Introduction: How to Make an Electric Hydrofoil Surfboard

By

What is this?
Its just like a surfboard, just with a wing underneath and a motor, and it flies. This instructable covers the surfboard part. The motor and electrics will follow very soon. Let me know if you have any questions!

Materials you will need:

Tools you will need:

Step 1: Plan Out Your Shape.

Obviously my board is a little bit different from ordinary surfboards because of the cavities for the electrics, but you could build any other surfboard the same way.

I decided to design everything in CAD, but you could also just grab a piece of foam and start cutting away everything you don´t need with a hot wire cutter.

Doing it in CAD has a lot of advantages though. You get a really good idea how all the features are coming together, you get perfect semetry if you use a CNC and you can take measurements to double check the model in the real world. CAD is also great to communicate your idea with others.

My surfboard is 150x50x14 cm big. This is the maximum size I could fit on my CNC. So this is about the size of a wakeboard.

The ESC that runs the motor gets very hot, so It will be submerged in a water bath in the small cavity. The big cavity is for the batteries in a waterproof camera box. That box does not need to be submerged, so I added 2 big drainage holes. The plan is that this cavity will drain as soon as I start lifting out of the water. (If I learn how to ride it at all).

Routing the cables is also easier to visualise with CAD. You can see on the third picture how the cables will be routed from the ESC to the mast.


The blank has to be cut in two layers of 40mm foam and one layer of 60mm foam in the middle.

Step 2: A Word About Safety

Just three things I want to mention:

  1. Wear dust protection! It is not difficult or expensive. Get a 20$ silicone mask and you are good to go! You need one set of filters for dust and another one for aerosols from spray cans.
  2. Wear hearing protection. Costs 15$ and does not only protect your ears, but also keeps them warm and makes the sanding process less stressfull.
  3. Wear gloves. I used about 50 sets of gloves for this entire process. Spray paint, and uncured epoxy are very unhealthy for your skin. That cancer type of unhealthy.

Dont save money on these items. They are cheap anyway. Extremly cheap compared to a single doctors appointment. This is an invesment for many projects to come.

Step 3: Cut the Blank

You could cut the entire thing only with a bread knife. I have a CNC, so I used that. It gives me perfect symetry and I don´t have to worrie about transfering measurements.

The Foam is EPS foam which stands for extruded polystyrene. XPS is expanded polystyrene. Could these names be any more confusing? E for Extruded and X for Expanded. Expanded is the rough stuff and known as styrofoam. You can use both. People are arguing which one is better. Apparently they both soak in water. I don´t care.

You can check out the CAM setup in the Fusion 360 Files provided. I think that is easier than me trying to explain everything.

Basically it was cutting out the shape with a 10mm 2flute endmill and then finishing the round surfaces with a 10mm ball nose bit. All cam paths combined took about 5 hours of cutting time. That ended up as 2 days of labour. I could clean the shop in the mean time though.

Step 4: Sand the Blank Smooth.

The blank should be smooth and free of big holes, but still with a lot of rough texture. There is no need to get this perfect. Every sanding process left behind a few more holes and its not worth filling and smoothing them again. You can fill in any cavities later on once you have some epoxy on the surface.

I used an orbital sander with 120 grit sandpaper. I also stapled some sandpaper to a flat piece of wood to sand the vertical surfaces. Sanding the foam is very simple.


Any holes should be filled with a mix of epoxy and micro bubbles. These are tiny tiny hollow glass bubbles that add volume but no weight. Also makes the epoxy less runny.

Step 5: Prepair for Glassing the Bottom.

I just didn´t know how much fibreglass I would need to handle the large forces of the mast acting on the board. So I took the safe route and submerged some 20x20mm aluminium profiles in the bottom. They add a lot of rigidity and give me something solid to bolt the mast head to.

The inside of the profiles was filled with wood. You could also use foam. Overall, I don´t think the extra weight matters a lot. I guess we will see once I go into the water.

The bolt holes received sturdy plastic sleves in foam of conduit pipes. These pipes protect the foam and of course they are also waterproof. You need to make sure that the fibreglass overlaps nicely onto those pipes to make them waterproof.

The washers are there to protect the aluminium from the bolt heads.

Step 6: Add Some Lights.

Why not add some lights? I used a USB powered LED light strip. This was conveniant since a 5V power supply is easy to get and I don´t need to use a remote.

These LEDs will not get painted over ans shite through the fibreglass. Maybe not the brightest LEDs, but I think they will be visible in the water.

Make sure to really secure everything down and test them before glassing. You cannot fix them aftewards.

You could also add them at the very end and glue them to the outside. Not as slick though.

Step 7: Glass the Bottom Patch.

I did this in two steps. First I glassed over all the aluminium at the mast area, and then (in the next step) I covered the entire surface.

Getting the glass to stick tight against the profiles was difficult. A vacuum bag would have helped a lot. Even a space bag! But my method with the cling film and the rubber bands also worked. Every layer of rubber adds a little bit more pressure. It wasn´t perfectly flat at the end, but the sanding and consecutive layers created a strong bond.

