My Hovercraft Build Diary

Specifications

Description: Twin Engine, Pointed Hovercraft

Length: 15' 6" Nose to Rudders Width: 6' 8"

Power Plants:

Thrust Engine: Briggs and Stratton 26 hp V-Twin (vertical)

2:1 Belt Drive Reduction to prop

Lift Engine: Tecumpseh 12.5 hp Single (vertical)

Direct Drive

Propeller: 54" diameter/ 50" Pitch Wooden Prop - Ducted

Lift Fan: 26" diameter / variable pitch Hascon 9 Bladed Fan

Max Speed (Estimated): 50-60 mph on smooth water

Dry Weight (Estimated): 475 lbs

Fuel Capacity: 6.5 gal (Primary) + 6.5 Gal Secondary (optional)

Fuel Consumption: 2 to 2.5 gph

Passengers: 3 adults

Constructing the Hull...

I used 2'x8' (2" thick) extruded polystyrene panels available from home improvement centers like

Home Depot and Lowes. This part is a lot easier if you are using 4'x8' panels though, so I would

recommend you locate the 4'x8' sheets and use them. On the left is the stack of foam I started out

with.

The base hull is two layers of 2" foam, with another layer of 2" around the outside which finish off

the bow planes. This makes the hull 4" thick in the middle, and 6" around the edges. I used epoxy

to join the first layer together on the edges, then reinforced the seams with 4" fiberglass tape.

You want to alternate seams so that they are staggered - this prevents a weak line that goes all the

way through the hull (from one side to the other).

Then I epoxied the second layer on top of the first, and to each other in stages. I did this one 2'

strip at a time, and weighed them down with heavy weights for 24 hours. It took 4 days, but it was

manageable in my workspace. When working with extruded polystyrene foam, I recommend that

you sand the surfaces being bonded with a random orbit sander and 80-100 grit disks. If you don't,

the extruded surface is very smooth and it will be a very weak bond. I did tests and the epoxy

peeled off the smooth extruded surface. If sanded, the foam ripped off, much better.

After the hull was completely cured I laid out the pointed shape of the hull. I used AutoCad to lay

out the shape on paper. I generated dimensions at 1' intervals to locate the spline points. I

marked the centerline first, then the rear edge of the craft, which is perpendicular to the

centerline. I used the back and centerline as datums to lay out all of the spline points.

I ripped some ferring strips to make long 3/8" wide strips to do the hull layout. I used some long

drywall screws as an anchor, I just lined up the wood strip on the mark and pushed the screw into

the foam right behind it. I also used the same strips to use as a guide for laying out the centerline

and wire cutting the bow plans. They worked great, I would recommend this method highly

Then I used a spring clamp to hold it in place. When done at each spline point, I had a nice edge to

mark the shape using a Sharpie marker. The thin wood strips work extremely well for this kind of

layout work.

When done, I had the pointed shape completely laid out. The layout only took about 45 minutes.

Once the shape was marked off, it was time to cut. I used a sawzall to cut the shape, trying to stay

about 1/4" from the layout line. You don't want to cut on the line, it is very easy to undercut the

shape, which is the final shape you need. It is very easy to take the 1/4" off using a Stanley

Shureform tool. Here is the hull with most of one half cut out. The big pieces left over in the

corners will be used for structural members later.

Here is a close up. This was too close to the line. I backed it up to 1/4" - 3/8" after this since the

sawzall blade undercut some of the hull here at the line. If you have a Shurform tool (I got mine at

Home Depot) it makes short work of taking off the excess, and it looks great after. It was the only

tool I needed for final shaping the foam.

After cutting the sides, I used a straightedge guide to cut the square back

edge. My circular saw was a little short of the 4" thickness, but a standard wood hand saw (which

turned out to be very valuable later) cut through the rest in a couple of minutes, using the groove

from the power saw as a guide. A couple of passes with the Shureform tool and it was done.

Here is the Hull, completely cut out and ready to put on sawhorses for final shaping.

Here are the tools you need for shaping the edges. I had two Shureforms, a framing square, a

carpenters square, a long straightedge, and a marker. I used the carpenters square to make sure

the final edge was square to the top. The short Shureform was the most useful, you don't really

need the longer one. It was very easy to shape the hull down to the marked line, the whole

perimeter took about an hour.

Another shot of the edge after final shaping it with the Shureform tool. These things work great

for this.

