Building a Hovercraft

Home
Flying
Science
Photos
Literati
Miscellanea
Curious Web Sites

This page last updated on 02/28/2014.

Copyright 2001-2014 by Russ Meyer


It was August 27th, 2005 and my daughter, Serena, and I were looking for something to do.  Preferably something out of the house and away from the rest of the family.  The whole family, except Serena and I, had the flu.  It was a viral maelstrom in there; what with the fevers, coughing, sneezing, snotting...well, you get the picture.  Anyway, we were determined to spend the day outside.  After talking about it, we decided to go flying.  Since it was almost time for lunch, we thought we'd fly over to Spinks airport South of Fort Worth and walk to a nearby restaurant.  After that, we'd just buzz around and do some sightseeing.

We went out to the airport, readied the plane, and launched.  The air was hot and bumpy.  Despite the Dramamine, Serena lost it justSerena and I at Spinks Airport as we were landing and puked all over the floorboards.  She recovered pretty quickly so we headed to a nearby Chili's for lunch.  Over lunch we talked about what to do when we got back from flying, but couldn't come up with any good ideas.  On the way back to the airport, we stopped by a hardware store.  We were walking the aisles looking at stuff when a leaf blower caught my eye.  I picked it up and told Serena that I sure needed one.  We talked about leaf blowers a bit and then an idea hit me.  I had heard you could make a hovercraft out of these things and told Serena we should while away the rest of the day trying to build one.  She was delighted, so that became the plan.  We thought we'd pick up the necessary materials when we got back from flying.  The project would occupy us the rest of the evening and keep us well away from all the virus infested natives.

After flying, we stopped by a local hardware store near home and picked up the following items:

  • Two rolls of duct tape
  • A gasoline powered leaf blower (30cc displacement)
  • Some 2 inch outside diameter PVC pipe, elbows, etc.
  • A sheet of inch press board
  • A bunch of inch carriage bolts and lock nuts
  • Four small castoring wheels

The only other thing needed was plastic sheeting, but I already had some at home.

The Mark I Hovercraft

The basic idea was to use the board as the body of the hovercraft.  The leaf blower would be bolted on top with PVC pipe ducting the air through the board so it would blow underneath.  The wheels would be bolted under the hovercraft to act as landing gear.  If the blower stopped for some reason, I didn't want the machine to lurch to a stop and throw the rider; the landing gear would allow a gentle rolling stop.  The hovercraft needed a skirt; it's a flexible sidewall that hangs down under the craft close to the ground.  It traps a cushion of air for the machine to ride on.  The skirt is the most important part of a hovercraft and has to be built right.  I figured we'd probably go through a number of skirt variations before finding one that worked.  My first thought was to just tape some plastic sheeting around the circumference of the hovercraft, reinforce it with duct tape, and put a drawstring at the bottom.  The drawstring would prevent the bottom  of the skirt from flairing out when the air cushion was pressurized.

Now for the dimensions of the machine.  I figured the leaf blower could easily generate PSI of pressure.  The total weight of the machine and rider (like me) was, at most, 300 pounds.  That meant, the machine needed to have a pressurized cushion area of at least 600 square inches (300 lbs PSI).  If the machine were circular, it would need to be to  28 inches (~2.5 feet) in diameter (diameter = 2 * SQRT(600 square inches π)).  Because my board was 4 feet wide, I made it easy on myself and built the hovercraft as a four foot square with rounded corners.  That gave me about 2000 square inches of area; more than enough to lift 300 pounds.  Here's a photo gallery showing the first prototype under construction:

Mounting the landing gear.

Plumbing the air duct.

The basic machine sans skirt.

The skirt laying on the floor to the right; assembled and ready for mounting.

Mounting the skirt.

Finishing touches on the skirt. Almost ready for a trial run.

After five hours of construction, we were ready to give it a try.  I started the blower and revved the engine.  The machine didn't rise an inch; and that was with no one on board!  The machine couldn't even lift it's own empty weight...about 30 pounds.  Heck, I couldn't even get the skirt pressurized...it just sat there limp and lifeless.  Well, that was a total failure!

