View Full Version : A question ...

Ross Floyd
19-Feb-10, 09:07 AM
Sir Christopher Cockrel's invention of the perhpheral air jet hovercraft was made obsolete in 1960 by the development of C H Latimer-Needham's flexible skirt.


John Robertson
19-Feb-10, 11:07 AM
Not sure what the question is ... but I think the answer might be efficiency :)

For the peripheral jets to be effective more than a few inches from the nozzle, the exit air velocity needs to be very high. Generating a high velocity air stream needs power (0.5 x air volume x velocity^2 - if I remember correctly?). Probably why the SRN1 needed a 450HP engine :)

Which begs another question - why does the peripheral jet theory live on only in finger skirt craft? The theory I've seen is that the finger feed holes and segment shape generate an inward pointing "jet" at the segment tips. I find it hard to believe this as the "nozzle" hole is tiny compared to the exit area of each finger - the air leaving the segment tip must be at almost the same pressure as the main cushion.

The plenum feed system on a typical small segment skirt craft can't be very efficient at all - the losses caused by pushing air around corners and through a tube ( the hull plenum) and then out small holes must be pretty large. Surely it would be far more efficient to just dump the lift air straight into the cushion plenum under the craft (as on commercial open loop/segment craft)? Some air may need to be used to initially "inflate" the fingers to get a decent seal but that could be done using very small holes and a small pipe/duct with the majority of the lift air directly feeding the cushion. I realise that some of this has probably already been tried in the dim and distant past but I can't find any examples - or the reason why it didn't work :(

Ross Floyd
19-Feb-10, 11:27 AM
Precisely my thoughts John - I've been lying in bed thinking about it while I've had flu and decided to pose an exam style question for the gurus of the club to answer.

After the VA1 and SRN 1 & 2 as originally built, I can't see the actual 'hovercraft' as developed today is the same thing as the design by Cockrel - and poor Latimer Needham sold his idea to Westland and got 'nowt in comparison.

I can't see much evidence of a peripheral jet on modern craft - directed air possibly, ..... but a jet ????

A sort of coffee break question :-)

19-Feb-10, 02:32 PM

Many years ago my first craft, designed by Jeremy Kemp, did exactly as you suggest and dumped it's air straight out the bottom. It worked fine. However it was a two engine craft so the lift motor was right at the front. I believe with integrated craft it is necessary to direct the air to the front to avoid plough in problems at speed.

Roger Drew

19-Feb-10, 02:39 PM
Not sure what the question is ... but I think the answer might be efficiency :)

For the peripheral jets to be effective more than a few inches from the nozzle, the exit air velocity needs to be very high. Generating a high velocity air stream needs power (0.5 x air volume x velocity^2 - if I remember correctly?). Probably why the SRN1 needed a 450HP engine :)

Which begs another question - why does the peripheral jet theory live on only in finger skirt craft?


not quite so, i believe vortex have small holes in the bag skirt, just inside the contact strip to give a peripheral jet effect.

surely tho the air gap between skirt and surface, is kinda a peripheral jet on all craft, in that a uniformed stream of air exits from under the craft all around the skirt periphery?

with regards a jet segment skirt, does this not have a curved panel inside which is designed to increase air pressure to an outlet point at the bottom, which should in theory give a higher pressure outlet compared to inlet?

i put high pressure skirts on the front of my ally craft, where the outlet size was smaller than the inlet size and the skirt was sealed up the planning surface to the inlet, this increase the speed before plough in:)

on a craft with skirts of segment or bag, or both does the peripheral jet/air blead/air gap blow out, not reduce surface friction, to therefore increase speed or decrease the amount of thrust needed for a given speed?

Ross Floyd
19-Feb-10, 05:27 PM
Think back to the original tin can experiment gents - I think the inward air jet ( although the original was vertical ? ) made a curtain that retained more air inside the plenum under the hull than would have been the case by blowing air in from the middle. That was the invention - retaining air with an inward pointing jet.