I sanded the top (which is actually the bottom of the craft) with an orbital sander (80 grit) to rough

up the surface and remove any high spots - mostly around where the edges butt together. I

fabricated the third layer of foam for the bottom of the bow planes. These will create an air

chamber under the craft, which will help prevent the craft from "sucking" down to the water

during a plow in (when you lose lift pressure for some reason and the hull plows into the water).

I cut these pieces to the exact shape I needed using a simple foam layout tool (see pic below) to

transfer the exact shape. After installing the thrust engine reinforcement plates (see Edge Layout

Tool below) I attached the third bow plane layer to the 4" thick hull with spray foam (Great Stuff).

I used the great stuff because I was low on epoxy at the time. It works great as an adhesive for

foam, but it does expand so you have to clamp the stuff down really well. I would prefer to use

epoxy for this layer as well. But many people bond all their form with spray foam.

Edge Layout Tool: These aren't the same layout lines for the bow planes, but the tool was the same

and it shows what I am talking about. Just drill a hole the right distance from the T, put in the

Sharpie and layout the lines. Worked great.

Here I laid out the lines for the thrust engine frame reinforcing plates. These are 3/4" plywood,

and the carriage bolts will go through here and protrude upwards on the deck. There will be

another one on the top side, installed the same way, then the plywood deck will be laminated over

that. This will make a very strong mounting structure for the thrust engine frame and air drive.

The engine mount/air frame will drop down onto the bolts. These reinforce the area and prevent

the carriage bolts from pulling through the hull. The wood strips are guides for the router that was

used to make the recessed pocket for the plywood

Pocket routed and corners squared up, ready for the plywood.

Here is the plywood plate dry fit into the pocket

Both pockets done.

Here are the plywood plates epoxied into the pocket, then overlaid with two layers of 2 oz

chopped strand mat and a top cover of 6 oz fiberglass cloth to seal and reinforce the area.

Here all the lower sections are dry fitted in place prior to bonding them to the hull. Once

everything looked good, I glued down the third layer, including rear section.

Note how it is tapered down to the craft bottom center area. This is so it will tend to deflect air

and obstacles down and away from the rear bow planes. I used Great Stuff to attach the third

layer, but I should have used epoxy since it expands a bit. It wasn't a problem, I clamped it down,

but epoxy would have been easier to deal with.

Same tool, marking out the bow planes on the sides of the craft.

These were marked all around.

Third layer glued in place. The same 3/8" strips used to lay out the centerline and spline the

pointed section of the craft are now used to be guides to hot wire the bow planes. every 12"-18"

or so I drilled a hole and put a long drywall screw in it. These hole the strip nicely to the hole

while you hot wire. I used some duct tape at the ends to hold them firmly in place. The guide is

placed so that the hot wire will exit the hull on the layout line, but sits on the wooden strip. The

wooden guide was close to the line on the sides, but had to be set back a bit from the line on the

top due to the angles. Easy to do by eyeballing it though.

Here you can see the side, and top guide strip in place. I did the left side, then the right, then the

rear bow planes.

Time to break out the hot wire and cut the bow planes! I use

0.015" diameter stainless steel wire (I got it for free, and it worked great) in my homemade bow.

Here is the hot wire panel I designed for working with foam. There is a page on my site here

dedicated to this item. A hot wire setup will save you a lot of time if you are building a foam

craft.

What's a Hot Wire System?

Thats a good question. If you are working with foam, you really should have one. Here I will talk about the one I built, and post some pictures and diagrams. A hot wire system is an electrically heated wire that you use to cut and shape foam. There are relatively safe ways to do this and there are less safer ways if you know what I mean. There are two parts to a hot wire system, a power source and a bow. The power source should be adjustable (voltage) to allow for different lengths of cutting wire and materials. The bow is generally a stainless steel or nichrome wire stretched between some non-conductive materials (bow).

DISCLAIMER: Here is the thing, electricity is dangerous. I am not responsible for anything you might try to do with any of the information on this web site. If you don't know how to work with electricity, get someone who does to help you or don't do it!

My hotwire bow. 12 gauge speaker wire (stranded) is great for this. I can adjust it to be 12" up to

about 28" or so.

Adjustable Tensioning Mechanism.

Cutting Wire Attachment. I use a screw with two washers. The wire goes in between the two washers

and I wrap it around 5-6 times before tightening. There is a nylock nut on the other side.

