Amazon river, Loadum up

By the way, I threw this problem out on a few other forums and you guys are the only ones who have tried to help. Just wanted do say that I appreciate it.

 

hovercraft float.

 

When a hovercraft breaks, it becomes a not-very-good boat. They do not sink unless you have breached the hull in a way that would sink a boat. The hull shape is not good for planing or even motoring in a straight line, and the skirt has a lot of drag, but it floats.

Noise, it is louder than a boat probably but if you use good mufflers this one will probably be fairly quiet at least from shore. My UH-18sp, people have commented that they don't even hear me coming until I'm within sight of the ramp and in the no-wake zone. Not all hovercraft are quiet, but they can be easily built to be quiet. You want large props turning slow for that.

You will definitely want to think it through before you try. The lack of others around you can look at would be a big problem for me, if I had never seen one before.

You would definitely want to park it for the night on dry land somewhere, that may be a problem for you. You know the places you would stop, I have never been to South America, let alone the Amazon.

 

The different payload is a sort of problem. The higher the maximum power output of any engine, the narrower the senseful band of operating conditions. Read: you´ll have no benefit by installing the EFI´s, the contrary, because as you mentioned, you have very different op. conditions.
I am not in the hovercraft business, but still think it´s worth a thought.
The weak point is the skirt as kroberts pointed out, but I do´nt see that to be more of a issue than the very weak outboard!
I have seen many outboards loosing their props due to debris in and around Manaus.

Regards
Richard

 

Why can't the skirts be rigid inflatable tubes instead? Then it can always float and could even have an outboard kiker engine. This is the idea I'm pursuing. Tubes can me made of very durable materials with an extra skid layer on their bottom and divided into 6-10 cells so one puncture or tear will not sink the boat. If it adds to much extra weight, fill the tubes with helium. Race cars fill there tires with helium. That amout of helium in say 22"-26" tubes or collar will offset the weight of the empty tubes. Just an idea I'm trying to find out if it works. I have seen military hovercrafts on the net done this way.

 

Inflating the skirt like an inner tube makes in no longer be a hovercraft.

The hovercraft without a skirt attached should have all the hull integrity and flotation of a regular boat. It will float unless you have a big hole in the side or bottom.

The skirt is not what supports the hovercraft at all. It is only a seal with the ground, and must be soft and able to flex without pressing significantly against the surface. It is open to the atmosphere in two places: The drain hole in the back, and the feed hole at which point it is right behind a lift fan.

The hovercraft needs obstacle clearance just like a car does. If you want to go over a rock or bump, or go up the ramp, you have to be able to do so without high centering on the obstacle. The bigger the craft the more clearance you need.

In the 60's hovercraft did not have skirts. They used an immense amount of power to make an air curtain or peripheral jet to contain the air pressure. In some cases, this was hundreds of horsepower, and you got a few inches of visible air under the craft. The more gap you needed the more power it took to create that gap.

Now they use a skirt of some sort, there are several kinds. The skirt provides a barrier to keep the cushion air under the craft. There is still a visible air gap, but it's more like 1 cm or less. The skirt tries to act as though it were air to any hard obstacle, and like a barrier to the air under the craft.

The pressure under an average recreational hovercraft is around 15 lbs/square foot. That is about 0.007 bar. This is less than the pressure exerted by a seagull standing on one foot.

The pressure in the bag on a bag skirt type craft is slightly higher than that, about 1.1 times the pressure under the hull. The bag must be able to breathe, so that when an obstacle is encountered the bag conforms to the obstacle rather than trying to buoy the craft over it. The air pressure under the hull is for buoyancy, the bag is only a seal. When the bag presses too hard on the ground, it's now like trying to drag an inflated boat across the beach, where it should be mostly frictionless.

If race cars use helium in the tires, they must do it for some other reason than buoyancy. Helium in a huge balloon only has minimal lifting force. http://www.balloonsdirect.com/weather_balloons.htm?gclid=CPuwx7XMspkCFSBinAod3xQa6Q

There is a hovercraft (http://www.hovercraftsales.com/) which uses what they call a permanently inflated skirt, but this is not what comes in contact with the surface unless you land. There is a finger skirt underneath that which conforms to the surface. They trade a lower hover height and the permanently inflated part is more like a semi-rigid hull.