I couldn't figure out what was wrong.  Everything was working right, but the machine wouldn't rise.  Finally, it occurred to me that perhaps the blower didn't have the ability to deliver a great enough volume of air.  The skirt had a total length of about 15 feet around the circumference of the machine.  That was a lot of area for the air to escape.  The air was escaping under the lower edge of the skirt faster than the blower could pump.  The blower needed to pump more air into the cavity than could leak out around the edges, otherwise the skirt would not pressurize.  Hmmm...guess I learned something about hovercraft design.  It wasn't all about pressure differential.  Air volume and outlet area around the circumference of the skirt were also important.

To test this theory of skirt outlet area, I got a small trash can, about 2 feet in diameter, and placed the hovercraft on top of it.  With the air duct outlet over the center of the trash can and the engine revved, the craft seemed to rise about ⅛ inch.  That indicated the theory was right.  With a reduced outlet area, the blower was able to develop some pressure inside the trashcan and lift the machine.  I needed to redesign the machine to better match the volume the blower could deliver; that meant a significant reduction in linear feet of skirt.

The Mark II Hovercraft

The machine had to be scaled down to minimize skirt outlet area.  At it's smallest, it would need space for the blower, some air duct plumbing, and room for someone's feet so a person could stand on it.  More like a hoverboard than a hovercraft.  It would probably be unstable and very difficult or even downright dangerous to ride.  Still, I thought I might try it as an interim experiment.  If the thing flew and held the weight of a person, I could scale it up.  I thought about it for a while and decide to ditch the plan.  The experiment would be a detour, burning time and attention span.  If I didn't get this thing working soon, the kids and I would lose interest and shelve the project.  Too much foolin' around would kill the project.  No, I had to make another direct assault on creating a rideable hovercraft.

My whole approach to skirt design had been bothering me.  I was trying to use a skirt hung like a vertical curtain.  If the hovercraft was cruising along and the leading edge of the skirt snagged on something, it could be pulled under the hovercraft.  That might make the skirt collapse as the pressurized air rushed out the resulting breach.  A sudden skirt collapse could be a problem.  It might make the machine come to a sudden stop and throw the rider.  Also, the drawstring at the bottom of the skirt was ideal for snagging ground obstacles.  It could snag on something and peel the whole skirt off the machine as it was moving.  Yet another catastrophic failure mode.  Nope...my skirt design was really lacking.  I needed to come up with something better.

In an attempt to pick up some tips, I Googled "hovercraft leaf blower" and found a handful of relevant web pages.  It turns out that most guys use a radically different skirt design.  The skirt is simply a sheet of plastic stretched across the underside of the machine; anchored in the middle and along the edges.  The blower exhausts into the space between the skirt and the hovercraft body, inflating the skirt like a balloon.  When inflated, the skirt looks like a bagel cut in half.  A series of holes are cut in the skirt near the middle to allow air to escape.  The air rushes out of these holes and flows between the skirt and the ground, lifting the machine.  Here's what it looks like in cross section:

I decided to rework my hovercraft to use this skirt design.  The landing gear and old skirt were removed and discarded.  I then hacked the hovercraft body down to a 3 foot diameter circle.  A center skirt anchor was made out of the bottom of an old Wisk liquid detergent container and a carriage bolt.  A shower curtain was pressed into service as a skirt and mounted in place with duct tape.  Finally, I cut six 1 inch holes spaced evenly around the center anchor for an air outlet.  Here are a couple of photos showing some design details:

New skirt design with blue skirt anchor made from a laundry detergent bottle.

New hovercraft circular platform; about 3 feet in diameter.

I started the blower and revved the engine.  The skirt inflated and the machine lifted off the ground!  It floated almost frictionless on a cushion of air.  Serena sat on the machine and it lifted her just fine!  I pushed her back and forth across the garage a couple of times.  The hovercraft worked!  Serena and I ran in and got the rest of the kids.  Katie and Annette gave it a try, but Nicky was afraid of the engine noise and opted out.  Throughout these first few "flights," the plastic skirt took a beating.  It tore very easily and I had to repair it with duct tape.  Finally, when I tried to get on the machine myself, the plastic sheeting blew out...actually ripped in two.  That ended our test flights.  However, I was determined to find an improved skirt material and repair the machine.

