1st time coosa board user

[Jeff79Sceptre]

Well, I ponied up and bought a sheet of 1/2" bluewater 26 for the fuel tank platform. I can say that it requires much more epoxy and mat to come close to marine grade plywood strength. I know the debate is endless on this but I'm glad I went with 3/4 marine for the deck on the Sceptre. It's very solid. I would use this for the transom but all the additional work for the deck to support the flexing on this stuff would shun me away. I do like it for it's light weight. 1 sheet was 156.00. Not bad considering I paid 106.00 for 3/4 marine grade.

[heynow2203]

Lay it up with 1808 and 11/2 oz chop on either side it will not flex. Or ever get wet

[erebus]

Originally Posted by heynow2203

Lay it up with 1808 and 11/2 oz chop on either side it will not flex. Or ever get wet

Thats what I did for my replacement fuel tank coffin lid.
3/4 coosa with glass on both sides.
I can stand on it all day in good chop and its solid as a rock.

[Blue_Heron]

Originally Posted by Jeff79Sceptre

...I can say that it requires much more epoxy and mat to come close to marine grade plywood strength.

Laminate cores aren't selected for flexural strength, they're selected for shear strength. It's the glass layup on both sides of the core that provides the bending strength. That's why you can core a hatch lid with a bunch of plywood squares and still have good stiffness.

Also, unless you have mat that's formulated for use with epoxy resin, you should stick with the biaxial stitchmat products when using epoxy. Most mat uses a binder that dissolves in poly and vinyl resins, but not in epoxy.
Dave

[martin]

I have never had an issue with the foam cores flexing At all. I used scrim and scored 13mm,or 1/2" on my decks from Diab I think there's call divinycell. Any way you can't get more flexible then scored foam. I cut the top skin off the deck and left the bottom skin on. Layed an extra layer of 1708 then 1.5 oz mat then wet out good and layed the foam while wet, scrim down score up. VAC Bagged it then one layer of 1.5 oz mat and a layer of 1708. I did sand the foam down to match my floor height . so the core is about 3/8 thick.. But It is extremely strong. No flex whatsoever. even out to the edges of the motor well (inboard) where there is no support. I will finish off with another layer of 1708. after the whole floor is done. and then 1/2" teak sole....all in all I am very happy with this stuff.. plus got the foam at surplus prices from a local guy...old core kits from boat manufactures...

[RidgeRunner]

I used Coosa 20 for core on the sole of my boat. Near zero flex with a couple layers of matt, but I do understand the concern before glassing it.

[Jeff79Sceptre]

I coated it first with epoxy and then laid 2 layers of 1708 on it for the new tank bed. It does flex a little bit I assume that's the glass flexing as it would be considered a finish glass. Am I correct on this? I will get some heavier mat to tab it in better since most of the weight of the tank rest on the outside edges of the bed. This tank holds 165 gallons so making sure it doesn't crash thru is important.

[CoreyTrx]

Jeff that 1708 isn't a finish glass. You're not going to get any mat that is stronger then the 1708. Did you put 2 layers on each side or just on one side? If its just on one side I would put a couple layers on the other and that will really stiffened it up. If its already on both sides I would put another layer or two.

[martin]

Originally Posted by CoreyTrx

Jeff that 1708 isn't a finish glass. You're not going to get any mat that is stronger then the 1708. Did you put 2 layers on each side or just on one side? If its just on one side I would put a couple layers on the other and that will really stiffened it up. If its already on both sides I would put another layer or two.

I agree I alway use two layers on both sides. i try to use alternating strands if i can.. But no mater what i use, I lay 1.5mat on the coosa side then 1708 or 1808 biaxial glass. I always sandwich biax inbetween mat layers. and finish off with an extra layer of 1.5 oz mat for a smoother finish.. this is for any sandwich core I do...

[Jeff79Sceptre]

Unfortunately, I only used it on one side. I really just mimicked the factory layup with the exception of the plywood. I figured I couldn't tab it underneath so why waste the epoxy and mat since there is really no weight in the center of the fuel tank. Most of the weight is closest to the stingers. I'll add some more 1708-6" toward the center from the stingers to help out. I did have Sunshine to weld some tabs for the new tank to keep from laterally moving but I know the will not help from vertical compression from slapping on waves. I can walk on the tank bed now but 800lbs of fuel may crush my layup method.