I think we have moved a long way from this - indeed with the jet reaction effect, could you have a peripheral jet from a flexible skirt without the skirt collapsing?

This came about as I have to give a talk to a local school - and then demo the craft I began to work out how to relate Cockrel to a BBV3 and decided it was difficult.


20-Feb-10, 12:49 AM
Ross you are right, there is little or no air jet principle on a segmented skirt craft unless those skirts are tubular and effect would be slight compared with the containment of the fabric skirt, really only a curtain principle is common between SRN1 air curtain and a segmented or bag fabric curtain with lower snagging properties than a straight perimeter fabric curtain, which must have tried out at some point and logically discarded.

There is a magnificent cut away drawing of the SRN1 which shows how the air was distributed under the skin to the curtain jets in individual channels, which is an important similarity between Cockerels SRN1 and a segmented skirt layout and the contact point air lubrication factor of a bag skirt. Air control with as few losses as possible is the game after Cockerel.

best of luck with your school talk.


20-Feb-10, 06:55 AM
On a segmented skirt craft, the inward pointing geometry of the skirt creates the peripheral jet effect. i.e. the airflow is curved inward onto the plenum to compress the trapped air.
There are two reasons for the ducted hull design, particularly on integrated craft. 1.to give an angled planing surface and 2. to duct the air to the front of the craft rather than dump it all into the rear of the skirt.
I have found that using pressure segments, which in theory give more of a jet effect and less airflow into the cushion, make the craft more "grabby" so it seems that having more air in the cushion is preferable to having a high pressure peripheral jet.

Ian Brooks
20-Feb-10, 07:18 PM
I would agree with Alan - there is little or no positive contribution from a curtain effect. The reason for this is to be found in submerged jet theory. A turbulent jet forms at the feed holes. The bit of the jet we are concerned with is the "potential core" - this is the bit that is at full velocity. Outside of the "potential core" is the "mixing zone" in which the surrounding fluid is drawn into the jet, reducing the jet velocity sharply. The bad news is that the potential core only last for 5-7 diameters, getting smaller is it nears the bitter end - and that is without the impact of the jet onto the skirt wall. By the time the air gets to the airgap, it is thoroughly mixed, very turbulent, and the velocity is reduced sharply.

So what all this means is that the velocity of the air through the feedhole is lost before it get to the place where it might be useful (the airgap under the skirt).

The unfortunate thing is, that the velocity raised through the feedhole is very expensive in terms of power. Taking an Osprey 5 as an example, this craft requires 24hp to power the lift system - of this, a whopping 16hp is used just to get the lift air through the feedhole! Only 8 hp is actually used to support the air loss through the airgap.

Practically then, the best thing to do with an integrated craft is to open out the feedholes as far as practical, to reduce the feedhole power loss.


20-Feb-10, 10:01 PM
I would agree with you Ian but after resizing the air feed holes on the Osprey to 120mmx90mm I noticed that the craft now gets over hump speed without a problem but one result of doing this was the engine would rev to 5500rpm before , after I opened out the feed holes it would only rev to 5300rpm flat out which suggested to me that more of the crafts weight was now feeding back into the plenum and pressurising it ,eg the fan has more load on it than it had with smaller air feed holes. this is the opposite of what i thought it would do. I am now thinking of fitting a engine that produces 130bhp at 3000rpm this would be the the highest rpm the engine would ever reach and would take all the stress loads of it and make it last for years. Well that's my 5p worth, hovers are not as straight forward as they look


21-Feb-10, 01:56 AM
Very interesting points raised by Ian and Trev, and imho Trev's experience of higher engine load once the feed holes were expanded is completely in line with Ian's point of high hp requirement to push the air through the feed holes.

In my mind, Trev's lift fan was previously skidding against the plenum pressure, now it can push the air through much more easily and so now has a higher air movement = load albeit at a lower pressure, and is more closely connected to the cushion pressure, all of which should equal a quicker recovery time back onto cushion. Entirely in line with Ian's recommendation, which I gather would relate also to separate lift fan segmented skirt craft as well as integrated segmented skirt craft.