The Mark III Hovercraft

The next day, Serena and I trekked to the hardware store and bought a 10 mil, fiber reinforced, contractor's tarp.  It seemed very tough; perfect skirt material.  I decided that along with replacing the skirt, I would rebuild a few components that had just been slapped together in the previous model.  The following improvements were made:

  • Replaced center skirt anchor.  I discarded the anchor made from the Wisk bottle and replaced it with a circular anchor made of sanded, ⅛ inch, press board, 6 inches in diameter.
  • Increased vent size.  The size of the vent holes was increased from 1 inches to 2 inches.  I had noticed on the previous model, that the skirt tended to over-inflate when the engine was running at full power.  Hopefully, the larger vent holes would reduce that problem.
  • Improved circumference skirt mounting.  I changed the way the skirt was affixed to the circumference of the hovercraft body.  Instead of being held on by duct tape, I made a 1 inch wide ring of inch thick press board to serve as a clamping retainer for the skirt material.  It was held in place by 35 screws spaced every 3 inches around the circumference of the hovercraft.  If the skirt ripped, I wanted to be able to pop off the retainer and replace the skirt without having to custom tape the new skirt in place.  Taping the skirt in place takes a lot of time; popping the retainer on and off is much faster.  Besides, duct tape just hadn't been doing a great job of securing the skirt.

Here are a couple of photos highlighting the design changes:

The underside of the Mark III. Note new center skirt anchor and the larger vent holes marked for cutting.

Close-up of the vent holes marked for cutting and the new center skirt anchor.

Kids love hovercraft! Serena riding the Mark III. Note the new circumferential skirt retaining ring.

After test flying the Mark III a few times, it became clear that the larger vent holes were almost perfectly sized.  Any smaller and the skirt over-inflates.  Any larger and the skirt would probably have some difficulty inflating.  The circumferential skirt retaining ring did a magnificent job sealing the skirt and holding it in place.  Finally, the heavy tarp held up well to the beating.  In fact, as long as the hovercraft is operated over very smooth surfaces, like tennis courts, the skirt should last a long time.  Bumpy concrete causes significant scuffing, leading to terminal abrasion.  Scattered rocks can critically damage the skirt.

The hovercraft appears to float less than ⅛ inch above the surface.  A variety of people, weighing from 45 to 250 pounds have ridden the hovercraft.  In its present configuration, it seems to have a sweet spot with respect to load carrying capacity.  Skirt dragging is minimized with 50 to 80 pound riders.  In fact, the skirt hardly ever touches the ground with riders of about 65 pounds.  Weights outside this range increase skirt drag.  The machine seems able to comfortably carry a maximum of 115 pounds.  Below is a graph showing how skirt dragging depends on rider weight.  Skirt abrasion, and therefore skirt life, is directly related to skirt dragging.

More skirt scuffing and dragging is expected with heavier riders, but I was surprised that dragging increased for light riders.  I'm not sure why this happens.  Perhaps the inflated skirt takes on an inefficient shape when there isn't much load on it.

In theory, a larger diameter craft should have better weight carrying capacity.  (I eventually tried a larger diameter; see the epilogue below.)

Other Ideas

If I get around to it, there are a number of things I'd like to try:

  • Change the center skirt anchor plate diameter to see what effect that has.
  • Instead of stretching the skirt tight when mounting it, leave some slack so it will balloon more when inflated.  I'm not sure what effect that would have.  When I used the shower curtain for a skirt, I noticed it ballooned out much more than the fiber reinforced tarp.  The shower curtain seemed to suffer less skirt dragging, so it had much more frictionless ride.  Perhaps that was because a more rounded skirt shape is better at sustaining the air cushion.  If I put slack in the tarp, maybe I can replicate that behavior.
  • Experiment with the number and placement of the skirt vent holes.  There are a number of patterns I'd like to try.  Maybe I can improve performance of the craft.  I also have some ideas on using well placed holes to provide a modicum of propulsion and steering.  There are lots of things to try here.
  • It might be possible to build a more serious hovercraft with steering and propulsion.  The current hovercraft uses a 350 cubic feet per minute (CFM) leaf blower.  With 700 to 1000 CFM, a large one could be built that could carry enough humanity and propulsion to really do something with.  Let's see, I'd need two or three leaf blowers for lift and another two for propulsion.  Sheezh...five leaf blowers screaming wide open would create quite a racket!