 

Thanks Ken, What about soft pressure tubes say inflated at 4psi under and at the top of the skirt? Helium is used in tires because the molecues are larger than air and in hot tires do not excape through the tire material as much as air. It is not a weight saving thing. What got me thinking of building one like a RIB was a video of a kid with a small tube raft. He turned it over and cut out the botton then sealed a piece of plywood to the tubes and drilled a 2" hole for a shopVac hose and it lifted up W/O any weight on it. I guess you could put tubes topside for 100% flotation. Darn, I hate to let go of this idea. Stan

 

If you would change that to 0.4 psi then maybe for the top of the skirt, if you wanted to make a more complex skirt design.

On the bottom, you want as thin and flexible a material as you can get. There is a balance you have to find between flexibility and wear resistance.

Too thin and it tears or wears through.

Too thick and it will not conform to the ground, which means too much air escapes under the skirt and you need more power for lift. If your requirement exceeds the power of your lift system, then you lose lift and land. Also, a crease will stay in the fabric in spite of inflation, and it will wear through faster than it should and you wind up with more holes and a shorter skirt life.

If it's just right, then it will conform reasonably without pressing too hard at the obstacles. The goal is to reduce friction as much as possible, and a lot of things come into play.

If you are interested in skirt design, some significant research has been made. This one is a bit math-rich for me, but this book might help:

Theory and Design of Air Cushion Craft (ISBN: 0340676507)
Yun, Liang Bliault, Alan

 

Thanks again, does the under side of a hovercraft hull have a design that is unique for the lifting abilty of the lifting engine? I have looked at many pictures but cannot find one where the hull is flipped. Should it be a V or a more flat surface? Stan
http://www.jameshovercraft.co.uk/hover/images/craft/tiger12/w59s.jpg This looks like a RIB to me but with a skirt
http://www.jameshovercraft.co.uk/hover/images/craft/tiger12/HIGH DEFINITION Pictures 005.jpg

 

A hovercraft has a bottom designed to facilitate lift air distribution throughout the cushion area, and plow planes to prevent submarining whether you plow in sideways or forward. Generally not so much for backward plow-in.

Each manufacturer designs them differently, and each gets different results when plowing in. My favorite is excellent for plow-in behavior, On my old racing hovercraft I could force the nose down at 60 mph (not knots) and spin it around and pick it back up at any time. My new one is a 6-place and much more stately, I could probably do all there is generally not enough room for that kind of stuff, and the consequences of something going wrong when you get crazy on an automotive-powered machine are too big for one guy to deal with. Other manufacturers you plow in at 35 and you can count on being ejected or injured.

Universal Hovercraft includes a step so that the water does not stick to the hull when you plow in. So it's like a seaplane pontoon, only the high part is in the middle and there is an angled part all the way around.

Like on a boat, hovercraft plans are worth their weight in gold. Buy a good set of plans and you will save several times that much because you don't have to tear it apart and try again. People can and do design their own, but usually they start with a decent set of plans so they can learn what is important.

 

Stan, thats just extra buoyancy for offshore use, shown in the picture you linked to.
Richard

 

To come to a conclusion (at least for me) here:

A Wood/Epoxy built semidisplacement boat using local timber, plank on laminated frames, with a heavy glass Ep. sheathing both sides, should fit the needs and stand the harsh conditions. Driven by a 140 -160hp diesel it should be possible to meet the requirements. Though not faster than the minimum requirement. Done in Iquitos I am shure it is possible within the price range mentioned.

A OTS outboard craft as the>>>25 foot heavy aluminum boat with vinyl roof, 60Hp 2-stroke Yamaha<<< ravencry used, does the job (as we know), but is prone to puncture, fatigue and has a vulnerable propulsion too. And it is not economical to operate

Logs, rocks, debris, flats, weed, sandbanks, rapids are an issue for both boats, as for every boat on duty on the Amazon river and his branches.

A hovercraft 24´of the design Ken recommended, is much faster than the boats, much more economical to operate, relatively easy to get hammered together with material of local sources (except the Prop/s and skirt). Moreover it is cheap to build using second hand car engines.
A hovercraft stays afloat if the skirt is kaputt, a boat does´nt if punctured.

Logs and rocks are still an issue but the other obstacles are not as much.

