What would be the strongest way to fit US boxes to mount a foil, like this

Thinking to epoxy 10mm divinycell/pvc foam? around the box and leaving the divinycell long enough to epoxy it to the underside of the deck like you would a Power or Tuttle box ?
Or would the conventional way, ie rout out a slot, line the slot with glass, fit Us box and glass over the top be strong enough?

A US box is not very deep, there'd be a lot of compression force against the side of your divinycell. I think you have the right idea, forming a strong channel between deck and bottom to hold the box firm, but I'm not sure 80kg/m2 foam will take the load at the top and bottom of the box.
I'd be happier using a light timber pine or ceder, with the grain running between deck and bottom, it wouldn't need to be as thick, 5mm probably would do if you made a carbon sandwich out of it.

A couple of layers of carbon over the box side and one layer over the board side of the divinycell may be OK, the carbon would spread the compressive load over a much bigger area, that would probably work in 5mm thickness as well.

I'm only guessing here, but it seems to me a foil places huge loads on the fin box.
I think what I would do is build a carbon divinycell box around the US box, so it fits between bottom and deck, making sure it's well attached to both.

^^^^ agreed

I do think lots of glass around boxes will help, especially if lapped over the divinycell. I'd be happy with divinycell but it is not as simple as just sticking the box in a channel and glass over. Check the last thread think put some pics of my rebate around the box so the glass comes up the side then lays over (ends up under the deck glass) it is a lot stronger

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decrepit said..
A US box is not very deep, there'd be a lot of compression force against the side of your divinycell. I think you have the right idea, forming a strong channel between deck and bottom to hold the box firm, but I'm not sure 80kg/m2 foam will take the load at the top and bottom of the box.
I'd be happier using a light timber pine or ceder, with the grain running between deck and bottom, it wouldn't need to be as thick, 5mm probably would do if you made a carbon sandwich out of it.

A couple of layers of carbon over the box side and one layer over the board side of the divinycell may be OK, the carbon would spread the compressive load over a much bigger area, that would probably work in 5mm thickness as well.

I'm only guessing here, but it seems to me a foil places huge loads on the fin box.
I think what I would do is build a carbon divinycell box around the US box, so it fits between bottom and deck, making sure it's well attached to both.

Mike - thanks for the advice.

It probably won't be long till I crack the deep Tuttle out of my Isonic.
When it happens I'll install twin US boxes as securely as I can - I'm not too bothered about adding weight or looking a bit rough.

Would you mind sketching a cross section - I can't quite picture all the details above. Cheers JJ

hmm, not much good at sketching, but here's a try.



So there's a 5mm divinycell box around the us box, the purpose of this is to stop any movement by tying it between bottom and deck.
I would normally router the hole all the way through the board. As Mark says it's stronger if you also rebate the bottom of the board about 0.3mm for 15mm around the hole, then fit the box in with glass into the rebate.

To make the box, I'd cut the 4 pieces of divinycell roughly to size, a bit bigger than needed. then laminate two layers of carbon on one side.
cut some lower density foam the same length and width as the US box. (maybe you can use the stuff you routed out of the board) Then stick the US box, foam and divinycell together, wrap the outside of this with one layer of carbon. Hopefully you've made the size of the hole, so the divinycell box is a neat fit. So it's just a matter of a liberal coating of resin on all surfaces, but a layer of glass around the bottom of the box lapping over into the rebated board bottom, probably easier to put this on the board first and push the box through from the bottom of the board, until it's level. Once resin has set, flush top of box with deck and sand the cloth for about 20mm around it. I don't think the overlap is necessary on the deck, as you can glass over a much bigger area of box. The only thing here is, that inside the divinycell you have soft foam, and it's an area that footstrap plugs are likely to be. This may need a bit more reinforcing, (a small strip of divinycell perhaps), but if not 3 layers of glass should be enough.

Great advice as always, thanx heaps Mike and Mark

Perfectly clear, many thanks Mike

Just thinking overnight. My method above probably doesn't have enough resistance to the box getting pushed in.
Mark's method of wrapping cloth around the US box and onto the bottom of the board, would be better than cloth from the outside of the divinycell onto the bottom of the board. But to achieve this, the US box would have to go in last. That makes the whole procedure a bit more complicated.
So instead of the bottom of the US box resting on soft foam, it needs to be against something hard and very well fixed to the divinycell carbon covering. Or going all the way through to the deck and the glass over the top.
I'll leave that to your imagination.

