Carbon Composite Hydrofoil From Free Junk!




Introduction: Carbon Composite Hydrofoil From Free Junk!

About: Tim Anderson is the author of the "Heirloom Technology" column in Make Magazine. He is co-founder of, manufacturers of "3D Printer" output devices. His detailed drawings of traditional Pacific I…

Here's how to shape a carbon fiber hydrofoil wing from a chunk of broken yacht mast.
The same techniques work for making centerboards, surfboard fins, or any other carbon thing you might want. Even propellers and windmill blades if you make templates to account for twist.
Here's the side view of the new hydrofoil blade I just made. It's a front wing for a Hydrothopter.

Carbon fiber costs way too much these days. Fortunately the free stuff is still free.

Step 1: Scavenge Carbon

Why is carbon fiber so expensive now? Stealth aircraft, superyachts, and even new civilian airplanes are using it up, and they didn't include "supply chain management" in their huge plans. So there's a shortage and a bidding war drives the price up.
The good news, you can get more of it for free than ever before, if you look in or near the right dumpsters. At the boatyard near me they have to pay people to cut the stuff up and throw it away.
Usually it's in the form of pre-cured composite structures such as broken masts.
There is more of this anywhere carbon gets made, fixed, or used. If it's fiberglass blanks you want, say for surfboard fins that have a little more flex, free fiberglass is also abundant. This same boatyard cuts up fiberglass yachts and packs the pieces into dumpsters to haul away. Those slabs of fiberglass are really amazing, seen on edge. You could make some great fins out of that.

Get your carbon and mark the outline for your blank. These chunks are a little shorter than the original wing, so I laid out a wider wing with slightly less wing loading.

Step 2: Cut Out Your Blank

Learn from my mistakes. Wear gloves, glasses, and full-coverage clothing.
I was in a hurry so I just wore glasses and a dust mask. It wasn't enough.

I used a circle saw to cut out my first wing blank. It cuts fast, but it puts a lot of flying crap everywhere.

This is a magnesium-framed left-handed circle saw. I can see the blade clearly, which means it can shoot tons of crap right into my face. Cutting the carbon with a carbide-tipped blade throws some sparks. I didn't know that would happen.

I spent the rest of the day using duct-tape to clean the prickly fibers out of my skin. I cut the next blank with a sawzall. There wasn't as much of a snowstorm, but I still got all itchy. It got in my sandals and made my feet prickly too.

Step 3: Edge the Blank

This belt sander has a dust-collection system, but it still kicks a lot of fibers out the back. I've gotten smarter, now I'm wearing gloves duct-taped to my longsleeved shirt.

The 50-grit belt on this sander really cuts fast.
I check the blank's edge with a ruler to make sure it's straight.

Step 4: Pick a Foil, Any Foil

The original front wing foil was approximately a "Clark Y".
So that's the foil section I'll shape onto this foil. Thickness is approximately 12% of chord, which is a hair thicker than this plot.

Notice that the bottom of this foil is mostly flat. Except for the leading edge area, the top of the foil has a gentle curve that's almost a circular arc. The existing outer surface of the blank nearly matches the top curve.

For a good time browse the UIUC foil section catalog.
Whatever odd piece of carbon you start with, you may find a foil section that matches it well.

Step 5: Shaping Stand

I drilled and countersunk the mounting hole that will bolt this foil onto the Hydrothopter. I screwed the blank to a chunk of 2x4 and clamped it in a vise.
I drank some coffee and started shaping the bottom of the foil.

Step 6: Shape the Tail

Using long even strokes, I belt sanded the bottom portion of the trailing edge thin.
It takes a lot of concentration to get the strokes even and at the right angle.

Foil shaping on wood using a planer is a lot easier than this. In that situation you draw the foil sections on the endgrain and plane down to it using full length strokes. It's pretty easy to shape the whole length the same way. Unfortunately the belt sander isn't a planer.

Step 7: Check and Recheck

You've heard "measure twice, cut once?"
For freehand foil shaping it's more like "measure 20 times, cut 19 times".
I checked my work many times between strokes with a ruler to make sure it was flat and planar.
I checked it with a caliper to make sure it was the right thickness.
I rubbed it on some flat pavement to verify that it was planar. I found out that truly flat pavement is a rarity.

Step 8: Leading Edge Marking

That was easy. Most of the foil is already shaped, thanks to the flat bottom of the Clark Y section. The blank wasn't quite thick enough, so there's some cloth weave that never got sanded.
I'll fill that in later with bondo. If I needed the strength of full carbon, I would have laminated two blanks together for thickness.

Now I'll shape the leading edge. I'm painting the leading edge with white correction fluid to make the guidelines easier to see.

Step 9: Scribe a Guideline

All the deep cuts are totally flat cuts. That's important. A flat facet is easy to measure and check that it's correct. So we cut down to the curve of the foil as a series of flat facets.
I use my calipers as a scribe to mark a guideline on the leading edge.

Step 10: Grind Down to the Guideline

I carve a flat facet down to approximate the leading edge shape.

Step 11: Top Surface Shaping

I grind another facet on the top surface, then I round the facets into the final curve.
The dangerous work is over. I switch to hand sanding with blocks, then hand-sanding with finer grits of sandpaper.
The foil is shaped.

Step 12: Mix Bondo

Warning!: MEKP catalyst is very dangerous to your eyes. Wear eye protection. In industrial arts class they told us it would destroy our optic nerves. Don't breathe it either.

Bondo or "body putty" is a mixture of polyester resin and fine glass beads. Boat shops call it "Fairing Compound" and use hollow glass microspheres instead of the heavier glass beads. Follow the directions. Add the right amount of MEKP catalyst to make it set up. More catalyst makes it set quicker, for small jobs you can use a lot because you don't need as much working time. Catalyst comes in clear and pigmented versions. The pigmented kind is easier to mix fully, but I'm out of that.

