Is a Gas-Powered Blower/Vac Enough for a Water-Capable Hovercraft?

[Imop45]

Hi, I was wondering how to make a fairly-good sized hovercraft that can go over water. I have a Gas-powered blower/vac 180 mph, and 32cc, but will this be enough power? Also, what body shape should I use, especailly for higher speeds (40+) and for water?

 

[Imop45]

Also, what should I use to propel it, and control it?

 

[FredGarvin]

Do some searching through threads around here. There have been numerous discussions about hovercraft.

 

[Danger]

I agree with Fred that there's no point going over the whole thing again until you've checked the other threads and maybe still have specific questions. One thing that I do want to stress, however, is that there are safetly requirements for a hovercraft that's going to be used on water (any HC is water-capable).
Many jurisdictions demand that it be outfitted as a boat, with floatation devices, bailing bucket, rope, maybe running lights, etc.. If yours is powered by a leaf blower, it might not be capable of carrying all of that stuff. Also, you want to make sure that it will float if you lose your lift curtain.

 

[Mech_Engineer]

Hi, I was wondering how to make a fairly-good sized hovercraft that can go over water. I have a Gas-powered blower/vac 180 mph, and 32cc, but will this be enough power? Also, what body shape should I use, especailly for higher speeds (40+) and for water?

There's no way you can make a hovercraft that can go 40+ mph with only a little leaf blower holding it up.

The kind of hovercraft you're talking about is big, about the size of a small boat, and you'll need a lot of power for it, like 50hp-100hp between the lift and thrust IMO. You need to try searching around a little before you go any further.

I don't think you know what you' re getting yourself into.

 

[Imop45]

I searched though other threads, and didn't find much, although I limted my search cuase this sites kinda slow.
"The kind of hovercraft you're talking about is big, about the size of a small boat, and you'll need a lot of power for it, like 50hp-100hp between the lift and thrust IMO."
I'm not looking for a gaint one, just, I guess to other HHC, it's pretty small. I didn't think the Gas Blower was going to work, but I knew a smaller electric blower could produce enough thrust to hold 175lbs. so I figured a gas blower would work.
And also, I've talked to different poeple who said that even a fire extenguisher made their small hovercraft go up to 20mph. And I've seen HC's go faster than 40mph, and their controlled well, so why couldn't I make one that can be controlled?

 

[Imop45]

I thought, though, that it requires more power, etc. to make a HC run over/on water?

 

[brewnog]

Imop, a hovercraft needs air to be moving in two directions. Firstly, you need air to be being blown directly downwards (to inflate the skirt), this provides the "cushion of air" that a hovercraft is known to float on. Secondly, you need a method of propulsion. This is usually a fan blowing air off the back of the craft. Directional control is achieved by either moving this fan from side-to-side, or by putting some kind of rudder behind the fan to control the direction of the air.

So you'll either need two methods of providing the necessarily air flow (ie two engines and two fans), or a single engine with either some sophisticated transmission to two fans, or some sophisticated air ducting system.

Have a look at other designs, and have a look what kind of powers you're talking. Estimate the weight of your craft based on its payload, size, and construction material, and you'll have an idea of what it'll take to lift it. Then think how fast you want to go, and you'll have some idea of what it'll take to move it forwards.

A mate at school built a small hovercraft in 2 years based on an old dinghy and a couple of motorbike engines. It can be done, however he failed all his A levels as a result (although he did get to drive it round the school field on results day).

 

[brewnog]

Oh, and this topic https://www.physicsforums.com/showthread.php?t=54808 is only one below the last one of yours, so no excuses with the search feature eh!

 

[russ_watters]

How big is the outlet on that leaf blower? You can easily use Bernoulli's equation to calculate how much lifting force (velocity pressure times area) it will generate.

 

[Mech_Engineer]

I'm not looking for a gaint one, just, I guess to other HHC, it's pretty small. I didn't think the Gas Blower was going to work, but I knew a smaller electric blower could produce enough thrust to hold 175lbs. so I figured a gas blower would work.