The necessity of in plenum air deflectors and the regulated size of feed holes has always played a part in the even distribution into the cushion, which was absolutely necessary with the SRN1 due to it's air curtain effect and to a degree reactive jet, but is this actually the main focus now or is keeping each segmented section inflated into its correct shape the major focus ? so the cushion from anywhere is maintained without spillage by a fabric curtain rather than the easily lost air curtain ?


21-Feb-10, 06:42 AM
I find Trevs experience very interesting and Alans assessment is probably right, there is more back pressure. Of course the restricted feed hole in the planing surface has many roles. In the event of segment loss you haven't got a gaping hole with the resultant loss of air and the peripheral inward jet of the neighbouring segments help to contain the cushion.
The hole size should be relative to the air loss from the segment.
The only feed holes I've ever enlarged are the front and rears.It's all about having enough pressure at the back to support the hull and compensating for the flow losses which occur when moving forwards. I'm convinced that the peripheral jet is only a bonus to maintain the cushion rather than the prime requirement for a hovercraft.
Maybe in larger craft there's a benefit to having some integrated lift air to pressurise the segments (particularly at the rear) and a separate lift unit for the main plenum air?

Paul Fitz
21-Feb-10, 03:29 PM
This is an interesting thread, but appears to be confusing technologies. A great deal has been written about Peripheral Jet craft (as SRN1) inefficiencies, much of it true. Ian's comments regarding air jet core effects and the degradation over distance are quite accurate and probably the main reason why PJ systems were inefficient and fell out of favour.

One thing I must disagree with is the statement "there is little or no positive contribution from a curtain effect"

A few years ago now, I suggested and described the 'Flexible progressive pressure jet' segment to another club member who tested the idea on a lightweight craft driven by a junior (Kristie Scales). The skirt used at that time was a simplified jet and it worked well with some quite staggering results.

In essence this segment was a pressure segment which had a well defined conical interior which increased the velocity of the air through the segment such that the static pressure at the feed was a little below that of the distribution plenum. As the air travelled through the segment the static pressure decreased as the velocity increased.

As Ian has pointed out, much of the energy lost in a skirt system is across the feed hole where the air transits from a high pressure to low pressure via a restrictive orifice. This loss varies throughout the system due to the difference in distribution Plenum pressure on a typical craft and the feed hole sizes. On a Jet segment the differential of pressure across the feedhole is relatively low so the losses are reduced at that point (but still substantial due to air turning and turbulence losses). The losses then occur as exit losses at the bottom of the jet close to the contact point.

During the tests, it was found that the segments generated lift as a direct result of the impingement of the high velocity jet with the surface. This was further increased by effect of additional air directed down the outside of the front segments due to their additional stiffness, (this would also be true of any high pressure segment). The most noticeable improvement was to the cushion containment. They improved the air curtain effect to the point where skirt spray was almost absent. I considered this to be one of the most important advantages of the segment type.

The test craft was originally fitted with jets on the front only. This proved troublesome as the craft flew with too large a hover-gap under the front despite Kristie leaning out over the front of the craft. It was necessary to balance the craft by moving some of the jets to the rear-sides, but ensuring sufficient rear drag to allow the craft to steer. We initially fitted too many at the rear and all steerage was lost. The craft were less likely to plough with jets fitted to the front. They gave clear warning of an approaching event. If this was ignored the craft then ploughed severely when the entire jet folded backward. A modification to the jet can help to prevent this.

The jet segments absorb additional power within the skirt system as does any pressure segment. The question one has to ask is "Am I gaining any advantage for the increased power requirement". If you feel that delaying the onset of plough-in at speed, additional stability or the absence of spray are desirable then the answer may be 'Yes' If you wish to squeeze every last ounce of thrust from an integrated craft then the answer is 'No'. Anyone that fits pressure segment to a craft or elects for a bag skirt in preference to segmented must ask themselves similar questions.

In my opinion, anybody building a segmented craft for the fist time should always fit simple segments and try the craft before going for any other segment type. The biggest practical problem with Jet segments is the additional cost in material and time and the high maintenance.