The apprehension of failure "in the middle of the river" is as valid (or not) for any sort of vessel.
Repair / Service problems are in my opinion much more serious if you have a broken outboard, rather than a broken "Toyota" engine in the middle of nowhere!???!
The fear to have a sort of "exotic" vessel I cannot share, I would see that as a advantage. Many poor people are living at the river banks as I remember, they might be more attracted to make their next weeks living by stealing a outboard, than by guessing what is of some resale value in such a "flying vacuum cleaner" monster.

So, if that was my job to transport the gold diggers to their claims, I would get me those plans, make shure that poor Ken will assist me during the construction period, and hammer the junk together.

If cruel life shows it was a misdetermination, sand down the silver metallic paint, spray it camouflage and sell it to the local drug supply ****** for twice the cost, there are some.
Then rent a >>>25 foot heavy aluminum boat with vinyl roof, 60Hp 2-stroke Yamaha<<<

Kindest regards
Richard

 

Richard brought up an interesting point by PM.

There are constraints on hovercraft which boats don't have, and constraints on boats which hovercraft don't have.

This post is something of a brain dump. I am not trying to toot my horn here, I am simply trying to give an accurate representation of what a hovercraft can and can't do. It is in the interest of full disclosure.

A hovercraft can go in some places where nothing else can go except a helicopter. For example, a mud flat that stretches on for miles, with 6 feet of mud in it. You can go out on that with no problem, especially if there is a skin of water over it or it is really soupy. If you go out on that, and you break down, they will have to send a helicopter to get you out, or another hovercraft.

Another situation is if you go onto flooded land with very little water, you may not even know you're on that little water, and if you break down you are again stranded. Hovercraft typically do not use sonar because they are generally driven up onto shore without pause, and the sonar transponder will not like that. I have often been on a river in flood stage, and have sometimes lost the river because its course was not evident. I wind up in some guy's field, and have to work my way back out to the water course.

That's another thing. Flooding is an extremely hazardous thing for boats, but not nearly so bad for a hovercraft -- unless you break down. Again, in that case, the skirt is hanging underwater and is more likely to either catch an obstacle or, if you get swept under a bridge or tree, you can be flipped more easily than a boat would be flipped.

Another situation is rapids or low-head dams. As long as the skirt can conform and you have the thrust to make the slope, you can go up or down rapids easily, you slow down a bit and make sure you won't jar the passengers and go, no big deal. If you lose power or have some other mishap on or just above the rapids, you are worse off than you would be in a boat because of the bottom hull design. Hovercraft operators often forget that low-head dams are a major cause of death in inland rivers.

The problem is, the places a hovercraft goes without concern it does not fuss about. I have been out of boats for so long I may have forgotten some of the things a boat can't do. I've gone out on a mud flat, came back fine. I have jumped over rapids or low-head dams, then turned around to go back up so I could jump back down again.

Depending on the nature of the mud, the hovercraft may have problems too. A bag-skirt craft would have problems if the mud is thick, wet and sticky. The type of mud that when you walk through it your feet get too heavy to move, with an inch of mud or more sticking to your shoe. This type of mud will simply grab the skirt and not let go. The good part of this situation is you will probably not be stranded, because you won't get very far into that type of mud anyway. You may be in for a very messy extraction, but you can probably get out.

If you have a large area of reeds or brush, you will probably not get through even if it is only a few inches high. A hovercraft relies on the skirt for lift, and the lift engine or lift fan, if it's a single engine design. There is only so much air the lift system can pump under the craft, and the pressure needed to pick the craft up is based on the weight of the craft. If the surface you are going over has lots of air spaces for lift air to get out and has hard protrusions which force the skirt up, the air gap will become large by artificial means, and if the lift system cannot keep up with it you will land and be unable to get back out easily. An air boat may power right over the reeds or brush, a hovercraft cannot necessarily do that.

Take a look on the net for anti-hovercraft barriers. They are similar to an STD (severe tire damage) barrier for cars. They tear the skirt off the craft. If you try to go over anything like that, a particularly nasty thorn bush for example, or a sharp tree branch pointing at you, or a piece of metal or similar, you will lose your skirt. In my opinion, you would not get a boat through there either.

A hovercraft may have trouble with wave action which would not bother a boat of the same size. I think this is mostly a driver skill problem, but I am not a "big water" person anyway, so use your own judgment and look at the pictures.