My point is that going that fast, you will want the skirt to be pretty thick, and a thick skirt will take a good seal and and a lot of air to inflate it.

Next, I know it isn't really scientific, but Mythbusters tried to make personal "hovercarft" to race against each other. They both used gas leaf blowers to "hover" and propel them. One of them had 4 or 5 leaf blowers, and it still didn't work... The fact is you need a lot of power to go fast over terrain that isn't perfectly flat.

It would be pretty much impossible to build a hovercraft under 175lbs or slightly over (including driver) with a couple of leaf blowers, that could go 40 mph. Impossible.

 

[Imop45]

Oh, and this topic https://www.physicsforums.com/showthread.php?t=54808 is only one below the last one of yours, so no excuses with the search feature eh!

See I look at that, but it was too small, the HC, so I made a new post to satisfy what I wanted too know.
Oh, I thought mythbuster did an episode on HC's. It really required that much power(for high speed)? So with water or un-flt areas, it takes a lot extra? So, I was wanting a hovercraft about...well no smaller than 3x4 ft. So, if its a flat area, my Gas Blower would deffinetly work, but when it gets too loose ground like water, it won't be enough power, correct?
Some other guy that posted on here, lifted his HC with a 325cfm blower. So, my 375cfm blower won't lift my HC over loose ground, only flat, smooth ground?

375cfm Blower=flat ground
?cfm=Flat ground and loose ground.

 

[russ_watters]

CFM alone is not enough information. You need to know how much pressure it will generate at 375cfm. Obviously, if the pressure isn't high enough, the hovercraft will sit on the ground and the fan will be spinning but producing zero cfrm.

 

[Mech_Engineer]

So with water or un-flt areas, it takes a lot extra? So, I was wanting a hovercraft about...well no smaller than 3x4 ft. So, if its a flat area, my Gas Blower would deffinetly work, but when it gets too loose ground like water, it won't be enough power, correct?
Some other guy that posted on here, lifted his HC with a 325cfm blower. So, my 375cfm blower won't lift my HC over loose ground, only flat, smooth ground?

From what I remember, Jamie had a blower for the "lift," and four blowers facing backwards for thrust. From what I saw, the skirt ended up being about 1" thick inflated, and got to <maybe> 10 mph (probably more like 6) on flat cement. As soon as there was something that hit the skirt (like a curb or rock) it deflated and he hit the ground.

Then, they tried going out to the beach, and the hovercraft couldn't float on the sand, no matter what they tried. In the end, they ended up just pushing sand plows, because they didn't have enough float or thrust (with 5 gas blowers!)

375cfm Blower=flat ground
?cfm=Flat ground and loose ground.

The fact is you need a blower system that has a very high static pressure for the skirt, it doesn't really matter how many CFM it does. As for the thrust, if you did a mass balance to see what the thrust was out of the blower was, you would see it was minimal. Even if "375 CFM @ 100mph" sounds like a lot, that's only 10 N of thrust. Not much to overcome the friction from your skirt and air drag! Even neglecting friction or air drag, it would take you 200 seconds to accelerate to 40 mph, but WITH drag I doubt you'd get past 5.

 

[Danger]

In all seriousness, it would be worth your while to try finding a used snowmobile motor. That's the powerplant of choice for most homebuilt HC's. A motorcyle engine might work as well, but you'd have cooling concerns.

 

[russ_watters]

The fact is you need a blower system that has a very high static pressure for the skirt, it doesn't really matter how many CFM it does.

Slight expansion: Say you have a 1'x1' skirt and 144lb of load on it. Lifting it off the ground requires 144lb of force or 144/144= 1psi of static pressure.

The airflow is only relevant for determining how high it will float (and thus, what terrain it will handle). That's a little more complicated, but let's just say you can get 144 CFM at 144 ft/min velocity and 1psi. Since the perimeter of the skirt is 48" and 144 CFM at 144 ft/min is 1 sqare foot of area, you will hover 144/48=3" off the ground. If you double the airflow but keep the velocity the same (velocity determines pressure), the height off the ground will double.