On a simple segment, air from the feed hole is constrained to some degree as it is guided down the inside of the shaped segment toward the contact point. It is completely wrong to consider this effect as any form of peripheral jet. The air reaching the contact point is of much lower velocity than the air exiting under the skirt from the cushion and therefore exerts no advantage, which a true jet does. It is also wrong to consider the lubrication holes in a bag skirt as peripheral jets for the same reason.

"Dumping" air directly into the cushion and using segment or a non-flow bag (very low pressure) for containment, has the advantage of being very efficient in terms of power use. I have yet to see a craft of this design successfully complete in a UK racing series where others have shown that pressurised bags and mixed feed (ie. both central and peripheral feed craft) have been used with both reasonable and high success rates. The non-flow bag craft I have seen working here appear to have a higher bag pressure approaching that of Keith's (Vortex) bag skirted craft.

John Scotney manufactures jet segments based on my original design and they have now been fitted to a number of different craft types. It is desirable to experiment with the jet diameter as this will vary, dependent on cushion pressure, craft size No. of jets fitted etc.

Paul Fitz
21-Feb-10, 03:41 PM
..eg the fan has more load on it than it had with smaller air feed holes. this is the opposite of what i thought it would do. Trev


By increasing the size of the front feed holes you allow more air to pass. The greater the volume of air through the front distribution plenum the greater the velocity (and velocity pressure) and the lower the static pressure up to the feed hole. So static load on the fan is decreased allowing the fan to rotate faster and the fan to supply a larger volume of air. As more air is being moved through the rear of the plenum to feed the front the same is true there. ie the fan has moved to a new working point on its static pressure curve.

21-Feb-10, 06:48 PM
I opened up all 88 air feed holes around the hover in a attempt to equal the plenum-skirt pressure, its not moving more air as the top engine rpm has come down 200 rpm with the same fan setup as before. The air must be moving through the feed holes easier as they are all larger, but the load on the fan is greater and is not allowing the fan to rotate faster I was thinking it was the weight of the craft lifting the pressure inside the plenum but not really sure what's going on but it works for me .Remember one thing my hover is heavier and has thicker fibreglass than a normal osprey 5 as it was built to go down hardcore roads in the bond film.


Ian Brooks
21-Feb-10, 07:36 PM

By increasing the size of the front feed holes you allow more air to pass. The greater the volume of air through the front distribution plenum the greater the velocity (and velocity pressure) and the lower the static pressure up to the feed hole. So static load on the fan is decreased allowing the fan to rotate faster and the fan to supply a larger volume of air. As more air is being moved through the rear of the plenum to feed the front the same is true there. ie the fan has moved to a new working point on its static pressure curve.

As shown here ?:

If I remember correctly, you are using a 16 blade 5Z. Most fan data suggests that for moderate static pressure to low static pressure, the power consumed by the fan is fairly constant or reducing as the pressure reduced - See fig 1. It seems that by bad luck you have a case where the power increases as the pressure reduces. See fig 2 - if I have guessed right, the expected reduction in static pressure in the plenum has increased the power consumption by about 8% - see graph. Whilst this is only for the 30% of the fan thats producing lift, it seems to be enough to knock a few hundred RPM off the top speed of the fan.

But notice the increase in air volume - a whopping 50% increase! That'll mean 50% more hover height and noticeably less drag on bad surfaces.

Is there a noticeable loss of thrust - my calcs suggest it might be 5% or 6% down? But thats within the error margins for the calc, so it could mean anything.


Fig 1 "Normal" characteristic - power (thin blue line) goes down with reduced pressure


Fig 2 16 blade fan around 40 degrees - power goes up a bit as pressure reduces from 2200Pa


21-Feb-10, 08:14 PM
Hi Ian
The lift was good on the hover last year and as you will remember it coped with the tricky conditions we encountered on Loch Fyne.The fan set-up is 42deg pitch with 16 5z blades fitted and the engine is 130bhp at 5500rpm supposedly. Top speed not sure about that one but I did have it doing 47mph on the gps at one point with no plow-in so if it is down I have not noticed.


