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Modelhovercraft - n/a
22-Dec-07, 03:09 AM
Whats the best skirt for a small 1 or 2 man Hovercraft.



So far previous experiences have told me that the bag skirts are far to bouncy, can easily bottom out your craft and even bounce you out when traveling at speed.



Segments and pressure segments are far more stable. very Little bounce, no wobble and no bottoming out.



what are your previous experiences?

tonybroad - n/a
22-Dec-07, 11:50 AM
light the touch-paper and stand back - it's an age-old debate



either skirt designed well should behave itself so any bounciness, wobbling or bottoming out is often a sign of poor skirt or hull design or too high or low hover height



craft weight and passenger weight all contribute to a skirt working well also



i find a segmented skirt does the job well and is lighter, easy to make and easy to fit and replace segments



just my opinion



Tony

jar2 - n/a
23-Dec-07, 08:39 PM
Whats the best skirt for a small 1 or 2 man Hovercraft.



So far previous experiences have told me that the bag skirts are far to bouncy, can easily bottom out your craft and even bounce you out when traveling at speed.



Segments and pressure segments are far more stable. very Little bounce, no wobble and no bottoming out.



what are your previous experiences?




A difficult question - and like a lot of hovercraft questiosn the answer is - it depends.



It's like trying to compare tyres - is a racing slick better than a tractor tyre?



I have no racing experience so I will limit my comments to cruising. For cruising use (water based) my personal opinion is that a properly designed low pressure bag skirt is hard to beat for stability, low cost, damage tolerance and reliability. All commercial hovercraft use bag skirts (usually with small fingers). They do this primarily to gain stability and damage tolerance over rough water.



A bag (or loop) skirt is deceptively simple - like a lot of simple things the way it works is actually quite complex. In other words, minor changes in geometry can end up with something that doesn't work at all!



If you cruise on water that has a lot of hard obstacles (spiky stuff liable to cause significant or frequent skirt damage), and you don't venture more than a few metres off shore in smooth water, then maybe you should consider a segment skirt.



My experience of bag skirts has been that you can expect around 250-300 hours cruising time from a skirt (with some minor repairs). If you are new to hovercraft then you could easily half that number http://hovercraft.org.uk/images/icons/smiley_icons/icon_surprised.gif

Ian Brooks - n/a
24-Dec-07, 04:55 PM
The skirt of a hovercraft is, or should be, a sophisticated system, and any "simple" skirt is likely to have problems. I have operated bag and segment types, but my experience is limited to a well-designed low pressure bag, and a relatively poor segment type - so I choose the bag!



As John says, what you need is very dependant on what you need it for. It seems clear that skirt drag on land is greater for a bag than a segment skirt, and this results in a majority of segment type on race craft. For this reason, the bag skirt seems to have been rather under-developed in UK light craft, whilst segment types have been developed extensively, but in the main, for racing use. In the US, more work has been done on bag skirts, and I am very impressed with the skirt on my SevTec, which, in my opinion, is a sophisticated and well designed skirt. I'll describe how it works here - but leave the details of segment and high pressure bags to others more qualified to describe them.



Beside retaining the cushion air, a skirt has to do a number of things, such as provide roll stability, pitch (nose down) stability and heave (up/down) stability; it must control plough-in, it must reduce wave (hump) drag to a minimum, it must control spray, be resistant to wear, deform readily over obstacles, etc.



The skirt on a SevTec provides roll stability by means of the large side bags, which are low pressure "deformable" skirts. As the craft rolls, the bag contacts the ground, and the geometry causes the bag to spread out under the falling side, moving the center of pressure to produce a restoring moment.



Pitch stability is provided by the use of the partition skirt. This is a segment of a truncated cone, similar but smaller than the front skirt. Together, they control the pressure in the front compartment to be slightly lower than the main compartment. As the craft pitches down, the pressure in the front compartment rises to provide a restoring moment. This actively controls plough-in and means that the craft is relatively insensitive to fore/aft load position in use.



Plough-in is further controlled by the inclined front curtain, which gives the characteristic high-bow appearance to the SevTec. This helps to control the effect of the wetting drag on this skirt - this can cause the front skirt to tuck-under in many systems, this being the final and catastrophic stage in a plough-in. However, the curtain, being mounted forward of the contact point, can't tuck under in quite the same way.