The Mark III in Movie Form

The first garage tests:

 

Parking lot fun:

 

Build Your Own Mark III

Why don't you try building your own hovercraft?  It's not hard; I think I could build one from scratch in 3-5 hours, now that I know what to do.  Below are some instructions you can follow to make one for yourself.  Be advised...the instructions are not super detailed because I'm just not up to writing a humongous assembly procedure.  You'll just have to fill in the little details yourself.  However, all the dimensions, components, etc. are not real fussy.  If you get it put together anywhere even close to what I describe, it should work just fine.  It's OK to make-up some stuff as you go along...heck, I built mine almost totally by guessing!

  1. Gather parts.  You need the following items:
    • Roll of duct tape.
    • The heaviest fiber reinforced tarp you can find; Home Depot, Lowes, etc. are good sources.
    • A 4' x8' sheet of inch thick plywood, particle board, press board, etc.  Press board is usually cheaper.
    • Three one inch long by inch diameter carriage bolts and lock nuts.
    • Thirty-five ⅜ inch long, flathead, wood screws.
    • A gasoline powered leaf blower of at least 30cc displacement.  I used a Homelite Yard Broom II.
    • Some PVC pipe, flexible dryer vent conduit, or something to duct your leaf blower air down under the hovercraft.  Leaf blower dimensions vary by manufacturer so you'll just have to ad lib this yourself.  I used 2 inch PVC components to plumb mine.
       
  2. Cut out hovercraft platform.  Mark and cut the hovercraft platform from your 4' x 8' piece of plywood.  The platform should be circular and 3 feet in diameter.  After you cut out the disk, sand the edges to remove sharp corners and burrs.  These can damage the skirt, if they aren't removed.

  1. Drill center skirt anchor mounting hole.  Mark the center of the hovercraft platform and drill a " hole for one of your carriage bolts.  This will be used to mount the center skirt anchor later.
     
  2. Drill various blower holes.  About halfway between the center of the hovercraft platform and its outer edge, drill a large hole to connect your leaf blower air duct.  Also drill a couple of " holes to bolt down your leaf blower.

    You're going to need to do a lot of eye-ball engineering to accomplish this step.  You'll have to mock up how you are going to plumb your leaf blower and make an appropriate duct hole in the hovercraft platform. 
    Just for reference, my ducting consisted of 2 inch PVC pipe and a few PVC fittings.  Below is an exploded parts diagram of how my ducting went together.  The top end of the PVC pipe was just jammed into the outlet of the leaf blower and clamped in place.


     

    I used a fly cutter to drill the duct hole in the hovercraft platform, but a hole saw would work too.  You could even use your saber saw if you're careful.

    At this point, you'll also have to figure out how to mount your leaf blower to the hovercraft platform.  I just drilled two " holes through the base of my leaf blower and then through the hovercraft platform, so I could bolt it down.  You could simply hold the blower in your lap and forgo the bolts, but you'd have to used some kind of flexible duct for the air instead of the rigid PVC pipe I used.  I think holding the blower makes it harder to ride the hovercraft later, because you'll have your hands full just hanging on to the blower.  At this point, your hovercraft platform should have several holes in it like this:

  1. Mount blower and ducting.  Bolt down your leaf blower and put all your ducting in place.  Note that you should do your best to minimize the amount of hardware you have projecting below the hovercraft platform.  Anything sticking down will eventually wear a hole in the skirt, so you want to be scrupulous about this.  For example, I always use carriage bolts with the smooth rounded head on the underside of the platform and the nuts on top.
     

  2. Mark and cut out the hovercraft skirt.  Spread the tarp out flat and put the hovercraft on top of it.  Use a felt marker and draw a line on the tarp about 6 inches from the edge of the of the hovercraft platform, as shown below.  Cut along the dotted line.

  1. Fabricate the center skirt anchor plate.  The plate needs to be about 5 to 10 inches in diameter.  It must be low profile...it can't be too thick.  I made mine out of a ⅛" thick press board.  It was circular and 6" in diameter.  I've also made one out of the bottom of a plastic liquid detergent bottle; I just cut off the bottom and used it.  The plate has to be fairly stiff though; for example, the bottom of a plastic milk jug would probably be too flimsy.  Drill a " hole in the center of the plate so you can bolt in onto the hovercraft later.
     

