Board construction tips -- and learning from failures :-)

I am thinking about building an e-foil board, trying to get my head around the construction of the mast attachment / battery box area. I have build 10-15 foil boards for surf, sup, kite, wing. The way I learned to do the foil installation is to fill from deck bottom to top with high density foam, so that there is a connection between top and bottom, effectively making an I-beam … so the internal foam is resistant to compression and spreading the load. Without that, the tracks can compress into board, then gain leverage, and then pull out. Or you can potentially push mast right through the board. Seeking lighter construction, people replace some of HD foam by making pillars/columns that connection mast tracks to deck.

For e-foils, all the builds I am seeing on here don’t seem to worry so much about these potential failures. Main construction method seems to be to embed two stringers of wood or aluminum in foam extending beyond the box, then inserts into the stringer that are screwed into. But the stringers are not embedded in HD foam, even directly below the box, which seems to present compression potential.

As well, if the battery box is just glassed in at its edges, that seems to present potential for the whole box to be popped through the board with weight of rider pressing down against mast top. I am guessing that tying this into the stringers is needed to spread the load across the board?

I haven’t seen any threads about board failures…so I am assuming these are working out.

If you have any tips or suggestions for such builds, I’d love to hear them. Also, any board failures you’ve experienced that we can learn from?

Yes! You are absolutely right! The best way is to link the lower and upper deck at the place where the mast is attached. But since the bulk of builders use xps blocks, and they are quite dense, even when using aluminum spars only from the bottom, they are not pushed through. On my second assembly with aluminum spars, I crashed into a sandbank at a speed of 30 km.h! And this system has survived! Only a small crack appeared along the spars, which I sealed with epoxy resin. Visiting my friend, who is engaged in the repair of windsurfing and surfing boards, I saw two takuma boards with broken mast attachment points in his work. He complained that even takuma does not connect the lower and upper decks at the place where the mast is attached, and over time they break out not even after an accident, but from time to time!


Thanks Anatoly. A few follow up questions:

re: “The best way is to link the lower and upper deck at the place where the mast is attached”. This is how I was taught to build foil boards. Specific to efoil boards, though, the battery box is above the mast. So does this advice mean using HD foam between the mast plate and the bottom of the battery box? Then the box itself becomes like a header over a window or door in carpentry, spread the load of the mast across the board…does that make sense?

re: “On my second assembly with aluminum spars”. Can you describe further what you did with the aluminum? I picture it as 1" wide strips of aluminum along board bottom, stretching from board tail to halfway up the board. With some kind of insert that that mast bolts attach into. Is that how you did it? Or something different? [related question: how to get the bolt holes watertight]

re: " I saw two takuma boards with broken mast attachment points in his work". Were these efoils? Or regular foilboards? I have seen failures of regular foilboards when the tracks for the mast plate are embedded directly in soft EPS foam, or where they are embedded in a layer of HD foam or divinicel, but then there is soft EPS below that. The EPS deforms over time with compression, causing void, then delam, then cracking, then collapse. Worst case scenario is aluminum and carbon foils sinking to the ocean bottom!

Yes, it was Efoil. Our spars are made of aluminum profile 20x10.In our assembly, the esc stands on the mast, and the battery compartment is shifted forward. And in order to reduce the thickness of the longing, we put a sheet of plywood 3 mm on the bottom in the battery compartment. The thickness of the bottom is equal to the thickness of the langeron plus the thickness of the plywood. Everything is glued together. We did this. Maybe someone else will give advice.

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As they say, a picture tells a thousand words…SUPER helpful. That’s exactly what I envision, along with your explanation … makes perfect sense.

I have read probably 20 threads on efoil builds on here, found it interesting that in the 100s of photos, all but a few skipped showing the actual mechanics of the mast connection point. I am guessing that it seemed obvious, so no need to illustrate it. :slight_smile:

I guess I am hyper focused on this, having experienced failures and not wanting to go there again. Particularly when a failure on an efoil may involve destroying or losing a few $000 in electronics as well as the foil. OUCH!

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This is what I did because I was also thinking of the weight against the foil that the compartment might pop out. And might replace with carbon fiber for my next board build. Haven’t tested it tho. Hope this will work.


That’s awesome!

Question 1) where did you get that black box? looks perfect, saves a lot of fussing around with building something equivalent. Even better if it came with a lid!

  1. Does the wood extend beyond the box? Or end at edges of box? Seems to me it would add strength if it is installed from the board bottom and extended beyond the box, as then the force of the foil pushes against the board’s foam, not just onto the box and the glass holding the box in place. Spreads the foil load to the board itself. However, I assume you have had no failures, so it worked (and so maybe my thinking is overbuilding, as I tend to do)

Thanks for sharing

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  1. I 3d printed then reinforced with fiber glass the outside part of the box. I made the lid using 1/2in marine plywood then wrapped with fiberglass and carbon fiber.

  2. The only thing in between the wood and the mast mount is the fiberglass. I installed threaded inserts to the wood then the wood is fixed to the black box and the foam using epoxy resin. It’s like, I can stand on the black box with lid even without the foam. Foam acts like a floater only like what others are doing for their inflatable boards I guess.


I made it exactly the same way with a 1 inch aluminium spar. However, if I were to do it again, I would do it as light as possible. Also have the electronics inside the board itself.

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Thanks for the input.

“have the electronics inside the board itself”…did you not have a battery/electronics hatch and lid, maybe instead mounted this on the deck like with FoilDrive? Or do you mean you have the hatch holding the battery but the other electronics elsewhere in another part of the board?

I did it in a foto case, however that is overkill, heavy and cramped.

I would build a lightweight hatch right into the styrofoam next time and make a very large hollow interior all glassed over. So you can mount all the electronics in there. That means you only ever need 3 wire throughputs that need to be waterproofed.

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I’ve done the mast connection with 25cm rails screwed and glued into a larger piece of 8mm glass fibre reinforced plywood. It has survived a crash into a rock that broke (off) the mast foot - without any signs of deformation at all so it’s proven to be strong enough.