 

[Imop45]

Oh, so no matter how fast the blower can blow,it doesn't matter if it doesn't have enough power/force. It's like a car that can go very fast, but can't go up a steep hill; it has the speed but not the power. Is this how it works with hovercrafts?
Could I use a pump then, to inflate my skirt, just reversing the flow?
Also, how could I rig a snowmobile moter to inflate the skirt(you don't have to answer this, by the way)?

 

[brewnog]

Oh, so no matter how fast the blower can blow,it doesn't matter if it doesn't have enough power/force. It's like a car that can go very fast, but can't go up a steep hill; it has the speed but not the power. Is this how it works with hovercrafts?

No, not really. Your car analogy is wrong; the top speed is dependent on power. A better analogy might be that the engine is capable of spinning really quickly, but doesn't produce enough power to get anywhere. We don't know your level of knowledge here so it's hard to pitch this at your level.

Could I use a pump then, to inflate my skirt, just reversing the flow?

Well you have to inflate your skirt somehow, but to maintain the pressure needed to keep your hovercraft afloat is going to take the same amount of air no matter how you do it. A pump? What sort of pump? Your fan is a type of pump, loosely.

I don't want to patronise, but do you know how a hovercraft works?

Also, how could I rig a snowmobile moter to inflate the skirt(you don't have to answer this, by the way)?

The output shaft attached to a gearbox to make the axis of rotation vertical, and a fan on the end, within a cylindrical casing, pointing into the skirt.

 

[Danger]

The output shaft attached to a gearbox to make the axis of rotation vertical, and a fan on the end, within a cylindrical casing, pointing into the skirt.

You certainly can do it that way, and keep your lift independent of the thrust. An easier way, though, is to use the motor/fan aimed backward and simply duct part of the propulsion air downward through a hole in the deck.

 

[Imop45]

QUOTE=Danger]You certainly can do it that way, and keep your lift independent of the thrust. An easier way, though, is to use the motor/fan aimed backward and simply duct part of the propulsion air downward through a hole in the deck.[/QUOTE]
So how would I do this. You can use paint if you want and e-mail me an attachment at Luc_iss@yahoo.com.

The dots are like spaces, don't pay attention to them
._______________________________ /\
|.........| /\ THRUST.
|.........| /\ F
|....Moter.....|=|| A
|.........| \/ N
|______________________________| \/ (Back)
..........\/

And I do know how a HC works, you force air into a "pocket" under the craft. High pressure air is trying to escape under the skirt, which cuase you to rise, kinda hard to explain, but I know how it works, yes.

...____________________________________________
...|||<<<<<<<<<<<<<Air...Air>>>>>>>>>>>>>>|||
<<<<Air...........Air>>>>>>>

 

[Danger]

Okay, Imop, I just did up a very rough sketch in AI 10. Unfortunately, I don't at this time have the means to convert it to a file that ImageShack can host. I'll work on it tomorrow.

 

[Imop45]

You can just e-mail it to me.

 

[Danger]

Unfortunately, I can't due to technical complications. As much as I hate Paint, I've just gone ahead and sketched it up in that. It's below.

"[IMG[/URL]

That's a pretty gross exaggeration of the duct size, just for demo purposes. You'd also want a guard around the fan.

 

[Imop45]

That would really work? I thought you needed a lot of pressure to inflate the skirt.

 

[Danger]

Yeah, it'll work, but you'll have to do some math to figure out just how big it needs to be based upon your fan output. Generally, it would be a rectangular duct, about the proportions of a legal-size envelope, that draws primarily from the bottom of the fan. The whole thing will be a lot more effective if you use a ducted fan (ie: put a shroud around it to keep the airflow axial).
Alternatively, you can bleed your propulsion air out of the cushion instead.