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First foilboard

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BWD
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Re: First foilboard

Postby BWD » Thu Aug 17, 2017 8:57 pm

With veneer or a thin plywood deck you could use lighter foam, especially in the middle.
1/8" divinycell is pretty standard for bomber decks over 2#/ft^3 foam, for the whole board to be stronger 2.5# foam.
If you want to push your evident expertise with machine-cut, hand finished boards to the next level, why not go with your usual cut blank and add d-cell composite deck, box insert and hull?
You could get super rigidity and strength if you, for example, laid a carbon patch both under and over the d-cell from rear to front foot positions, with a similar approach on the bottom.
And still use 2# foam.
Or for a quicker build, depending on your hand shaping preferences, the 3 layers of d-cell would be built like the transom of a speedboat, very strong. And stiff if not carved away into deep concaves top or bottom.

TomW
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Re: First foilboard

Postby TomW » Fri Aug 18, 2017 12:50 pm

BWD,

thanks for the suggestions. Im wondering if adding the veneer or D-cell with sandwich would be weight efficient compared to just increasing the laminate thickness on the surfboard foam.

I dont think the the local impact strength on deck is needed, but overall strength of the board to take the stress of foil hitting bottom - board nose hitting water, with my weight driving down on board through my front foot.
forces.png
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TomW
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Re: First foilboard

Postby TomW » Fri Aug 18, 2017 1:54 pm

Using my basic physics. a 85kg body travelling at 30km/hr will exert a force of 705N . But body is rotating around an axis formed by foil wind hitting sandbank under water and body has both forward and downward falling trajectory.
I dont really know how to calculate that, its unclear how much body is following the rotation of the board. But if we assume 50% of the force we are looking at 350 N on the front foot. Thats 350kgs.
Has to be too much because my knee would surely explode if i had 350 kg on it ?

Anyone that can give an estimate of the force on the front foot during this fall ? My guess the board needs to take 250kgs. ( 250N)

BWD
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Re: First foilboard

Postby BWD » Fri Aug 18, 2017 2:07 pm

Thicker skins give resistance against buckling boards. In buckling a board from impact whether foil on reef or wave lip on board, what happens is the skins are sheared from the core and deflected from being in column, allowing a crease, delam or snap.

Composite skins with fiber/structural foam/fiber are just a way to get thick, stiff skins at less weight penalty than glass. Stiffer skins are harder to bend so their fibers get out of column and break. Composite skins also make it ok to use lighter blanks keeping weight under control. Use heavier glass and obviously it is still thinner than light glass sandwiching a couple mm of structural foam. For each hoard size, shape and use there must be sime tipping points that point you toward one build technology or another given your goals for flex/stiffness, durability, weight etc.
From my point of view the challenge would be building composite skins without the whole thing getting too resin-heavy....
I wouldn't sweat the exact physics modeling of a crash snapping board, 250kg and you're likely to be broken too, plus every crash is different.
But consider the board will try to snap when the front edge of the plate makes a stress concentration on the bottom skin. When running aground the front plate edge tries to crush the bottom skin. So Imagine if the plate on your foil stuck out 50cm in front of mast instead of 10. That would decrease the leverage of the mast in a crash from ~10:1, to ~2:1 no snap. But very awkward to carry around. So make a very strong stiff reinforcement around the mast attachment running well forward if it. Ketos boards are a clear example of this...
Last edited by BWD on Sat Aug 19, 2017 9:37 pm, edited 1 time in total.

cosmo
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Re: First foilboard

Postby cosmo » Fri Aug 18, 2017 2:17 pm

When I hit the bottom my front feet always flew out of the strap before the board hit the water.
The force of rotation moment of the board on the mast broke (happend twice) on the front of the tuttle box not on the front strap.

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Re: First foilboard

Postby faklord » Fri Aug 18, 2017 3:25 pm

Hi Tom,
For what its worth I'd go for something based on your "With sheets of Divinycell, I was thinking of gluing together 3 sheets of 10mm to form rocker, cut outline and then shape some bevels on the bottom rails. Otherwise flat bottom and deck"
For a general freeride board, once your flying I don't think the board shape matters too much. Obviously a wide nose with plenty of scoop helps with touchdowns and sharp edges should be avoided. Maybe have a look at the Ketos boards for some inspiration?