The critical parts of a hovercraft in that respect are:

1. The inlet to the lift and thrust systems. Generally you can suck in spray or even a wave, but if you take "solid water" in you will very possibly damage the lift fan, stall your engine or similar. Wood props and fans are more durable in this case, and once you understand the process they are easily made locally.
2. The bottom of the lift fan. In a no-flow bag skirt design such as the Universal Hovercraft products, you can look through the lift duct and see water. If a wave comes up and makes hard contact with the lift fan, that is a "prop strike." A prop strike can stall the lift fan (and engine if direct drive) or shatter the prop if bad enough.
3. In a full-flow bag skirt or a finger skirt, the wave can enter the lift system from the top and will have to wait to get out through the feed holes for the finger. That can stick you to the water very hard, and possibly throw your passengers and payload around pretty roughly, depending on how fast you were going.
4. The back of the thrust prop. This I think is the equivalent of free board on a boat, where water rushes over the back of the hull. If that water hits the prop hard enough, it can shatter the prop.

In the interest of fairness, there is another North American design that should be considered. The Sevtec brand claims to be more of a "big water" design. It is not as fast, but may handle wave action better. I have never driven one or ridden in one, so I have no experience directly. It is a reputable brand. The Sevtec founder recently retired, I believe. I do not know who currently owns the business, but it does exist.

In my opinion, a lot of these problems are solved by common sense. If you are in waves which might swamp the craft, then slow down. If a huge boat wake is coming toward you, then slow down and turn to quarter the wake in a way that least jars your load. If you got the job because you are the only one who can get there, then tell people where you will be so they can look for you, or consider not going without backup support. Make sure you have a radio with good coverage of the area, and that somebody is listening to it.

A hovercraft differs in how it handles wave action than a boat does. You have the air cushion area, meaning the top view footprint of the craft's cushion area, and the height of the cushion, and the air reserve under the hull due to the hull shape.

When you hit a wave, the skirt is instantly pushing on it, but as the wave comes the cushion air and the bag air escape through the lift inlet. This is by design. Then the wave hits the hard hull, and is jarred like what would happen if it hit a regular boat hull. I think it would be jarred less, but in my 10+ years of hovering I have never really been in that much trouble on the water.

Trying to think here, if a boat's wave handling were a mass on a spring, then I think a hovercraft would be a mass on a spring on a mass on a spring, with the smaller mass in the middle. The hovercraft will react less violently to an initial wave because of the skirt, and due to that fact will react strongly after the wave makes contact but AFTER a regular boat hull would have reacted. So I can see where a large wave which would cover the hull might be more directly hit, where a boat hull might ride up the wave better.

Keep in mind that I'm a recreational hovercraft user, who has never hovered on any ocean or in a bad storm. In the event that I see weather coming, I generally turn around and go very fast in the other direction. I have seen small chop (estimate slightly more than 1 foot chop, maybe 30 inches from crest to trough) with an approximately 8 foot wavelength, in rain and wind. This on a 12 foot long craft with enough speed to blow over under its own power. The craft had relatively poor characteristics for rough water, but if you are paying attention and driving like you would drive a boat in that water, it is manageable. I had waves continually entering the lift duct, but slowly enough that they did nothing but spray water around. I had waves entering the thrust duct from front and back, but likewise could keep the volume of water low enough to not cause permanent damage. Keep your priorities straight and you can get through. The critical bits are slightly different than they would be on a boat, is all.


The maintenance, I try to do a sort of pre-flight inspection every now and then. Before I go out, I feel around under the skirt looking for holes. They can be patched fairly easily, and are best caught sooner. It is a pain until you figure out how best works for you, and then it's just maintenance.

I always park on shore unless I must park in water. I pull the drain plugs and make sure there is no mud or leaves plugging things up. I make sure there is enough slope to drain the craft out, and scoop out any debris. Every so often I pick it up and look underneath for worn parts, rotten wood or missing landing skids, or any other type of damage. This can be done with a hoist, or with a log on a rock to tilt the craft up.

Every now and then I take the skirt down and look at the hull that way. A lot of punctures come from hitting a rock at speed, and that would happen on the plow plane.

The parts recommended for a Universal Hovercraft are what would normally be used on farm equipment or any other general industrial design. They take some maintenance, like greasing bearings or whatever. Keep on top of it and you will generally not have a problem.

