:43 PM
Flexibility at contact line is very important. I once tried placing a rather stiff pool noodle at the bow, it was a heck of a cow catcher with a bow wave like the Titantic and unable to get over the hump.

Link to 6 videos:

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

The thing about the Sevtec skirt is that the bag pressure is just about equal to the pressuer under the craft. The ratio is 1: 1.2 which may qualify it as a "responsive skirt" as described in the Yun and Bliault book.

Ian Brooks is right about the importance of "flexibilty", but I don't have a good idea of how deep the proposed attached fingers/segments are. As they are not shown in the first sketch.

If the fingers account for all of your cushion height then their upper attachment point can be stiff as a fiberglass hull, right?