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Airex/carbon kiteboard build

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phuqit
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Airex/carbon kiteboard build

Postby phuqit » Fri Aug 10, 2018 11:12 pm

I've tried another build yesterday, it's a simple asymetric-shaped clone of my Flysurfer Split2 board, 139x43. I like it, and use it as my main board, but I wanted to see what could be achieved with different materials. So I've ordered some 6mm Airex C70-90 foam, and cut out the outline using the FS board as a template. Marked the holes, used SS flange nuts (which happen to be 6mm tall), covered by masking tape on the bottom and filled the thread with wax for inserts/handle, and poured some epoxy with 20% of milled carbon fiber powder into the fin holders. The core weighed 330g without inserts, and with inserts and epoxy it was 400g.

Then, I've cut the board into 5 pieces, like brokites g5 (look it up on utube), and wrapped each of them on the sides with 5cm wide unidirectional carbon strips, that are almost long enough to reach the fins. This is supposed to form an I-beam which strengthens the construction. While this looks trivial in their videos, it turned out quite tricky to do, since the spray glue I had wasn't doing very good job at fixing the carbon onto the foam. Devil is in the details, I guess. So after each of the 5 core strips was wrapped in carbon on the edges, I decided to simply add another layer of unidirectional carbon on top of the board instead of wrapping each individual core. The 5cm wide strips were 104cm long, so I added 85cm wide layer of 250gr carbon on both top and bottom on the middle of the board. Then, I covered both top and bottom with 300gr biax, oriented diagonally to account for torsional tension. Peel ply, flow mesh, vacuum, started the infusion, and like 1 hour later the whole thing was thoroughly infused. I think I used approx 1kg of epoxy, but with proper alignment of materials, I think it could have been something like 800g.

Pulled the board from vacuum earlier today, and boy it's awesome. Flex is pretty much the same as the original board (I didn't want it to be as stiff as the previous experiment I've done, it's really hard on your knees). Rocker is pretty much spot on. Original board had a convex bottom (why would they do that???), so I chose to do a flat bottom instead. Concave was considered, but I decided not to deviate from the original too much.

Anyway, the board weighs 1781g now. I tried standing on it and flexing it, with ends placed on a few bricks, and it didn't break, so I assume that's a good sign. My wife stood on it and the rocker pretty much evens out under ~50kg pressure. The original bends like 1cm more under similar conditions.

Plan to test it out on Sunday, but winds may be too crazy (24-46knots)... If the thing doesn't break on landing the first jump, I think I'll try another similar build with proper bottom sheet and rails. This was merely a test for flex, so I'm not too concerned it'll break or chip.

There are a few obvious issues though:
1. Bottom has few tiny dryspots (~1x5mm). My guess I should have turned the vacuum pump off as soon as it infused, but I kept it going for another 8 hours or so.
2. The rails are... missing. I wanted to use FG rails idea, but as I started laying it up, it became apparent I have no clue how to do it properly - glass doesn't really bend. I'll need to reiterate the process again.
3. No bottom or topsheet. Top is probably easiest covered by UV-stable epoxy, as it sticks nicely to the top once you remove the peel-ply, and it doesn't add much extra weight. I've ordered bottom nylon sheets from junksupply, but they managed to fsck something up with their shipping provider, so it ain't happening until mid next week. I don't expect this board to last.

Other than that, I'm quite amazed at how much better foam is compared to wood.

Some pics:
IMG_20180809_105920.jpg
IMG_20180810_155137.jpg
IMG_20180810_155146.jpg
IMG_20180810_155052.jpg
IMG_20180810_204517.jpg
IMG_20180810_203857.jpg

BWD
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Re: Airex/carbon kiteboard build

Postby BWD » Sat Aug 11, 2018 2:16 am

Very nice!

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rynhardt
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Re: Airex/carbon kiteboard build

Postby rynhardt » Sun Aug 12, 2018 12:23 pm

Well done on a nice board! Just needs some colour now 8)
phuqit wrote:
Fri Aug 10, 2018 11:12 pm
Then, I've cut the board into 5 pieces, like brokites g5 (look it up on utube), and wrapped each of them on the sides with 5cm wide unidirectional carbon strips, that are almost long enough to reach the fins. This is supposed to form an I-beam which strengthens the construction.
I'm not sold on the engineering principles of the brokite construction, but hey, if it works it works. An I beam needs a web to connect the outer layers, but the web doesn't contribute much apart from keeping the outer layers apart and preventing shear. A core does exactly the same thing, which is why you probably don't need the carbon web (stringer).
phuqit wrote:
Fri Aug 10, 2018 11:12 pm
Pulled the board from vacuum earlier today, and boy it's awesome. Flex is pretty much the same as the original board (I didn't want it to be as stiff as the previous experiment I've done, it's really hard on your knees). Rocker is pretty much spot on. Original board had a convex bottom (why would they do that???), so I chose to do a flat bottom instead. Concave was considered, but I decided not to deviate from the original too much.
My opinion is that slight concave or convex has little effect of the dynamic behaviour of the board, apart from stiffening it. Sure a significant concave will cause the edges to provide more lift than a flat board, but a high aspect fin does a much better job. Besides, the stiffness provided by the concave messes with the flex I try to get, so I prefer flat boards.
Convex hulls would theoretically disperse water better to the sides than a flat or even worse a concave board, so landings would be softer. But that only holds true when you land the board perfectly flat, which almost never happens. If you land even with a slight angle on the board then the water disperses in that direction.
phuqit wrote:
Fri Aug 10, 2018 11:12 pm
There are a few obvious issues though:
1. Bottom has few tiny dryspots (~1x5mm). My guess I should have turned the vacuum pump off as soon as it infused, but I kept it going for another 8 hours or so.
2. The rails are... missing. I wanted to use FG rails idea, but as I started laying it up, it became apparent I have no clue how to do it properly - glass doesn't really bend. I'll need to reiterate the process again.
3. No bottom or topsheet. Top is probably easiest covered by UV-stable epoxy, as it sticks nicely to the top once you remove the peel-ply, and it doesn't add much extra weight. I've ordered bottom nylon sheets from junksupply, but they managed to fsck something up with their shipping provider, so it ain't happening until mid next week. I don't expect this board to last.
1. It sometimes helps to massage during infusion, using a foam roller. This promotes resin flow into all the fibres.
2. I'm not convinced rails are really necessary. I've been building my wakeskates without rails and they get pounded at the park, without any edge issues.
3. Bottom sheet is nice if you ride boots. If you ride straps and don't do beach starts then the bottom sheet is probably not required.