21-Feb-10, 10:23 PM
Wow this is a very interesting thread, but while you guys were getting stuck into the numbers I finally got to see Avatar. Some very nice contra-rotating ducted fans in it, all in 3D. Philip

Paul Fitz
21-Feb-10, 11:07 PM
It looks as though we have hijacked Ross’ post here.
Sorry Trev, I obviously misunderstood your post. If you enlarged all the feed holes then the reason for the higher fan loading is a higher fan duty requirement. Ultimately, the pressure requirement is determined by the craft mass (cushion pressure) + system losses. So the fan pressure must be higher than cushion pressure. The fan duty is the combination of output pressure and volume of air. Ian’s Graph demonstrates this admirably. If you push more air through the system than actually required the Hover Gap increases as Ian said, usually with an attendant increase in power consumption. Unfortunately in an integrated craft, there is a further complication of having 2/3 of the fan delivering a high volume of air at low pressure for thrust which may limit the maximum power absorption of the fan.

You had it spot on when you stated “hovers are not as straight forward as they look”. It is not always easy to get the fan duty right.

Philip, The only real advantage of a contra rotating prop on a hovercraft is to stop the hull rotating when you leave the ground. My advice, don’t fly higher than 8 inches. :o)

22-Feb-10, 06:14 AM
WOW, what fantastic reading. A lot can be learnt from all this. Great stuff guys, I will ponder it a bit.:)

Ross Floyd
22-Feb-10, 08:03 AM
Don't worry about hi-jacking or changing tack Gents.

This has become a fascinating and evolving discussion which has kept me riveted to the computer. Although my interest is more towards historical aspects, the aim of the first post was to pose a serious question and to hope that the gurus of the Club would get stuck in. They did !

I think the answer has been found - what is hovering today is not what Cockrel invented, but a logical progression, made possible by an equally brilliant invention that was not credited to the person who developed it. That dosen't take anything away from Sir Christopher, but perhaps we should do something to recognise Latimer-Nedham? I'll start another thread on that as this one is purely tech and it is a pity to spoil it.

All that aside. let this fascinating discussion continue!


22-Feb-10, 09:07 PM
This is something else I started to experiment with last year 7 x 72mm pipes fitted to the front plough plane to see if I could lubricate the bottom of the hull when the inevitable plough-in happens.The idea behind this one was discovered after reading a article in a 18 year old club MAG that John R sent to me, I decided to do my own take on it and fitted the pipes as i had them lying around ,I reshaped the bottom ends with a blow torch so that they did not stick down below the bottom of the hull .
Now the idea is to stop the hull from sticking to the water surface when a plow-in happens ,the bags collapse against the pipes and the air is diverted down to the bottom hull edge causing bubbles that stop the water suction affect that stops the hover so quickly.
As far as I can tell it works as the hover now recovers quicker without coming to a stop allowing me to keep going at a good pace:cool: its a sort of jet air effect to get us back on track



23-Feb-10, 07:48 AM
Nice work Trev,

This is an area I have been interested in for some time and had precisely the same thought re the tubes. I guess a few more have arrived at similar solutions as well over the years.

Hman (Ralph) USA, has done a lot of experimenting also with shapes attached to the plough plane of a scat to hold the front skirts away from the hull to keep them flowing and lubricate the hull bottom in the event of collapsed skirts and "plow-in". Scats have really steep plough planes and his additions made a lot of difference.

I have also had thoughts of applying this principle to front lift fan craft with a splitter at the back of the lift duct sending a small portion of lift air into a stepped cavity in the bottom of the craft. The cavity could actually either be like a neoteric hull bottom or a series of saw tooth cavities which would not affect side slipping as much as the neoteric cavities potentially could. The saw tooth pattern need not cover the whole base area either, this is kind of along the same principle as the golf ball dimple effect.