One of the things you never want to have happen is to have a ground loop in the bow. This would be plugging the power right from your wall outlet into either side of the bow. This is extremely dangerous. In my design, the cutting bow is isolated from wall current via an isolation transformer (in my case, a 24V AC-AC / 5 Amp transformer used for slot machines which I got on eBay). I used aVariac autotransformer to control input voltage between the wall current and the 24V AC-AC Transformer in my hot wire panel. So 110V AC comes in from the wall, goes through the variac, which varies the voltage from 0 to 110 V (or whatever you have coming out of your walls). The voltage coming out of the transformer is proportional to the voltage going in. At 110 V into the transformer it is 24 volts out. At 55 Volts in, it is 12 Volts out, etc. This allows me to set whatever output voltage I want (0 to 24 V AC). I also used digital panel meters to monitor voltage and amperage out of the transformer so I could exactly control the transformer's output voltage. Incidentally, I bought all of the major parts for my hotwire on ebay for around $75, which includes the new in box variac (6 amp), the transformer and both panel meters. NOTE: If you use an amp meter (panel

meter), you need a current transformer or a shunt in line or you will blow the amp meter. The hot wire power source is shown below. Mine is a little fancier than necessary, but it was worth the effort. Here is a PDF document that really helped me to design my hot wire setup (Credit to William Bellman / Sightlines) -HotWIreCUtterDesignWithVariacTSG34.pdf

I also have two fans in the back to vent the box. My box has two electrical outputs, the banana jacks on the right is the output from the 24V AC-AC transformer and goes to the hot wire bow. The switch above the panel meters switches the output from the variac between the 110V plug and the transformer. This allows me to use the box to dim incandescent lighting or control RPM on some AC motors. I probably wouldn't do it again (the switched outputs) because it really complicated the wiring, but it works great. Everything barely fit in the cabinet I made. I used wall transformers to power the meters (5 V DC and 12 V DC) from the 110V AC wall current. However, I plan on changing these out for a small PC power supply I got from a junker computer at work - this will be simpler than what I did. NOTE: The PC power supply will only be used to power the panel meters!

You can also use a cheap hand held volt meter or amp meter to monitor your output. Some people use a doorbell transformer and a dimmer switch for their hot wire setups. If you are only cutting small pieces of foam it's probably OK. You don't need a fancy cabinet, you could get these components and mount them on a piece of plywood to keep it simple. No matter what you do, remember to use fuses rated to the components you are using! You don't want to burn your house down or electrocute yourself!

HOTWIREDIAGRAM.jpg

There are a lot of online resources, just Google "hot wire foam cutter"

Here is an example of what the foam looks like after cutting with the hot wire bow. They are very

smooth and required very little shaping with the Shureform tool to finish them off. Notice the

strips at the top and side. The bow rests on them and you push down and forward at the best

speed.

Here are some pictures of the finished bow planes, prior to glassing the bottom of the craft. This

is the back of the craft with the rear skids just placed near their final installation points. This

finish is from the Shureform tool, takes just 5-10 minutes per side.

Here is a shot from the back looking forward. Notice the side bow planes. The skirt will be

attached at the bottom of the bow plane (the angled surface), just after the transition from

horizontal to the angled surface. you can see how I have rounded off the inside corners of the

area in preparation for fiberglassing. You also see another shot of the lower reinforcing plates.

A Big Oops: When I cleaned up the hot wire surface on the bow planes, I added in the front plow

surface (the flat area) without thinking about it. In my haste, I removed the front point area of the

craft (this is where the skirt gets attached!) To fix it I cut out a strip, 2" deep and about 12" wide.

I glued in another piece of foam and re-shaped it. The fixed (and glassed) nose shape is shown

below.

The nose area restored. Now there will be something to attach the skirt to!

Next, I cut out the hole for the lift duct. I will be using a 26" diameter lift fan (Hascon), installed

at a 15 degree angle (forward). Tip clearance and the duct material (marine plywood) will add

about 1/2" to that. You want enough room to get the duct installed though, so I cut the hole about

27 1/2" in diameter. That is OK, you use polyurethane 2-part pour foam to affix the duct and it

will fill any gaps. It is more important to have enough room to easily position the duct in the hull.

That foam block is cut at 15 degrees to act as a guide for the 1" drill, and the special hot wire bow

I made up out of PVC Pipe.