phuqit wrote:
Fri Aug 10, 2018 11:12 pm
Other than that, I'm quite amazed at how much better foam is compared to wood.
shhh.. it's a secret :thumb:

BWD
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Re: Airex/carbon kiteboard build

Postby BWD » Sun Aug 12, 2018 2:23 pm

When considering dynamics of concave and convex shapes you also need to think about how much a change in angle of attack or angle of heel moves the center of lift. Some shapes will help keep the center of lift from moving as far, others may exaggerate the movement, so which shape to choose affects feel. Extend this thinking to where lift is developed along sides of concaves and you can see how concaves can give lift and stability, depending on the specific design.

phuqit
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Re: Airex/carbon kiteboard build

Postby phuqit » Mon Aug 13, 2018 3:26 pm

I've tested the board yesterday, in 30-40 knots wind using Flysurfer Soul 8m kite. Took me like half an hour to get used to a smaller size - smallest I've ridden so far was 12m and 9m inflatable in a couple of occasions. The general impressions are:
-The board is super light. From carrying to the beach, to turning on the water, or during the jump. It feels like a feather. Heel to toe switches are super easy.
-It skids downwind for lighter riders - board is 140x43, so they had harder time applying enough pressure. Was just fine for my 100kg.
-It's still quite stiff. The flex is there, loading and popping is a joy, but you can feel everything that's happening under it. The flysurfer pads are quite hard, so I'm thinking about using RRD RadPads with soft cushions.
-Nothing broke. I've applied a cheap aliexpress protective 'rhino' film on the bottom (my skill sucks, lots of air bubbles left...), and it got scratched just very mildly. Not a good idea to use on corals/stone beaches, but works just fine with sand.
-No rails meant I just joined the top and bottom laminates. The resulting thickness is just over 1mm, and even though I used sanding paper to dull it, it's still somewhat sharp - I wouldn't want to be hit by the board going with speed. This needs some kind of resolution.
-Stance was great, max speed was 46km/h, did few jumps over 7m (could have easily done twice that, but new kite/board/extra strong wind kinda messed my courage up).


In general, I'm quite happy with the result. I pushed the board hard, really hard, and it held. I'll build the next one with a proper bottom sheet from isosport, abs rails, and one less layer of unidirectional carbon - 250g/m between the outsides of the pads + 5cm, and 300 biax over the whole board.
IMG_20180811_150651.jpg
IMG_20180811_150627.jpg
IMG_20180811_150618.jpg

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rynhardt
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Re: Airex/carbon kiteboard build

Postby rynhardt » Mon Aug 13, 2018 9:08 pm

Seeing as you have a working build process, you should be able to prototype new boards fairly easily.
One thing that I think is worth trying at least once is a an oval shaped board like the Nomad wave twinny.
Keep the flex and flat profile.

phuqit
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Re: Airex/carbon kiteboard build

Postby phuqit » Mon Aug 13, 2018 9:30 pm

One more question - would it make sense to make the rails using a square fiberglass rope as a filler? Glue it to the edge, make sure it's flush with the edge, and laminate with infusion. I'm thinking, something along the lines (pun intended) of 4x4mm that people use for fireplaces:

Image

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rynhardt
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Re: Airex/carbon kiteboard build

Postby rynhardt » Tue Aug 14, 2018 9:47 am

phuqit wrote:
Mon Aug 13, 2018 9:30 pm
One more question - would it make sense to make the rails using a square fiberglass rope as a filler? Glue it to the edge, make sure it's flush with the edge, and laminate with infusion. I'm thinking, something along the lines (pun intended) of 4x4mm that people use for fireplaces:
Sounds like it could work, if it doesn't collapse under vacuum. Do some test pieces.

Something else I've been thinking of using for rails are hot melt adhesives, but I haven't tried this yet.

phuqit
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Re: Airex/carbon kiteboard build

Postby phuqit » Tue Aug 14, 2018 10:16 am

Hot glue is crap IMHO, at least the ones I've tried - it's soft, hard to apply uniformly, expensive and probably won't stick to epoxy. There might be different kinds though.

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rynhardt
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Cloud C2 17
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Xelerator door
Hoff DD140
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Re: Airex/carbon kiteboard build

Postby rynhardt » Tue Aug 14, 2018 2:13 pm

What about something like Soric, which is a polyester bulker mat. It should fill up nicely with resin and won't compress under vacuum.
Comes in a variety of thicknesses as well.
https://lantor.com/lantor-soric/


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