Having a dedicated air path to the hull floor cavity area could be wasteful of lift air otherwise required to keep skirts in shape. Using the tubes as you have done eradicates the waste as they are only effective when required which is why I like the next logical step.

Putting a splitter at the front of the lift duct dedicated to supplying the front skirts and tubing some of that flow in the event of skirt collapse directly to the saw tooth cavities in the hull floor and some of it to the front plough plane in a similar fashion to your craft is where I am currently at with preparations for a build. One way to achieve this would be a double walled front plough plane with internal cavities doing the directing when the skirts collapse. Another way which I prefer atm is building the tubes part way into the plough plane so they increase the strength of the panel as well as diminish the ground resistance potential near the hull floor in the event of ground contact.


24-Feb-10, 10:52 AM
Hi Alan
The tubes are a low tec and inexpensive way as a starting point to see if you can improve the hovers plough characteristics, I'm not really sure if anyone else has tried it in the UK if they have they are keeping tight lipped about it.

I have been keeping an eye on what Ralph has been up to with his scart over the years and say well done to him for having a go and trying to improve its built in failings.

The stepped cavity on the bottom of the craft with a ducted air supply to it is something I would build into a cruiser if I was going down that road ,I think it would help stop the hull-water hydrolyc forces on flat calm water when trying to get over hump speed.

I would not try to pump extra air pressure into the front segments on my craft as all it would do is spray water from the front skirt and soak me in the process, I can do without that!

I have no experience with twin fan craft so cant really comment on them but pipes built into the front plough plane would make it stronger and I think they help to burst the water surface up when the front of the hull hits the water at speed preventing it from digging in and rapid slow down of the craft.


24-Feb-10, 01:11 PM
Hi Trev,

You have done your low tech tubes almost exactly the way I had envisioned my thoughts as being the easy way to go 12 months ago and I am glad they work as well. You have even made the ends square, I had used square tubes all the way as mine were glued on and I wanted the option of having room between the tubes for the skirt flanges to lay in if need be. I think I had three developmental ideas of what improvements could be made before settling on the low tech approach you took as a starter, with the part embedded tubes as a build option in a subsequent craft. One of the developments had the 'tube sides-only' extend up beside the feed holes so when the skirt was drawn tight against the hull, in a plough in, there was no reduction of air passage diameter to feed air down the tube. It was a minor development that lent itself to the square tubes I had planned and was a development as the embedded tubes came to mind.

As part of presenting the idea to other hoverers to get their reaction, I had a few text chats with Ralph (Hman) and Barry Palmer and a guy named Steve and another factor that came up was the plough plane to floor transition should be a sharp edge and not a smooth rounded curve. I notice your craft is trying to make it rounder, :) At the time I thought a nicely curved surface to make glancing contact with what ever would be the way to go, but was assured that it had been tried often enough and the sharp edge was needed because a curved surface when it touches the water sucks the water up as the water flows around the curve and that draws the hull closer to the water and then compounds itself into a rapid increase in surface contact and a plough in occurs. Whereas the sharp edge or a sharp edge with cavity behind stops this from happening to a degree by creating an air pocket behind the edge which helps break the water up. Ehh ??, that's what the tubes are supposed to fix, by that time the topic had been bashed enough that no-one was making sense of it and everyone agreed that Sevtecs are highly suitable for having comfortable garden chairs installed..... .......you know how threads go haywire sometimes, olay.

It's a good relief to see that the idea works and I thank you for that, I can assure you we are on the same page. Now I can get on with the curved surface instead of the sharp edges to bash into whatever, like the undersides of a few more recent craft I now notice.

You say the original article came out of the club magazine and is 18 years old, sometimes good ideas take longer than expected to get attention.


John Robertson
24-Feb-10, 02:37 PM
Most of the theory and practice behind water attachment to "hulls" is well known in the boat/ship world. It's very easy to do your own simple experiments - just fold a bit of card into the shape of the front of your hull and "hover" it vertically next to a smooth stream of running water from a tap to simulate a hull travelling over water (turn your head sideways if you are having trouble visualising it :)) - then move the top of the card toward the water stream (simulating skirt collapse pulling the nose down) and see what happens!