Looking back over this massive post one more time, you will see that most of the problems come from a failure of the craft. Perhaps the superior ability that you get when the sun is out and the birds are singing makes the failures that much more bitter. Generally, those things which can be fixed on the river can be fixed with a very small set of tools. I bring a screwdriver, some skirt screws, some drywall screws, some safety wire and a handful of wrenches. And some skirt material, skirt glue, lots of zip ties and a multipurpose plier. And a knife. The most capable hovercraft driver I know is Bob Windt, who is the founder of Universal Hovercraft. He thinks I bring too many tools. He can fix anything with a screw, a piece of wire and a stick he found floating in the water.

PS
I guess there is still more. I don't intend to overload this thread with a bunch of hovercraft information, if a moderator wanted to move these posts to a separate thread that would be fine with me, although it would probably make this thread harder to read.

I run inland water. More than that, I tend to drive on fairly shallow and winding rivers. I do not approve of drunks in control of boats, so I go places where they are unlikely to be. My river encounters are more often a fisherman standing in the rapids than they are another boat. Water which is too shallow for a canoe or inner tube is just fine for me.

I know a lot of guys who also run inland water, not many who run where the big water is.

The biggest complaint I hear from these guys is about wave handling. However, I think the waves they see are a barge wake or similar, on a very small craft, and they are probably going around 40 knots. They are cruising along on a glassy-smooth surface, they see the wake and just hit it like a lawn dart at the same speed. You need to slow down and hit it at a rate you could handle a mound of dirt of the same size and shape.

Another problem I hear about with regards to wakes or waves, is the angle to hit it at. Coming at it straight on will possibly submarine the bow, and then you are airborne on the other side to submarine even worse when you hit the next wave. Similarly, trying to parallel the wave and slide over it sideways will cause your moment of inertia (roll) to be so low that you will rapidly flip to the side, throwing your passengers around and possibly even flipping the craft. I hit the wave at an angle, and while I can drive it I don't really know what angle it is. I also SLOW DOWN enough to see what the next wave looks like before I have to hit it.

Another problem seen here is wind. It causes several issues.
Many hovercraft have a problem going down wind. If you have a thrust duct, the craft will weather vane strongly, always pointing up wind. You can go up wind easily, and sideways with some difficulty. You crab like an airplane, which makes some drivers nervous. There is nothing wrong with crabbing. Going down wind, you have problems and the wind keeps trying to flip you back around. Given that a lot of inland drivers don't like waves much either, the wind and the waves make them put their toys away.

The UH-26s is shown as a ductless craft. The weather-vane effect will not happen in an uncontrollable fashion. Maintain enough rudder authority (surface area of rudders and the number of degrees of travel) and you will be fine.

A single-engine craft tends to have problems with wind too. Going down wind, the thrust needed is minimal and the lift significant. If you set the throttle for the lift requirements then you are going at unsafe speeds, and if you try for the thrust requirement then you will plow in. On a Universal craft that is not so much of a problem, but it can jar the passengers and shift the load.

Likewise, going up wind can be the opposite problem. You need a lot of thrust and not much lift, so you get a lot of spray. The larger Universal Hovercraft machines use a variator for lift control, which is a continuously variable belt transmission which allows a workable variation of fan speeds to adjust lift relative to thrust.

Ideally, you have a twin engine craft. The UH-26s is definitely large enough for this to be a minor modification, and in fact the build site I linked to in this thread shows a twin-engine build. That negates the upwind/downwind lift and thrust ratio problems, as well as making it possible for one man to move the craft around in a parking lot or shop.

Sorry this has turned into a whole book.

Thanks.

 

I WOULD VOTE FOR A CATAMARAN. Built heavy out of local woods and epoxy and glass (dynel) or (xynole ) powered by 2 High Thrust outboards which can handle high current. I bet this will be the most economical way to achieve what you want. my 2 bits on the subject. Stan I'm building a 30' cat and it only draws about 9"

 

Thank you Ken, that was worth reading!

And here are the links:
http://video.google.com/videoplay?docid=7256418213112104934&hl=de
https://www.amphibiousmarine.com/catalog/
http://www.hovercruiser.org.uk/component/content/article/27-latest-news/41-sevtec.html
http://www.hovercruiser.org.uk/sev/
Regards
Richard