The complete glassing shedule is like this:

  • All 5oz or 160g cloth.
  • 1 Layer underneath the aluminium.
  • 1 Layer around the aluminium.
  • 1 Layer on top of the aluminium.
  • 2 Layers around the entire bottom going all the was over the rails onto the top deck.
  • 2 Layers on the top deck going all the way over the rails onto the bottom deck where the mast will be.

    This way there will be 4 layers on the rails which are likely to get damaged during transport.

Step 8: Glass the Entire Bottom.

After the bottom had coured, I sanded it smooth and added 2 more layers of fibreglass. This time going all the way around towards the top. This created a nice homogenous and smooth surface. The red marks are areas that were proud and had to be sanded down. Just a refference for me.

Step 9: Glass the Top.

The top layer was easier than the bottom because there weren´t all those components involved. It was just glass and foam.

I would advise to first fill up the cavities with glass and then add 2 big layers to cover everything a few days later.

To get the glass to cling to the sided of the foam, you can fill up a trash bag with water. The water pressure gently presses the glass against the foam. This worked very well.

The pink die is only to differentiate the different layers. And for the camera.

Step 10: How to Glass the Drainage Holes.

It is almost impossible to get the glass to stick to these holes without air bubbles. I used some balloons that were blown up inside to apply pressure. Worked fantastically well. The surface was extremly smooth. Obviously there is a lot of sanding involved after this.

Step 11: Sanding Sanding Sanding.

50% of this build was planning, 30% was sanding and only 20% the building.

I used a cheap orbital sander with 60 grit sandpaper throughout. I only switched to finer sandpaper for the very last layer of epoxy.


You need to send everything perfectly smooth. Not glossy, but smooth. Feeling the surface with bare hands tells you how much you have left to go. Then you can apply the hot coat.

Step 12: Applying the Hot Coat.

This coat is not really hot. Its just a coat of epoxy. Or rather several coats. This will fill up the fibres and give you a smooth texture that you can then sand flat. You need to add epoxy to the outside, because otherwise you would have to material to sand it flat.

Start by cleaning the smooth surface. Then add a drip edge in form of some masking tape. Without it, the drips would flow to the underside where you would have to sand them flat again.

After everything is clean and prepared, you can add about 250g of epoxy per side. The hoat coat is just like a filler. Repeat this process on both sides of the board.

Step 13: Fill Any Mistakes.

You will probably end up with a few areas with holes. These are created where epoxy does not adhere to the foam. During the sanding process, these bubbles become holes. Sometimes you have to cut them out liberally with a box cutter. I just filled them up with epoxy and micro bubbles. Big areas like the one on the last picture should be filled with a patch or two of glass fibre. Once the epoxy is cured, you can sand it flat again.

Step 14: Insert Screw Insertes

The camera box needs to be strapped to the board. This connection has to be pretty tight. The board will likely fly through the air and experience some heavy impacts. You could use rivets or rivet nuts, but I wasn´t convinced they woul hold up in the fibreglass. If they ripped out, I would probably break some electrics and get water into my board. So I decided to insert long aluminium tubes into the sides of the electronics compartment. Just take a 10mm aluminium rod, cut it to length, drill a hole and then tap it. The fibreglass around the aluminium will bond it well to the foam. Wax protects the threads and can be easily removed later on with a needle.

Step 15: Add a Final Gloss Coat.

Once you think you patched all the holes, you should sand everything smooth again. Chances are, that there are going to be a lot of areas that are not really smooth yet. Thats what the gloss coat is for. Its just like the hot coat, but a second time. Mix epoxy and apply it evenly to all surfaces like you did before.

Step 16: Final Sanding Before Paint.

The brush strokes will probably be visible. My garage was almost freezing over, so the epoxy was very thick. Thats not a problem, but it also did not flow smooth. The sander equiped with 120 grit and then 240 grit paper made every surface super smooth but still matte.

If you are a pro, you would polish it after sanding and reveal a perfectly glossy and sea through layer of fibreglass. I did not. Knowing that you will paint over any patches or mistakes makes this build a whole lot easier!

Step 17: Apply Paint!

I did not use a primer, the paint I bought was made for graffiti, and I am sure that those trains don´t get primer either.

One thing to make sure is to get all the paint from the same brand and to follow the instructions. A test spray would not hurt either.

After the paint had dried for a few days, I covered everything with clear coat from the same company. The texture I got is rather mate than glossy, but I think that works.

Step 18: Add the Wing.

My wing has 4 bolt holes that get screwed to the aluminium rails. I am using a long socket wrench to tighten them. I actually filled these holes completely with epoxy and drilled them out again to make them waterproof.

You should also apply copper grease to all threads to protect them from corrosion.

Step 19: Enjoy Your Work!

All done! Just make sure to cure the paint for a few days. You don´t want to ruin it because you were impatient.

Overall this project is not that difficult. You can learn all the techniqued on the go. The paint will hide any mistakes at the end.

Step 20: Drivetrain and Electrics.

The drivetrain will follow in the next instructable. Stay tuned! More videos coming out too. Thanks guys!

Epilog Challenge 9

Runner Up in the
Epilog Challenge 9