[Hoverwing]

My UH18SP (have Wings , but haven't flown yet) would randomly and in any conditions, go into a * "high drag" or "plow-in" or "front bag collapse" or "whatever else we can call it" condition until I moved the craft’s center of gravity rearward (moved battery & fuel tanks). With my craft, it was just an annoyance since the front of my hull looks, and the craft acts, like a regular boat during these "high drag" events (rear bottom of lift duct to hull is also angled).
Darvis

*PS. I like the "high drag" description.

[sevtec]

Two types of plow occur, spontaneous or forced.

In a spontaneous plow, where a craft is trimmed for maximum speed for the conditions, that is with the skirt as dry as possible with the usuall craft attitude of parallel to the surface, a small disturbance can cause the bow skirt to contact the water and the bow skirt will then try to attach itself to the water surface, exasperating the problem as the nose of the craft continues down, more forward skirt gets wet, and in a "positive feedback" control condition the plow will continue until the hull bow strikes the water and the craft comes to a stop, sometimes an abrupt stop. Spontaneous plow is usually caused at craft high speed in calm water.

Hold a strip of skirt material right next to a flowing water faucet and one will see it move over into the water stream and try to attach itself, this is what causes a lot of the extra drag.

To counter such a plow condition, if it is not so quick that the operator can detect its initiation, weight is moved back, or an aft wing in the propeller slipstream is moved (trailing edge up, UH, SRN4) to gain a craft nose up force moment, or a system such as on a Sevtec is used. However, there is frequently not enough time to move the weight aft or move the wing before the plow is all over, and the operator has reconfigured his face on the helm or has done a half gainer off the craft's bow and is swimming in front of the craft. (Experienced operators gain a feel for an incipient plow and can react in time, sometimes.)

Sevtec solves the spontaneous plow problem by automating the nose up forces so the operator does not have to react. The cushion is divided into fore and aft compartments, and the aft cushion is charged by the lift fan, and the forward cushion is charged by air passing under a thwartwise skirt that divides the cushion into fore and aft compartments. As the forward compartment is lowered to the water surface, the lower pressure in the forward compartment automatically rises toward the main aft cushion, thusly restoring the craft attitude to level to the surface, as air flow out of the forward cushion is suppressed.

The forced plow is a different case, and is caused by water surface roughness simply being too great, causing a need for more cushion air that simply is not available. As a result the skirt contact with the water becomes higher, and that force couple as above noses the vehicle into the surface, whether or not the Sevtec system or any other system is employed. (If the craft was in rough water to start with, it probably will not be going fast enough to do a forced plow or would have been slowed by doing a forced plow.)

The solution is to slow the craft down before coming to the rough water, as would occur if one was running up a smooth river stretch and into rapids. Sevtecs are slowed with a braking system, a foot pedal or handbrake opens up a vent to dump air from the forward cushion only, controllably lowering the bow for the braking action. Such a control need be applied so lightly to produce useful slowing forces that only the air gap under the bow skirt is diminished, (the kill switch is faster even if the hull is lowered clear to the water surface, in an emergency.) (If the craft is going so fast that the bow skirt is blown against the hull underside, the forward cushion is at craft free stream dynamic pressure, and the plow brake will no longer be functional. Presumably at such speeds the craft will not be near anything, so controlled braking is unnecessary. One still has the kill switch, not very controllable, but powerful.)So far, the bow skirt seems to redeploy automatically when I have been in this region, so far!)

Another type of forced plow is when a craft is driven at a speed higher than normal on land or ice, and enters water at a speed above the craft capability.

Just about all well trimmed craft can plow, so the next step is to shape the hull so that the resulting hull plow is gentle. There are some examples where this aspect was never considered by the craft designer (Scat, of note, has a bow similar to the head end of a bathtub and very poor plow characteristics). Sevtec puts a planing surface on the bow that when the craft reaches a nose down attitude the craft has a boat like planing surface that is set up at the proper angle to plane on the water surface as soon as the craft bow strikes the surface. A craft will usually maintain its aft fully inflated cushion near the water surface and the craft nose down angle at hull strike can be established with this as a nominal nose down angle. The plow plane should be such that the forward deck of the craft cannot be submerged. (The Sevtec plow plane is also located aftwards so that the brake system is more effective as the plow plane can take more vertical loading.)

The fact that the Sevtec hull is sharp edged and angular is no accident, as those very same effects that attach the bow skirt to the water in the unforced plow can occur in the hull to water contact areas during a plow, and even on a small Sevtec, thousands of pounds downward force can be generated by contact of a rounded surface to the water. (I have had builders round the section between the plow plane and the bottom panel, such could result in a dangerous breakup of the hull, the downward forces could be so great.)

Barry Palmer, for Sevtec

[jarrob]

Excellent description Barry!

I have, however, experienced another kind of spontaneous plough that isn't included in your description. The circumstances were a little unusual. I was around 3 miles offshore in very calm seas (no wind and, at worst, a 2" swell!) and travelling at around 20mph or so. I then passed into slightly ripply water - not chop or a swell but no more than 2-3" ripples on a short wavelength (probably caused by tidal flow around some deep underwater object). The craft started to vibrate as it passed over these ripples - the vibration or bumping built up until the nose dropped and a plough started. This all happend in a few seconds - the craft self-recovered even though the plough was quite severe. It felt a bit like an oscillation had built up and the positive feedback eventually resulted in the plough.

Anyone else experienced anything like this?

This post has been edited by jarrob: 05 February 2008 - 05:07 AM

[HoverTrekker]

Excellent description Barry!

I have, however, experienced another kind of spontaneous plough that isn't included in your description. The circumstances were a little unusual. I was around 3 miles offshore in very calm seas (no wind and, at worst, a 2" swell!) and travelling at around 20mph or so. I then passed into slightly ripply water - not chop or a swell but no more than 2-3" ripples on a short wavelength (probably caused by tidal flow around some deep underwater object). The craft started to vibrate as it passed over these ripples - the vibration or bumping built up until the nose dropped and a plough started. This all happend in a few seconds - the craft self-recovered even though the plough was quite severe. It felt a bit like an oscillation had built up and the positive feedback eventually resulted in the plough.

Anyone else experienced anything like this?

John,
I've experienced this plenty on rivers mostly (in my Neoteric). The short wavelength ripples that cause the front skirts to bounce and tuck. I can always see them coming (if I'm paying attention) and slowing down a bit always solves the problem. The problem is more exaserbated if I have a heavier load (passenger). Heavier skirt material also contributes to the problem. Now that I've switched to a lighter PU coated fabric, I don't notice it too much. This seems to be mostly an issue with finger skirts. Pointy nose bag skirt craft like Starcruisers and UH craft seem to be unaffected by the small ripples. Interestingly my finger skirts do amazingly well in 1-2 foot chop just bouncing over the waves.

[kach22i]

The circumstances were a little unusual. ...............

Anyone else experienced anything like this?

No, that's a new one to me.

However once on Ford lake about one hundred large carp (fish) stuck their heads about six inches up out of the water, my hovercraft came very close to coming off cushion as I knocked them in their heads. I felt bad, but it was them or me.

[HoverTrekker]

No, that's a new one to me.

George that's because you only have 100 hrs in 18 years. You need to get out and hover!

[sevtec]

Excellent description Barry!

I have, however, experienced another kind of spontaneous plough that isn't included in your description. The circumstances were a little unusual. I was around 3 miles offshore in very calm seas (no wind and, at worst, a 2" swell!) and travelling at around 20mph or so. I then passed into slightly ripply water - not chop or a swell but no more than 2-3" ripples on a short wavelength (probably caused by tidal flow around some deep underwater object). The craft started to vibrate as it passed over these ripples - the vibration or bumping built up until the nose dropped and a plough started. This all happend in a few seconds - the craft self-recovered even though the plough was quite severe. It felt a bit like an oscillation had built up and the positive feedback eventually resulted in the plough.

Anyone else experienced anything like this?