  2. Cut vent holes in the skirt.  Mark the center of your skirt with a felt marker.  Now center your skirt anchor plate up over the mark you just made.  Mark the cutting location for six 2-inch diameter holes around the circumference of the anchor plate.  The center of these holes should be about 3 inches from the edge of the anchor plate.  There are close-up photos of this step in the Mark III photo gallery above.  If you need more detail, go take a look.  Overall, here's what it should look like:

  1. Mount the center skirt anchor plate.  Push a " carriage bolt through the skirt anchor, the center of the skirt, and finally through the skirt anchor mounting hole you drilled earlier in the middle of the hovercraft platform.  Note that the skirt anchor plate goes on the bottom of the hovercraft.  Use a washer and locknut on top of the hovercraft to secure the assembly.  The nut should only be snug; not super-tight.
     

  2. Fabricate skirt retaining ring.  Mark and cut out the skirt retaining ring from the remaining portion of the plywood sheet.  It needs to be about 2' 11" outside diameter and 2' 9" inside diameter.  This results in a 1-inch wide ring.

  1. Drill retaining ring mounting holes.  Measure every three inches around the center the skirt retaining ring and mark a hole for drilling.  Drill a pilot hole for your wood screws at each marked station.  Also drill a counter-sink for the screw heads.
     

  2. Get the skirt ready for final mounting.  With the hovercraft sitting on the ground, right-side-up, grab one edge of the skirt, wrap it around the side of the hovercraft, then tape it in place on top.  The skirt doesn't need to be pulled real tight, and you don't need to do an elaborate taping job; just use a little piece of tape to hold the skirt.  The tape only holds the skirt in place until the skirt retaining ring can be screwed down in the next step.  Work your way around the hovercraft, tacking the whole skirt in place.
     

  1. Mount the skirt retaining ring.  Get the retaining ring and center it on top of the hovercraft platform.  The edges of the skirt that you tacked in place should be underneath the retaining ring.  Screw the whole works in place with wood screws.  The screws go right through the skirt and into the hovercraft platform.  The screws and the retaining ring hold the skirt in place around the edges of the hovercraft.  You can see the retaining ring in one of the photos in the Mark III photo gallery above.  Below is a close up, cross section view of the retaining ring assembly:

  1. Fly that puppy!  Well, you're all done and your hovercraft is ready to fly.  Just start the engine, rev it up and enjoy!

If you make a hovercraft using my plans, please let me know.  I'd love to hear about it.  Click here to send me E-mail.

 

Epilogue - The Mark IV Hovercraft

The kids had a lot of fun running the Mark III around, but I was really too heavy for it.  I wanted to try to increase it's load carrying capacity so I could experience the fully glory of hovercraft piloting.

I reasoned that the leaf blower appeared to have more than adequate volume, but at low pressure.  If I increased the surface area of the hovercraft, I should be able to reduce the load per unit area and make better use of the low pressure air cushion.  I rebuilt the Mark III, increasing hovercraft platform diameter from 3 feet to 4 feet.  This almost doubled the platform area from 28 ft to 50 ft.  In all other respects, the Mark III and Mark IV were identical.  The net result was a big improvement in load carrying capacity.  The Mark IV seemed immune to the skirt dragging that plagued the Mark III.  It carried me with ease and I got to have my share of hovercraft piloting.

I experimented with one other improvement; changing the shape of the skirt.  I did this by leaving a lot of slack in the skirt when I screwed it in place with the retaining ring.  This had the effect of making the skirt billow out more; it shaped it to look more like a bagel cut in half.  That shape didn't work at all.  It was horribly inefficient.  Ultimately, I came back to the shape I had used in the beginning, by just pulling the skirt taunt when screwing it in place.

Here is a video of the Mark IV in operation.  Jane is pulling Nick around on the end of a rope.  Nick weighs about 50 pounds.  Swinging the kids around in a big circle with the rope became the favorite hovercraft pastime.

Jane pulling Nick on the Mark IV.