Here you can see the main shaft of the PVC bow. Remember that the duct is tilted forward, it

would be very easy to do this in the wrong direction, especially with the craft upside down! You

can see the pipe is angled back, which is forward on the other (top) side. I drilled a 1/2" hole at

the perimeter of the lift duct hole to run the wire through. I installed the center pipe, then the

elbows and horizontal members. Then attached the wire and alligator clips on either end for the

power. I then rotated the bow 360 degrees (very slowly) until I had the entire circle cut out.

I had to slide the bow up and down to get it to cut in some places, but it cut through the fiberglass

tape I had on the seams. I only broke a wire once. The surface on the ID was a little rough, but

that doesn't matter. In fact, all the little groves should make a great bonding surface for the 2-part

foam. The wire gets hot, and it melted through the PVC pipe at the ends of the bow. You really

need to use another material at the contact points if you are using PVC pipe. Some people use

spark plugs, which are a great idea since you can just attach a clip to the top of the plug and the

wire on the base. The PVC bow lasted long enough to get the hole done, then I threw it away.

Another view of the lift duct hole prior to cleaning it up.

The next thing is to install the landing skids. I used 2x4 lumber to make them, they are 21" long

and angled at 30 degrees in both directions. I originally cut them at 45 degrees, but after some

input from other builders cut them down to 30 with a hand saw. You don't want them to be too

blunt - if you hit an obstacle you want the skid to tend take a glancing blow, not a blunt one. I

used a router to make 1" deep pockets at each location. I laid out the outlines with a Sharpie,

then just routed them free hand. The pockets are about 3/4" over sized all around. When

installing, I made up a mixture of epoxy, 1/4" milled glass fibers and cabosil which I poured into the

cavity prior to putting in the skid. This will form a wide reinforcing pad under the skid. Then I

poured a mixture of epoxy and cabosil to fill the pocket flush with the foam. This produces a large

reinforced area around the skid. Once this cured, I mixed up some epoxy, cabosil and

microballoons into a paste and filleted around the skids to reinforce them. Another Oops: I made

the skids too high. They should be no more than about an inch above the bottom of the craft. I

made up a simple wooden router jig to shave them down to the right height after installation,

which worked really well. In fact, this may be the way to do this so they are all exactly the same

height, serendipity baby. You can see how this all turned out on the fiberglassing page. In the

picture here, you can see the first layers of chopped strand mat and resin.

Fiberglassing the Bottom of

the Hull..

For the first layer of fiberglass on the bottom of the hull I used overlapping 1 1/2 oz chopped

strand, 10" wide mat to cover the bow planes and bottom edges. It overlaps 4-5 inches along the

length, and twice for the entire length of tack strip area. I rolled out epoxy on the surface to wet

it, then laid down the mat. Then I rolled more epoxy onto the mat, just enough to wet it fully and

no more. Of course I pre-cut all the pieces before laying them up. After laying down the 1 1/2 oz

mat, I added another 2" strip of 2 oz chopped mat at the inner tack strip area (you can see it in

the pictures as the additional material at the bottom of the bow planes - the hull is still upside

down). This will provide a thick, strong area to attach the skirt using screws.

Here is the other side.

A view of the other side, with a close up of the nose area.

Here is a close up of the bow planes, prior to cutting down the skids. One layer of 1 1/2 oz chop

mat applied, overlapping at the tack strip area.

Rear Bow Planes, with additional 2 oz chopped mat tack strips clearly visible.

A bigger view from the rear of the craft.

Skid cut down to 0.9" height with a router. Also (prior to glassing) cut the leading and trailing edge

of the skids to something closer to 30 degrees.

The following 3 pictures show how the skids are reinforced with mat and cloth prior to the final

glass application. 3 layers of 2 oz chopped mat for bottom reinforcement.

Reinforcement Mat over that for top and side, also helps tie the skid to the bottom of the craft,

and spread the load over a larger area.

A couple of layers of chopped mat applied, with a layer of 6 oz cloth over it to neaten it up and

provide further reinforcement prior to the final application of 6 oz fiberglass. This will help to

spread the skid loads over a wider area.

Cutting the 6 oz cloth to finish the fiberglassing on the bow planes. I was able to do each side with

one piece. Same process to apply it. I use a roller to wet out the entire surface. Then carefully

apply the pre-cut fiberglass and roll it out with the same (near dry) epoxy foam roller. Once I see

how it is wetting out, I re-apply epoxy to the roller and completely wet out the cloth. You can

really control the amount of epoxy this way, and only apply enough to wet it out and get a good fill

on the weave.