For the core I'd go for the H60 or possibly H80 for improved crush resistance.
A while back I built a board similar based on your suggestion but using only 2 layers of 10mm foam (75kg/m3) http://www.easycomposites.co.uk/#!/core ... -core.html
the layup is describe here http://kiteforum.com/viewtopic.php?f=19 ... 95#p956395

This board is still going strong but I have decided it is just too short/light, for me....combined with a light foil (Ketos) I think the lack of inertia makes for twitchy ride for an old man with slow reactions. I'm fine on the same foil on a longer heavier board.
So, I'm now in the process of making another (135 x 48). The main differences are:
1) 1 layer of 200gsm glass rather than two.
2) Carbon patches for plate mount reinforcement now 1000gsm ! and about 50cm long on top, 40cm long on bottom.
3) Core is from one 25mm sheet with nose scoop built in using kerfing.
The lams are complete and now it just needs a filler coat, sanding and painting. Currently just under 2.5kg.

With both this board (and the last) I can support the board at each end and confidently bounce around in the middle, with the board either way up...Not a very scientific test but it does give some confidence.

Finally I suspect that with most foils/boards a real hard grounding will result is something giving, be it the foil, board or you! Given that you have the skills to build/fix a board, it seems that this would be the best bit to 'give'

Anyhow good luck, whichever way you go.

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Re: First foilboard

Postby TomW » Fri Aug 18, 2017 8:58 pm

Thanks people. It's pretty clear my first board failed because of to light top lams. If I'd used my normal 1mm veneer sandwich, it most likely would be alive.

My options are pu surfboard blank with sandwich skins or all H80 foam.

Keep you updated.

TomW
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Re: First foilboard

Postby TomW » Sun Aug 20, 2017 2:51 pm

I rode the Dwarfcraft today. It was very choppy and a messy windswell as wind has been SW 1-11ms for several days. I was on a 7m kite, as usual everyone else on 9s.
At first I could not get up. Nose of board is submarining directly, then if I try to compensate foil is trying to lift but too low speed, so stalling. I'm trying to get going in 60-70cm wind swell that is sometimes breaking. After a few attempts I get going. Ride an hour.
If I do a skimming touchdown, it works great, but a more abrubt touchdown, the nose dives.
I could not do any surface transitions, board nose submarines. Granted it's really rough, my roughest session ever. But still, this board is not forgiving for surface transitions.
In the air, it's great. Good length and width, and I find I can ride much lower to water because touch downs or rather, wave tops coming up to meet the board don't effect forward momentum.

I come in for a break and then go out for another 40 min. Starting is now much easier, but it's still hard to keep the nose above water and not stall the foil.
I do a few heel to toeside carves in the air, but it's so rough, and the 7 m kite is a bit too fast for my timing.

Overall, this is not a board to begin on, and I'm wondering if I made the right choice. It seems board should have 50mm more nose rocker.

I also think the boards angle to water during flight is too much " nose down ". I'm probably used to a more " nose up" angle. This board has no tail rocker at all, so the fuselage is parallel to the bottom. then there's only about 60-70 mm nose rocker left. The deck is also pretty flat all the way forward and this increases the perception that nose is low.
I have my 2016 Fluente strut plate at "2" marking. My rear foot was a bit in front of the rear hook as usual, perhaps I will move plate back 1cm.

I'm thinking to make a plastic shim for mounting under the plate to raise front edge of plate 3-4mm. This will raise nose in flight

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Re: First foilboard

Postby Jyoder » Mon Aug 21, 2017 1:11 am

For anyone still trying to make a board from plywood, I figured out how to add a decent amount of rocket/nose kick without laminating pieces. 1/2 inch Baltic birch. I dunked it in the bathtub for a minute and then propped it on a curb and drove the car on it and let it dry for a couple hours.
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Kamikuza
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Re: First foilboard

Postby Kamikuza » Mon Aug 21, 2017 4:30 am

Gold :lol: good job @jyoder!

Tom--what about allowing just the nose of the board to flex and absorb some of the impact? Is some sort of dampening like they possible?

DC had little volume in the nose, so I'm not surprised it's tough to learn to ride underpowered. I liked it, but for ease of light wind riding my Axis boards is better--wider up front, noisy of the volume under the front straps it looks like...


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