Bounce (or heave stability) seems to be a function of bag pressure, geometry and damping, and it's true that a segment skirt has intrinsically more damping than a bag skirt - which means its harder to get wrong. But a well designed bag shouldn't bounce - SevTecs don't.



Wave drag is reduced by the low pressure deformable bags. As a wave is encountered, the bag tends to ride over it rather than plough through it, and in addition the bag has no corners or edges that can "scoop". The same feature provides for the skirt to deform readily over obstacles.



Spray control is a function of craft weight/area as much as skirt design, but it is clear that skirt design is important. Two craft of similar size and weight can and do have very different spray characteristics, and the low pressure bag seems to be excellent in this respect - I suspect this may be to do with the relatively smooth air escape path under the bag, which may reduce turbulence and hence there is less spray generated.



So in answering the original question - whats best for a small craft - there is no simple answer, as the craft should be designed in conjunction with the intended skirt and with the end use in mind - there are good craft with both types. But - for my money - the SevTec skirt system is my choice.



Happy christmas to all!

Ian

kach22i - n/a
28-Dec-07, 02:56 PM
The skirt on a SevTec provides roll stability by means of the large side bags, which are low pressure "deformable" skirts.

Ian


What you wrote is a great description Ian, I had to read it several times to get it all, well written though.



The quote above is important, and let me explain why. In my hovercraft skirt experiments which somewhat resemble a Sevtec skirt (a little bit), an early test flight experience with way too much pressure in the side skirts (I'm not the engineer Barry Palmer is) had some serious turning issues.



What was happening is that early in a turn the inside of a turn side skirt would not deform and yet were too soft to plane on the water's surface (like an RIB) and would "dig-in" the water. This slowed down the craft right away and brought the craft below hump speed in a hurry much to my disappointment.



I was later able to close off much of the air to the side skirts which also gave me better plenum air flow and everything worked much better from then on.

Vortex - n/a
2-Jan-08, 10:08 PM
Thought I would post this, now that most people have had their say.....horses for courses....!!!



Bag skirts are lighter than segmented skirts, the average segment skirt takes 30M the average bag 10m, bag skirt material is thicker therfore heavier, even so recent research indicates a 30% weight saving



Bag skirts are more difficult to design correctly, but much quicker to make and fit



Bag skirts are cheaper



Bag skirts should not 'bottom out' unless the balance ratio is very wrong.



Bag skirts do have more drag, which is why I developed the contact strip system which has less drag than a finger skirt.



High pressure bag skirts ( as used on Vortex craft) are very stable, efficient and use a minimum of lift air. On our F35 twin engine craft we only had a 5 hp engine and had more than enough air even for racing.



Problems...



Bad design makes for a very unstable skirt



Large rips >300mm do cause major problems



At high speed >40mph you can get excessive skirt bounce on very rough ground especially if balance ratio is too high



Skirt does not collapse when cornering this can make cornering more difficult, there is an easy technique to compensate for this.



Keith

pest619 - n/a
3-Jan-08, 12:00 PM
definitely right about the ease of manufacture- i started my new bag skirt after boxing day and it was done by the 30th and the 25 yr old singer is still managing it... although it needs rests every now and then...



Has anyone measured the weight of water taken on by skirts through normal use over water? I would have thought that since there is less surface area of material on a bag skirt less would be taken on. (Not thinking about the water taken on while stationary in the water but the water that makes the material look wet) Or is this an insignificant amount?



pete

Don83000 - n/a
4-Jan-08, 01:04 AM
<table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">John Robertson wrote on Sun, 23 December 2007 21&#58;39</td></tr><tr><td class="quote">




A difficult question - and like a lot of hovercraft questiosn the answer is - it depends.








No it doesn't - Bag Skirts Suck Ass. http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif
</td></tr></table>



Funny that Russ it was a total novice in a BAG skirted craft that whipped your ass in a offshore event http://hovercraft.org.uk/images/icons/smiley_icons/icon_biggrin.gif

sixpackpert - n/a
4-Jan-08, 11:19 AM
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keith_b - n/a
4-Jan-08, 08:20 PM
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