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: