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Hovercraft SOG question.......

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Hovercraft SOG question.......

Old 21st Jun 2020, 11:29
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Hovercraft SOG question.......

I'm looking at acquiring a small "sports" hovercraft (350KG payload) for use on longish trips (400 km) on a frequently very shallow, very fast-flowing continental river.

I can't get my mind round the matter of whether the SOG will be faster when travelling with the current (typically 6 - 10 KPH) than against it.

The immediate thought is that the craft is flying, so airspeed is what counts, and what the surface is doing is irrelevant.

But it's flying in ground effect, so maybe surface movement does affect the hovercraft. The cushion of air held within the skirt is pressing on the surface; would it not move with the surface?

It's a conundrum. Can anyone help? What is the scientific answer?

To avoid accusations of introducing aviation content, let's say it's a marine matter. Per might know the answer.
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Old 21st Jun 2020, 11:46
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I suspect this thread will turn into a version of the "aeroplane on a conveyor belt" conundrum!
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Old 21st Jun 2020, 12:16
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Having "flown" a small hovercraft, the main two influences are the wind and the slope of the terrain. You also learn a great deal about inertia, and the absence of directional stability when not "flying" at speed, pretty quickly. Definitely an acquired art being able to control one safely, especially an integrated craft, where the throttle controls both lift and propulsion. I found it a lot easier "flying" a larger craft, with separate lift and propulsion motors. The golden rule is that if it starts to get away from you (and it will) kill the lift and come down with a bump on the skids. As long as you're not going sideways at the time this is pretty much always safe.

As for the effect of the surface you're travelling over, the answer is that it's negligible. At low speeds over water the skirt can pick up a bit of water for a short time as you transition, but giving it a bit of welly soon blows that out. Big waves can make the ride a bit choppy, as the craft will fly over them, trying to maintain a near-constant gap around the peripheral air cushion leakage area, so a small craft in a chop will buck up and down a fair bit.

My recommendation would be to try and get the largest craft you can. Something as small as a one or two man integrated craft is going to struggle, whereas a large cruising type craft, with separate lift and propulsion systems, will be a lot easier to handle, and probably have more hull clearance too, especially at low speed.
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Old 21st Jun 2020, 12:20
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The cushion of high pressure air leaks out at a steady rate, so which side does it leak out like a balloon?
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Old 21st Jun 2020, 12:30
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Originally Posted by lomapaseo View Post
The cushion of high pressure air leaks out at a steady rate, so which side does it leak out like a balloon?
Varies a lot. Most craft now use segmented skirts, that have multiple inward pointing "jets" of lift air around the periphery, so that there's an air curtain effect helping to hold the air cushion in place. Any slight difference in attitude between the craft and the ground or water underneath results in more lift air spilling out of the point with the biggest gap. A segmented skirt tends to be a lot better at conforming to the contours though, so minimises this effect. A simple bag skirt can get unstable without any sort of external influences, as it sort of "burps" air our from one side, making the craft rock or pitch, which then cause a bounce that lets air out the other side. I'm not sure that there are that many bag skirt craft around, though, as the simplicity of the skirt design is offset by the relatively poor efficiency as we ll as vulnerability to damage. You can lose a few segments with a segmented skirt and the craft carries on behaving pretty normally, as the adjacent good segments just expand a bit to fill in the gap. Rip a bag skirt and that often means a serious loss of lift.
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Old 21st Jun 2020, 13:23
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We had a lad in the year below me at school who designed and built a hovercraft in metalwork... OK it was Engineering Design really, and he rapidly got very proficient at flying it around the school precincts. A very clever and likable individual. I don't know what he got up to after leaving school but I do know that he died fairly recently having been in a coma for a number of years following a climbing accident. So sad.
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Old 21st Jun 2020, 13:34
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Originally Posted by treadigraph View Post
We had a lad in the year below me at school who designed and built a hovercraft in metalwork... OK it was Engineering Design really, and he rapidly got very proficient at flying it around the school precincts. A very clever and likable individual. I don't know what he got up to after leaving school but I do know that he died fairly recently having been in a coma for a number of years following a climbing accident. So sad.
My introduction to hovercraft was at school, too. We built a Daily Express Air Rider hovercraft, a design that was promoted for educational projects. The wood work lads built the plywood hull, us metalwork lads built the engine mounts and all the hardware and the girls in needlework made the skirt*.

We had three Sachs 200cc two stroke engines donated to the project, salvaged from old Invacar invalid carriages. We used one of these engines for lift and two for propulsion. We finished it, complete with a paint scheme in the school colours, ready for the end of year masters versus sixth form cricket match, and used it to transport the sixth form team across the playing field (it struggled - not enough lift power), together with a 1938 Morris 8 restoration project that the upper sixth had worked on. I left school then, so I don't know what happened to the project. I think the idea had been for several schools to build these craft then compete in them. It had a simple bag skirt, probably because this was 1969, so perhaps the benefits of segmented skirts weren't yet that well known. IIRC, the early SRN craft used a sort of hybrid skirt, a bag feeding smaller segments fastened to it. I'm pretty sure that the 100% segmented skirt that's used almost exclusively on small craft now either hadn't been invented, or hadn't been refined.


* yes, I know this is sexist, but it's a fact that back then boys did wood and metal work and girls did needlework and domestic science.
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Old 21st Jun 2020, 15:00
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My son and I built a small hovercraft. This one doesn't have sufficient thrust or lift to operate like the OP intends. But I'd like to do my part on the thread drift:

Someday I'd like to get it on water and see how it does.
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Old 21st Jun 2020, 15:02
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Originally Posted by lomapaseo View Post
The cushion of high pressure air leaks out at a steady rate, so which side does it leak out like a balloon?
You have some control over this by shifting your weight.
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Old 21st Jun 2020, 15:40
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Originally Posted by IFMU View Post
You have some control over this by shifting your weight.
Great! that answers the OP questions as well

Just like I do when farting while sitting ,only mostly it's the people nearby that actually move the most
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Old 21st Jun 2020, 15:45
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We watched the Joint Service Hovercraft Unit doing trials at RAF Ballykelly in 1965. As VP said, its low speed directional control was tricky. We were parked on the end of the runway. They mounted the runway at the lough end and hovered down the runway, or should I say lurched, to reach their dispersal behind us. At one point it started vere off so the chopped the lift and it flopped then they restarted and made it safely passed us.
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Old 21st Jun 2020, 16:50
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You might get some idea from the mini, lightweight canal boat which seemed to have a hovercraft-style skirt that could be towed on a trailer ... I believe the make of the one I saw was called a Toby, but couldn't find anything to reference it by googling it.. Watching one prepare to enter the lock we were just vacating, it went sideways across the pound at the speed of light with no friction and no obvious brakes. Mind you this was in the 1980s and technology has certainly moved on since ... and what I saw is either a museum piece or extinct.
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Old 21st Jun 2020, 18:18
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Originally Posted by lomapaseo View Post
Great! that answers the OP questions as well

Just like I do when farting while sitting ,only mostly it's the people nearby that actually move the most
Exactly! Excellent explanation.
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Old 22nd Jun 2020, 16:03
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Originally Posted by old,not bold View Post
I can't get my mind round the matter of whether the SOG will be faster when travelling with the current (typically 6 - 10 KPH) than against it.

The immediate thought is that the craft is flying, so airspeed is what counts, and what the surface is doing is irrelevant.

But it's flying in ground effect, so maybe surface movement does affect the hovercraft. The cushion of air held within the skirt is pressing on the surface; would it not move with the surface?
A hovercraft hovering stationary over water creates a depression in the water beneath it due to the increased air pressure within the skirt that supports it. Not surprisingly, the mass of the volume of water displaced from the depression matches the mass supported. So a stationary or slow moving hovercraft over water behaves (somewhat) like a displacement hull.

As the craft picks up speed beyond the hull speed (and there's a concept boat people love to argue about) of the equivalent hull, it ends up ahead of the depression it creates, and "flies" above undisturbed water. Given it has no physical contact with the water, the drag should be the same regardless of the movement of the water, provided that the relative velocity of craft versus water is fast enough to exceed the "hull speed".

So only the (ground) speed of the transition from sitting over a depression in the water to moving ahead of that depression should be affected by the speed of any current. Going upstream, the hovercraft will climb out of its non-contact hole in the water at a slower ground speed (but the same water speed) than going downstream.
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Old 22nd Jun 2020, 17:46
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Originally Posted by IFMU View Post
Exactly! Excellent explanation.
Thanks