Taking it all in - thanks again Mike and Mark

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decrepit said..
Just thinking overnight. My method above probably doesn't have enough resistance to the box getting pushed in.
Mark's method of wrapping cloth around the US box and onto the bottom of the board, would be better than cloth from the outside of the divinycell onto the bottom of the board. But to achieve this, the US box would have to go in last. That makes the whole procedure a bit more complicated.
So instead of the bottom of the US box resting on soft foam, it needs to be against something hard and very well fixed to the divinycell carbon covering. Or going all the way through to the deck and the glass over the top.
I'll leave that to your imagination.

Yes it does add a couple of steps but not hard. I should try and find those pics to assist in visualising

I've seen people make custom aluminum power plates, about 1 cm thick, that utilized all the multiple boxes on the board for connection, with a fitting on the surface of the plate for mounting the foil. But you have to be comfortable working with aluminum, you can get it cut to size at a metal shop then drill holes in it yourself for bolts into the various boxes, two bolts per box. I've only seen them, so all the details are beyond my pay grade. The plate spreads out the pressure across the entire tail if you make it rail to rail, but the ones I've seen are held back from the rails about 4 cm. min., but you can make it rectangular like the factory power plates. If the tail is flat around the boxes, it makes it that much easier.
Here's a factory power plate, but it's meant for a power box conversion, just to give you an idea of what they look like, the foam strips are for boards that have some 'V' in the tail.


Here's a foil box that can be fitted onto a custom power plate.



These are from Windance in the U.S., but you may have sources where you are.
Using this method, you wouldn't have to mess with your board at all, unbolt it and you're back to windsurfing.

we have a local sup repair guy who retro fits foils on SUPs for all the shops between newcastle and sydney been doing it for 12 months (over 50 conversions) and not one has pulled out yet..

he buys 2 x www.sanded.com.au/collections/surfboard-fin-system/products/futures-strong-box-fin-box-single and glues them into a square block of divinycell then routs out the hole to fit the block then glasses the block in with a layer of carbon and layers of glass.

it looks neat and works! The futures box was mentioned on the sup foil thread about that it wasnt as strong as chinook, but when speaking to Futures the box has more fiberglass content than chinook and is a one piece molded just like chinook. plus looks a lot neater as its designed to go under glass

worth looking at before going with the US finbox.

^^^^ Yes - but for a beginner the routing is harder and then the sand back along centre really needs a variable speed rotary sander /polisher. Some skill there to keep it all flat.

Same strength with a standard US box and none of those hassles.

I understand if you use a powerplate method with US boxes, it is recommended that you use two boxes minimum, which he has. A single US box isn't quite enough,,,,,,, so I was told.

Has anyone tried one of these ?
Looks like they would solve a few issues with stress distribution, easy alignment and install.

www.rideengine.com/Surf-Foil-Track-Box

I have just started using a PowerPlate in a Tuttle box with the Windance Foil Works Foil (rebranded Slingshot). The PowerPlate does a good job spreading the load along the green foam pads. There's a small hack to be done to it to prevent dimple marks on the base of the board, I can detail them it anyone is interested.

I wouldn't bother with the divinycell.
I would just sink in 4 x 3mm ply wood stringers slightly longer than the 10" boxes just use a circular saw 3mm blade, then epoxy them in, router out the foam and resin in the boxes to the ply, then finish covering over the top with a few layers of carbon.
Done, easy and cheap, the key is surface area, put in plywood atleast 50mm deep.

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scubaste said..
I wouldn't bother with the divinycell.
I would just sink in 4 x 3mm ply wood stringers slightly longer than the 10" boxes just use a circular saw 3mm blade, then epoxy them in, router out the foam and resin in the boxes to the ply, then finish covering over the top with a few layers of carbon.
Done, easy and cheap, the key is surface area, put in plywood atleast 50mm deep.

scubaste the strongest way would be to make the cuts though the entire thickness of the board. That way the timber stingers touch both fibreglass skins of the board.