Finished applying 6 oz cloth to the entire bottom of the craft. The center area was covered in one

layer of 6 oz cloth, then following the recommendation of another builder (Thanks Jerry!), a layer

of 6 oz Bi-Axial cloth. The weave was then filled with an epoxy/cabosil mixture and should be

pretty bullet proof. You can see that the duct transition area has been tapered to facilitate

smooth airflow back from the lift duct.

A close up of the fully glassed nose area where the patch had to be installed to provide a tack strip

area at the point. The hull is now ready to flip over and start working on the top!

Here is a shot of the rear bow planes and skid after the craft was turned over to start the top

area.

Installing the Lift Duct...

After turning the hull over (my wife helped me - it actually was not all that heavy) I started with Installation of the lift duct, which was very straight forward. First I placed the pointed piece of 3 mm plywood decking into position on and marked the lift duct cutout from below. A couple of minutes with the jig saw and a little sanding and it was ready to epoxy down. Then I epoxied down the plywood decking on the nose of the craft. Once it cured (I gave it a couple of days) I started to install the lift duct.

Getting ready to locate the lift duct assembly into the hole. The thrust frame and rear decking

aren't installed yet, just positioned on top of the hull.

A close up of the hole, prepped for installation of the duct. You can also see the tapered area on

the back edge to facilitate air movement under the craft.

Closeup of the transition area, It is tapered about 1 1/2" down to the bottom of the 4" thick area of

the hull.

The lift duct positioned properly, measured from both sides, and checked for squareness. Since

the nose is pointed, and left to right there isn't a good reference, I used the diagonal

measurements from the back corners to the rear engine mount blocs (left square for this very

purpose!). I did the same from the point to the front mounting blocks. I used some 4" deck screws

(you can see they are positioned in the image) in pre-drilled holes to screw down the block after

epoxy was applied underneath. They are really only there to clamp down the duct assembly

during bonding, but they are permanent.

Another shot from a bit further back.

Here is the lift duct epoxied down to the deck, with a bunch of chopped mat, 6 oz cloth and resin

applied to lock down and reinforce the pointed area. Once this cures I will pour 2-part urethane

foam (2 lb) into the gap around the duct. I used duct tape around the bottom of the duct to seal it

and hold the 2-part foam until it expanded and cured. There was some leakage, but no big

problems.

Close up of front mount block, attached to the hull. This area will be backfilled with 2-part pour

foam after the sidewalls are installed.

You can see the foam block I made to hold up the back of the duct while it was being installed. I'll

just leave it in place when I pour foam around this area later. You can see the 2-part foam

expanded up out of the hole around the duct. The stuff on top (underneath the bottom flange) is

Great Stuff. I don't know why I did it, just got a little silly with it after sticking the foam block in

place.

Close up of the foam location block. It is cut at the right height and angles to support the duct at

just the right position.

Foam has been trimmed all the way around now.

Now doing the same thing to trim the duct wall. Carefully go all the way around.

More trimming of the duct wall. In this shot, I hit the duct seam so it folded out at 90 degrees.

Excess duct wall completely trimmed from all sides, including the rear transition area. I used a

hand held belt sander to get the rear transition area flush with the bottom of the hull. Looks

good!

A view from the side, prior to glassing the duct in place.

The duct is permanently installed, time to knock out the former. It came out pretty easily with a

shot hammer from below. Took about 15 minutes to get it out. I am glad I didn't spill any epoxy in

there!

The Lift Duct former is OUT!.

Not a lot pf pics from the last one to here. I put a ring of duct tape inside the liner to give me a

cut line for the fiberglass. I wrapped many pieces of overlapping 6 oz fiberglass cloth 6-8" wide

from the duct wall to the outside surfaces all around the bottom. This integrates the bottom of

the duct to the craft. Then I applied two layers of wide strips all around the inside of the duct

wall, which covered the vertical pieces. Once the epoxy was tacking up, I used a utility knife to

cut the cloth at the duct tape line to get a clean edge.

A view from the top down. The engine mounts are in position (it is a good place to store them).

View from bottom ofter duct installed - after glassing the duct.

Yet another view from the bottom with the nose area - after glassing the duct.

That's it. The lift duct is now installed. It actually went really well and only took about three nights after work to get it in place and glassed. Next is finishing the decking, and installing the thrust engine mount/air frame.