I tried stuff like that in TV interviews but they always edited it out
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Old 22nd Jun 2020, 18:27
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Thank you, Nonsense, as well as all the other contributors. That makes a whole lot of sense, and is what I'll take away from the discussion. Re hull speed; I've just swapped a RIB with 115HP for a decked fishing boat that we can sleep aboard if we feel like it. It is the same length as the RIB, 6.6m, and has the same Suzuki 115. But I reckon its hull speed is about 1 Kt below what it takes to get it on the plane, which can be frustrating.
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Old 23rd Jun 2020, 04:20
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Originally Posted by old,not bold View Post
very fast-flowing ... river. ... the current (typically 6 - 10 KPH)
Maybe check how much slope the river has and what it's effect on the hovercraft might be.
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Old 23rd Jun 2020, 07:09
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Originally Posted by old,not bold View Post
........ Bdut I reckon its hull speed is about 1 Kt below what it takes to get it on the plane, which can be frustrating.
Found a cure for that when on an R&IT week on the Norfolk Broads way back (Resource & Initiative Training = a weeks "jolly" - just to explain!). Had 2 groccle boats. Luckily had 2 Marine Engineers with us. Boats chugged out with speed limit rules ringing in our ears from the boatyard owner. Opened up throttles once out of sight - only slightly faster chug! Stopped mid-stream while MEs vanished below. A couple of minutes later started up, throttle fully toward - bingo! Ok, we never got it on the plane but, by ek, we rendered the speed limit redundant!
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Old 23rd Jun 2020, 12:15
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Originally Posted by old,not bold View Post
Thank you, Nonsense, as well as all the other contributors. That makes a whole lot of sense, and is what I'll take away from the discussion. Re hull speed; I've just swapped a RIB with 115HP for a decked fishing boat that we can sleep aboard if we feel like it. It is the same length as the RIB, 6.6m, and has the same Suzuki 115. But I reckon its hull speed is about 1 Kt below what it takes to get it on the plane, which can be frustrating.
My 6.5m yacht has a 5hp outboard. Hull speed is roughly 6 knots (11.1km/h); I find 10km/h a useful and convenient trip planning speed.
The fastest I've ever seen on the GPS was a bit over 11 knots. The boat was unambiguously planing, and the motor was definitely not running.
I would have though 115hp would be enough to make even a floating 6m shipping container plane!
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Old 23rd Jun 2020, 12:38
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Originally Posted by jimjim1 View Post
Maybe check how much slope the river has and what it's effect on the hovercraft might be.
Assuming a non-integrated craft, then with the lift engine running and the craft up on the cushion, even the slightest slope will cause the craft to start to drift down slope. The camber on a road or runway is enough to cause such a drift on a windless day, for example. When underway then this sensitivity to the slope of the terrain is easier to control, but is always there, along with the effect of the wind.

With regard to hump speed, and the ability to climb out of it, then this is very different to the way that wave length affects a displacement boat. A hovercraft at very low speed over water certainly displaces a similar mass of water under the hull as the mass of the craft, but that's pretty much where the similarity with a boat hull stops. The shape of the displaced water under a hovercraft is a shallow and even depression, and there's no real speed/wavelength/waterline length relationship. Getting over the hump in a hovercraft is not much like getting on the plane with a boat, as there's no dynamic lift from the water, really. All that has to happen is that there needs to be enough propulsion force to get the craft to climb up the fairly shallow hump formed at the edge of the displacement depression, and this is rarely more than a few tens of millimetres deep for a small light hovercraft, and overcome the inertia of the water that wants to try and refill the hole. Once over the hump then the propulsive power needed to maintain a given speed is pretty low, far lower than that needed to keep a similar size boat planing.

I've "flown" directly from land into water at a modest speed, maybe 15mph or so, and there is no intermediate displacement mode, the craft will never really create a displacement "hole" and will be over hump speed before it touches the water. The biggest issue when transitioning from land to water, and vice versa, is usually the slope of the junction between the two and the wind. Any cross wind can make the transition challenging, especially if there's not a lot of space to drift across. The bigger commercial craft were all fitted with propulsion systems that could rotate around to provide thrust vectoring, primarily to allow them to handle crosswinds. Smaller, light hovercraft rarely have this feature, so can be a lot trickier to manoeuvre at low speeds.

There are a few places that hire out light hovercraft and provide training. Well worth spending a day at one of these places and getting a feel for the skills needed to "fly" one. The smaller craft are a bit like riding competition sidecar motorcycles, in that weight shift is essential. You have to lean right forward when accelerating, as well as lean hard into turns. Until you have a go, it's hard to appreciate just how much weight shift is needed to get good control. Larger, cruising type hovercraft are far less weight shift sensitive, although they do need a similar degree of C of G adjustment to fly well.
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