Bondo is cheap. Do a test first to see if you guessed right about how much catalyst to add.
Bondo is free for me because I rescue it from toxic dropoff reuse piles.

Step 13: Spread Bondo

Bondo is cheap. Spread it thick. Polyester curing is an anaerobic reaction. The stuff on the surface doesn't cure as well. Work it into the pinholes of the layup.

Step 14: Fair the Foil

The bondo stops flowing before it gets fully hard. You can start carving it when it's still like cheese. That's one reason panel beaters like it so much. They'll use a "surform" tool like a cheese grater on the soft stuff.

I did the rough work with that bondo rasp. I used the wire brush to clean the rasp's teeth. When the bondo was hard enough I switched to the sanding block, then hand sandpaper of finer grits.

Use your hand to check for smoothness. Your hand can detect microscopic features that your eye won't notice.

But It doesn't have to be perfect, because next there's...

Step 15: Spot Putty!

Also called "glazing putty", it's made of nitrocellulose!! A.K.A celluloid, just like:

ping pong balls
old movies
smokeless gunpowder
aircraft dope

It's dissolved in some brain-killing solvent. Wear your carbon mask and leave if you smell it.

This stuff sets up faster than bondo and is much smoother. It's even easier to sand. Use it to fill in the small imperfections left from the bondo. Sand the hell out of it with your various sanding blocks and grades of sandpaper. There's no such thing as a permanent mistake anymore. You have entered the "java virtual machine" of bodywork.

Step 16: Faired!

There it is!
It looks like cancer, but it feels amazing. It's also the proper foil shape.
Next comes paint. In a body shop they'd use three different primers.

Sealer primer: to cap the pinholes in the bondo and keep it from drinking paint.
Filler primer: a high build primer for more smoothing, sanding between coats.
Some other primer: I don't know, these dudes worship Buddha and he gets mad if it's not perfect.

Step 17: Paint!

I used "quick drying fast build blah blah primer" to paint mine. "quick drying" is important, because it's going in the water today. Then I worried about losing it, so I painted a coat of dayglo orange over that.

And then it's off to the dock for...

Step 18: Testing!

The new wing works great!
I rode across to the public dock, explained what I was doing to a toddler there, and rode back.
Thanks to Star for great photos!

Epilog Challenge

Participated in the
Epilog Challenge

Be the First to Share


    • CNC and 3D Printing Contest

      CNC and 3D Printing Contest
    • Puzzles Challenge

      Puzzles Challenge
    • Lamps Challenge

      Lamps Challenge



    11 years ago on Introduction

    I was surprised at the end when you put this foil on the commercial one. I thought you were making this for the Hydrothopter. Did you break the factory foil?

    Is the wooden Hydrothopter so bad you never rode it again after the photos?


    Reply 11 years ago on Introduction

    so many toys so little time. Actually the wooden one is so pretty I hung it up. I left the aluminum ones out on the deck of the Free Yacht, where they get abused and broken, hence needing replacement parts...


    13 years ago on Introduction

    You've been busy with those things! The bike nut in me wants to build a pedal powered one of these, a pedalo bike hydrofoil perhaps...

    There's no such thing as a permanent mistake anymore. You have entered the "java virtual machine" of bodywork.

    I have a CS degree and I'm not sure I understand that >_<


    Reply 12 years ago on Introduction

    what about virtualbox, linux, or BSD

    linux and BSD are very stable, and with the good user permissions system, then you could virtualise one of them in virtualbox!


    Reply 12 years ago on Introduction

    you have a counter strike degree?


    Reply 13 years ago on Introduction

    which is precisely why CS is so over rated a degree. ;) I keeeeeeed


    Reply 13 years ago on Introduction

    I had to look up my own metaphor on wikipedia!: "A basic philosophy of Java is that it is inherently "safe" from the standpoint that no user program can "crash" the host machine or otherwise interfere inappropriately with other operations on the host machine, and that it is possible to protect certain functions and data structures belonging to "trusted" code from access or corruption by "untrusted" code executing within the same JVM. Furthermore, common programmer errors that often lead to data corruption or unpredictable behavior such as accessing off the end of an array or using an uninitialized pointer are not allowed to occur. Several features of Java combine to provide this safety, including the class model, the garbage-collected heap, and the verifier."

    hi Tim Anderson my email is send me thank you for the information you need here from you thanks for helping me ....tim Riohacha City, The Guajira, Colombia

    hello timanderson could you give me the measurements (length, width and thick) of the piece on these developing aguaskipper on this page .. foil profile aguaskipper have a team and I have lost and I must foil manufacture propyl thank you very much for helping me

    piece lost aguaskipper need help.jpg

    13 years ago on Introduction

    Remember to wear a respirator when sanding carbon and fiberglass people, the dust and fibers are really, really bad for your lungs (like can cause chest infections and collapsed lungs). Good to see you wore yours TimAnderson! I'm amazed that so much valuable carbon and fiberglass gets thrown out!


    13 years ago on Introduction

    lol, i didn't even know who wrote this, and when i read it, i already knew it was the guy who did the kombucha video podcast because you write like how you talk. haha!


    13 years ago on Step 12

    Unfortunately the cheap-charlie auto Bondo has diatomaceous earth in it, which absorbs water. (That is why it falls off your car repair if there is rust and wet underneath). Marine Bondo should have fumed silica gel in it- which does not absorb water. Or glass micro balloons, possibly resin microballoons.


    13 years ago on Step 2

    Use Ceramic cutting wheel not carbide tipped steel blade

    That gives me some ideas for when i go to Westport, Also, whats the thing you're riding and what makes it move?