As you get close to the water, it will bend toward the card and, once it touches it, it will flow down the entire card length. You will need to move it a lot further away to detach it again. It's similar to magnetic attraction.

Try it out with a "stepped" hull shape and you will notice that, even though the water touches the step edge, it's much harder to get it to attach itself to the "hull" surface behind the edge.

None of this is new - look at powerboats, float plane's etc, etc and you will see plenty examples of hulls designed to reduce water attachment.

In the hover world, UH have used stepped hulls and front cushion feed for 30 years or more (the main cushion feed lubricates the underhull behind the step and prevents it attaching to the water). It is extremely effective at reducing the impact of a plough in and allows plough in to be safely used to slow and turn the craft

Plough-in has two distinct stages - skirt attachment and hull attachment. There is no discernible gap between them with some craft :o In theory, if your prevent skirt attachment you will never see hull attachment. However, in reality it's impossible to prevent skirt attachment completely (although the self-recovering front bulbous skirt used on all large hovercraft has almost eliminated it). Minimising hull attachment is the only ultimate solution - this will reduce the rate of deceleration to a level similar to gentle braking on a car.

This begs the obvious question - why have a lot of hovercraft designers completely ignored this issue (and the very simple solutions) for so long and still continue to manufacture hulls and skirt systems that are, quite frankly, dangerous.

24-Feb-10, 05:45 PM
The original source of the story was first released at the Australian Hovercraft Federation National meeting in 1990 by Kevin Dixon, and the hcgb club mag did a article on it in the January 1992 edition on page 19

To install a step on the bottom edge of my hover would reduce the front contact clearance and I am not prepared to do that so I will have to stick with what I have, the pipes are just longer than the tips of the skirt segs pulled back against the hull so they cant block the air flow,
The pipes are fitted every second air feed hole on the front

The diagram in the article shows a plough plane that looks almost the same shape as a Sev


Ian Brooks
24-Feb-10, 06:43 PM
I echo Johns surprise that manufacturers have not dealt with this issue. Some claim to, but few have, and it is a real safety issue. Especially when there are simple ways to fix it, which if incorporated at design stage would cost little or nothing and be utterly reliable.

Another simple and remarkably effective way to solve a plough-in problem was invented by Bryan White for his GP Cruiser - a very fast craft that could plough-in badly at high speed.

Inspired by the Sev curtain, he fitted a loosely fitted partition about 1/3 ways down the hull. This allows the front 'partition' to run at a pressure slightly higher than the rear partition - but note that the fit is not tight, and the 'partition' is well clear of the ground.

As the air is fed from a high pressure source (the plenum) any restriction to the airflow out of the front partition results in the pressure rising. Once a plough-in starts, the front skirts seal against the water. This creates a dramatic reduction in the air outflow, and the front partition pressure rises, preventing the plough-in from progressing to hull contact. This works a treat on Bryans craft.

The only issue is creating a robust partition that will resist the impact of the hull when landing. It needs to be able to fold back against the hull, being made of stiff rubber (conveyor belting) and tensioned by shock chords.

This solution is applicable to any craft with a plenum-fed skirt, either integrated or twin engine.


24-Feb-10, 11:22 PM
As John says the stepped hull has been around quite a while see 0.35 on this: http://www.youtube.com/watch?v=ZJH24Y3_Y9c didnt quite work at 3.00.

As Ian says the partition skirt system works on the GP, another GP owner will be trying a similar system this year combined with other ideas mentioned on here. :):) Philip

25-Feb-10, 06:47 AM
So I guess then that a step with a chamber forcing air like in the diagram will not work, or would it. I would think that at the hull drops and the bags close air off from the feed holes the chamber will allow that build up of pressure to escape down the chamber and into the step, stopping the water from grabbing onto the hull and thus allowing the front of the craft to recover without slowing down. With Bryan's plenum as a backup this could get rid of plough-in all together.

Or am I getting it wrong again?


John Robertson
25-Feb-10, 08:57 AM
You've got the step going the wrong way - it should step UP into the hull. Your version will probably make things worse (a well known commercial craft has a very similar setup under the hull :o). If you spend a little time with card and a tap and you'll see exactly what's going on - the basic principles are really simple.

As has been shown on this thread, there are many ways to reduce the effects of plough-in but the objective is the same with all of them - reducing or stopping the hull sticking to the water.

This actually brings us back to the origin of this thread - Christopher Cockerell - who started out trying to develop underhull air injection system on boats to reduce water drag :)

Trevs or Bryans methods seem to be good ways to cure the problem on an existing craft but it would be a lot easier (and cheaper) to fix the problem properly at the hull design stage.

25-Feb-10, 09:11 AM
I've been following this thread with great interest, but i think we still fail to solve the root of the problem of ploughing in merely cover it up with devices

that said i do not have the ideal solution, i've changed my mind on the inflatable sausages under craft creating different pressure 'zones', i have a 1960 publication with the idea clearly outlined and they have worked for me improving stability and resiting plough but steering is affected and more rudder is needed

but again it's covering up the root of the problem, i strive for a hull/skirt design that incorporates the pressure zones to stop the nose diving


John Robertson
25-Feb-10, 09:52 AM
You are correct to some extent Tony - it's all over by the time the the hull gets near to or touches the water - it's just a damage limitation exercise at that stage!

I can't see any way to avoid major skirt-water contact in the real world (hitting a 3ft wave head on with a low pressure hovercraft skirt/cushion can only have one outcome!). It's what happens just after the skirt is "wet" that seems to be the key to all this (and I don't have the answers either!). The segmented cushion "zones" (Sevs, Bryans, SRN4, etc, etc) increase the pressure at or near the nose as the bow drops (the bulbous bow skirt resists the drag force pulling the skirt backwards by deforming upwards) by moving the cushion centre of pressure forward. This produces an upward force at the bow and, if properly designed, should be large enough to overcome the downward force caused by the wetted bow skirt pulling the nose down. In other words the system should self-stabilise

It's still crucial that the under hull is designed to minimise the effects of water contact - no matter how good the cushion/skirt is at preventing the hull coming into contact with the surface it can never be 100% effective.

25-Feb-10, 10:42 AM
A great feature so far is at least 90% of the suggestions are suitable for hulls that presently have no built in stepped cavity and can be applied in Low Tech Mode with commonly available materials to any hull. The essence is hull lubrication that occurs naturally and automatically as a plough in develops followed by fast recovery solutions to the point hull contact becomes almost unnoticeable.

I think that at least one of either the partition or the tubes is necessary and that specific lubrication points and a cavity or two could only add to the benefit.

I cannot see any conflicts between the elements so far which means they could all appear together on a craft IMV. Keeping the front air feeds operating at normal capacity under all adverse conditions would possibly be the common theme that would help all craft. The number and spacing, diameter, shape, length, end details, fixing methods are all variables.

Using a partition to increase front cushion pressure sounds like it would assist both avoidance and recovery but does not guarantee the front air feeds remain working at all times. Using both makes sense to me.

Cavity steps appear possible made of a tapered triangular rubber section and glued onto the hull - nice and disposable.

Individuals applying these mods in a way that does justice to the available airflow and pressure will be the tuning element and variable degree of success achieved so an element of skill and application will prevail in the spirit of the game.

The rubber conveyor belting partition does multiple tasks IMV and I love solutions that cover the same problem two or more ways with the one piece of stuff. I like the way that it creates a step in an otherwise dead flat hull surface. Even as water is pushing it flat against the hull, just because it is there it is creating a small pocket space behind itself as well as increasing front cushion pressure.

A thank you goes to John for dispelling the myth that a picture paints a thousand words, re explanation of the surface tension flowing on a curved surface. That is how I understood it worked, some experiments are in order to test various surface shapes and textures full size in foam board and I will have to use a fast flowing water channel to do them.

Thanks to all for their contributions