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Ok, so I am about ready to start laying up my first cedar strip board, but I am a little lost as to how much glass to go for.
It is a 12mm core thinning to 4mm at the tips and will be vacuumed on a rocker table
Any suggestions?

Thanks in advance

Take an extra millimeter of thickness off the tips!
This will help give a better flex and reduce springback off the rocker table.
3mm cedar at tips is enough.
As little as 6oz glass on the bottom is enough, but an extra 6oz patch on the middle of the bottom between feet will give a little more resistance to running over shells and rocks, and add a little stiffness, but not too much.

Springback is an issue, cedar is springy.
Plan to lose 1/2 to 3/4 of the rocker off the table.
Make the deck 12oz total, maybe more for reinforcement areas.
Make sure the longitudinal fibers run very straight on the deck and all the way from tip to tip, to help against springback and give the board strength and pop.
Use fast resin and leave the board on the table for 36hrs plus.
If you can postcure on the table, do
Postcure appropriately for your epoxy, typically something like 4-8hrs at 120-160F.
Don't ride it or aggressively flex it until it has cured for a week.
If you have the patience, this will help the board keep its rocker, maximum strength, and springiness.
If you don't have the patience, it will probably be ok, but you might flatten the rocker a little.

any chance that it already has rocker?

i am happy with 4oz, top and bottom on my 12 mm cedar core.
i have not tapered as much as you so
...your mileage may vary

the cedar will also suck up alot of epoxy
if you are not careful, the bag will be counter-productive.

it does smell really nice while you shape it

good luck,
-bill

Thanks for the info.
Unfortunately it has no natural rocker to start with so it will all have to happen under the bag. I didn't realise springback would be so high - from reading other posts I thought it would lose around 25% not 50% plus?
With the glass would 0/90 degrees be the best option or would the use of triaxial help hold the rocker in albeit make the board heavy?

i dont think the amount or flavor of glass will be the biggest factor in the net rocker you end up with.

the cedar is pretty strong and doesnt need much glass to reinforce the board for kite surfing.
it seems like a waste if you add alot of weight just to bend it.

at this point i agree with BWD.
double the rocker of your table compared to the desired value.
let it fully cure on the table before taking it off.
consider glassing the rails,if you can.
post cure, if you can.

i believe the shear modulus of the epoxy provides the force which keeps the 3 layers from sliding past each other and returning to their original position (straight). the longer it cures, the stiffer it gets.
layup a small test piece (glass/wood/glass) at the same time you do the board.
that will let you "keep score" without disturbing your board while it cures.
......or do the test piece first, before wasting lots of expensive epoxy

-bill

It will spring back about 50% from what I have experienced. If you put a big 9mm concave it will hold the rocker better. I think it is very hard to get reproducible rocker with this material unless you really take the time and make a few boards until you get the recipe down.

Good luck.

the longitudinal axis (0 deg) fibers hold the rocker.
People often say expect to lose 20-30% rocker, but they are talking about foam, balsa or paulownia boards.
Foam is not so springy, so only the precast rails are really making foam cores want to spring back to flat.
Cedar, spruce, fir etc. will spring back much more.
You might consider adding a layer of unidirectional glass under the deck glass, or just using "warp cloth" for the whole build.
Warp cloth is fiberglass that has more fibers running the long way.

I've been building a few cedar strip boards lately. My tips are about 3mm, and the center is only 9-10mm. I've added a 9cm wide strip of hemlock down the center of the board for extra strength around the inserts.. I'm using a 19oz triaxial glass and vacuum bagging, and the boards are still fairly light, but not overly stiff. My rocker flattens probably 60%, but I may be impatient and am taking them off too early.

I've built a few 10mm birch plywood boards with 6oz per side and they are very stiff. The cedar seems to have more flex even with more glass, and a nice feel. I'm curious how they'd be with less glass.

Lots of good advice, thank you all very much.
I will ponder it some more whilst making my rocker table and sorting out a pump and let you know the results

Cheers

For a 12mm core 6 ounces of glass on top and bottom will be plenty strong,
The problem is you are trying to put it on a rocker table and it will just spring back almost flat with that thick of a core. When you put a board on a rocker table type mold you want the board to come off exactly the same with no spring back when you pop it off after curing.
I have to go less than 3/8 of an inch core and around 20 ounces of glass on top and bottom.
That's very close to standard for my wood twin tips.
And if you want to use carbon you can go even thinner.
6 ounces of glass on top and bottom is good for a cedar board that's made not to flex at all
like a thick surfboard type hollow cedar boards.
If you do add more rocker to make up for the spring back and the board comes out the way you want, it might twist really bad after ridding it a few weeks with so little glass.
I don't know everything this is just what has happened to me.
Looks good!