You were at low power and therefore you did not have much air under the skirt. The ripples were requiring more air, and possibly you entered a wave train that caused some sort of oscillation in pitch. A two to 3 inch ripple can be an ideal situation for low drag, the solution for getting into such a problem amy have been application of power to get cushion air volume up.

On the plow post, I forgot to mention the part craft inertia plays in a plow, rather than just the thrust and drag force couple. inertia and center of gravity height usually ends up producing vastly more nose down moment than propeller thrust as the plow progresses, while the plow is induced by the thrust alone, as the craft is just barely starting to slow.

Barry Palmer, for Sevtec

[sevtec]

No, that's a new one to me.

However once on Ford lake about one hundred large carp (fish) stuck their heads about six inches up out of the water, my hovercraft came very close to coming off cushion as I knocked them in their heads. I felt bad, but it was them or me.

I once was going directly into a morning sun, to discover a 3-4 foot dorsal fin aimed right at me. I was at a 20-25mph cruise and did not see it until it was very close. I put the craft into a 45 degree side slip and missed the fin by maybe 3 feet. I was so close that I noted some markings on the fin, as it sliced by. I tried to identify the creature from texts, but it became clear that the "experts" must have never got that close to such a critter alive in the wild. It would have been hard to describe the event as a plow, or a crash, had I hit it.

Barry Palmer, for Sevtec

[Rocket man]

Following large boat wakes and powering over them in a StarCruiser will also create a loss of lift that will sometimes lead to plow in at speed. I think of it as a teeter totter effect. When topping the crest the craft balances on the wake and causes air to bleed out that can not be replaced fast enough.

[kach22i]

I once was going directly into a morning sun, to discover a 3-4 foot dorsal fin aimed right at me.

Same story?

http://members.aol.c...over/craft.html

While I still had excess speed, "Whale!" I yelled at Reed, as I was standing (as usual) at the helm and he was seated. A whale was taking us head on, coming out of the suns glare, grey in color, and it would have been all right as its body was only a foot out of the water, but for a dorsal fin, a HUGE dorsal fin, extremely fine in section and standing a full 4 feet above the whale's body. I threw the sev into a quarter sideslip, to straighten it out as the fin sliced by about four feet away. I carefully noted very subtle light markings on the aft base of the fin and graceful fin profile shape as I passed by so I might identify the creature upon getting whale literature.

The identity was clearly that of a mature sei whale, probably about 50 plus feet in length. The experts who wrote the book did not mention the light markings, nor did their illustration show them, but I suspect my close encounter of the finny kind was much closer than any they might ever have had. Curiously, the whale did not appear to react in the slightest way to my passage. While the sev does not put noise (in human aural range) into the water, the boat I had just passed was nearby and certainly was putting out noise.

[jarrob]

Following large boat wakes and powering over them in a StarCruiser will also create a loss of lift that will sometimes lead to plow in at speed. I think of it as a teeter totter effect. When topping the crest the craft balances on the wake and causes air to bleed out that can not be replaced fast enough.

I've had the same thing happen when driving into big standing waves. The cushion loses lift as you crest the wave, the nose drops like a stone into the trough and the hull gets whacked by the follwing wave peak - nice!


You guys probably aren't familiar with the River Tay stone shark - it's dorsal fin (see pic just behind the hovercraft)) isn't quite as big as Barry's but it's a damn site harder. That has to be the worst obstacle I've come across on water - only 2" thick and virtually invisible on still water (no tell-tale water disturbance). It could have sliced a craft open from bow to stern. Luckily everyoen missed it -although there were a couple of close shaves

 stone_shark.jpg (39.95K)
Number of downloads: 24

This post has been edited by jarrob: 05 February 2008 - 12:00 PM

[ken]

Ripple: I have seen this several times, again on rivers since that's mostly what I drive on. They can cause a lot of drag, and could cause a plow-in. Usually I just tweak the trim wing so the nose comes up, lean back a bit and get a little spray. The craft will slow down a bit, and then everything gets back into equilibrium.

Barry, that was a really good explanation of plow-in.

One thing to be said for hovering on rivers is that the obstacles, while being unforgiving at times, are almost never vindictive. I can't imagine having hit that dorsal fin only to see the huge tail come up and get me.