Two Layers of 3/4 CSM versus One Layer of 1.5 CSM

[76SeaRay]

Is there any issue with using two layers of 3/4 instead of 1.5 CSM on the floor? I just realized that I had bought a couple of rolls of 3/4 by accident and its not returnable. Will it cost more in resin or will two 3/4 come out about the same as one 1.5?

 

[Lectro88]

Is there any issue with using two layers of 3/4 instead of 1.5 CSM on the floor? I just realized that I had bought a couple of rolls of 3/4 by accident and its not returnable. Will it cost more in resin or will two 3/4 come out about the same as one 1.5?

Not sure why you haven't gotten any replies.
I'm not 100% sure, but here's my 1/2 cents opinion.
There is a binder in csm that is dissolved by poly...
and 1 layer or 3 whatever. you're building to get a certain amount of material.... and resin.
And it requires "X amount" of resign per said ounce. (self explanatory)
The only issue I SEE and that's a Maybe...
If you can get the layers down while still wet or green and you get a good chemical bond, and Not a mechanical bond., I would think the difference in resin would be minimal and not enough to worry about.
The key would be laying in a manner to get both layers down at the same time and wet out properly. not too lean or rich in resin.
And you should be golden.
If your thinking 1 layer and let it dry/cure then the other layer that's a whole different story.

maybe others will chime in and confirm or have a different or better opinion.
This is just my thoughts.

 

[froggy1150]

As long as they are laid as one. The weight of the glass is the same. My biggest issue was doing by myself and keeping a wet edge without a mixer/helper. I had to really hustle but got it done with one minor flaw I had to fix where I lifted the glass back up to reposition and got a permabubble I couldn't roll out

 

[Scott Danforth]

I have used the pour and spread method. pour resin on the cloth, spread with a 12" bondo spreader. for two layers of 3/4, you may have to back off on the MEKP just a bit to slow the kick to work fast enough.

 

[Chris1956]

The binder in CSM is not dissolved by polyester resin. I have heard that it is dissolved by epoxy resin.

Mix the polyester resin light on the catalyst, so you have extra time. A short nap roller works to get the resin worked into the cloth. Fibers of the mat will move around, somewhat, so work carefully. A laminating roller also helps work the resin into the cloth and the air out. It also doesn't disturb the mat strands.

 

[76SeaRay]

Thanks. I was planning one layer of 1708, one layer of 1.5 ounce CSM, and Gelcoat. Eventually I want to get snap in carpet for the high traffic areas. This was before I discovered my mistake with the 3/4 ounce. So, sounds like I can use up the 3/4. I try to use wet on wet as much as possible with 1708 but by wet I mean highly tacky.

Now the question is seaming the glass as it is laid. I plan to use butt joints on the glass to avoid a build up creating a ridge and wear point. I plan to offset the seams for the CSM over the 1708 so guessing if I use two layers of 3/4 that I should offset the seams on those as well as offsetting them from the 1708 seams. Again, I plan gelcoat over the layered glass.

 

[JASinIL2006]

The binder in CSM is not dissolved by polyester resin. I have heard that it is dissolved by epoxy resin.

I think you might have that backwards... I was always told the binder didn't dissolve in epoxy, not poly or vinylester resin. US Composites lists CSM as useful for those resins, but less so for epoxy.

 

[Chris1956]

I would recommend the cloth go over the CSM, for areas that will show, or skip the CSM entirely. CSM can easily develop bumps, compared to cloth. CSM is fine under carpet, as the bumps will not show.

Overlap your seams for strength. You can fill and smooth them later. You don't need to overlap seams on decking, as long as it is not near a joint.

On my recent transom replacement, I used CSM on the inside and between the 2 plywood sheets. I used 4 layers of 10oz cloth on the outside, 1 extra layer on the bottom seam and covered it with gelcoat. After some filler and sanding, it will be nice and smooth.

 

[todhunter]

On the floor I did butt joints on my 1708 where the tabbing met the main piece of 1708 that covered the floor, and overlapped my CSM edges by about 2 inches with torn / hairy edges. You can't see the seams if you do it this way. The gelcoat will fill in the CSM fairly well, though for perfection you'll want to fill and sand. Otherwise, just knock down the high spots after the first layer of gelcoat. And the final gelcoat layer will smooth it out even more.

 

[mickyryan]

just remember all you are gonna do is seal it with the resin and cloth /matt there is no strength gained to speek of the strength of floor is the wood/whatever material is used, so don't go with 1/2 or anything that has alot of give or you risk cracks and ruining your work. csm should be the last top layer because you dont want weave blead through and csm you can knock down pretty quick with a sander to smooth out

 

[Chris1956]

Fiberglass cloth and resin do add a lot of strength, plus waterproofing. Not sure where you get your info.

1/2" plywood is usually strong enough for boat decks. Remember, it is the span of the plywood across the stringers and frames, that dictate the thickness required. Most stringers are less than 2' apart.

 

[76SeaRay]

I went with 1/2 plywood on my deck with floatation foam underneath. No give to the deck at all. I only have the deck over the fuel tank left to put in. I may go to 3/4 inch there as I won't have floatation foam on top of the tank but I am also trying to figure out how to put cross ribs over the top of the tank. The tank has neoprene pads on the bottom and sides. It has tabs in 4 places for attaching to the stringers at the top so maybe I can run cross ribs where those are attached to the tank.

 

[Lou C]

I used stainless L brackets to secure Mahagony cross pieces over my tank. Supports the hatch cover very well. This was done way back in 2006/2007.

 

[Chris1956]

You will want access to your fuel tank. On some newer boats, the OEM casts a cover cored with 3/4" wood, over the fuel tank. That way, about 8 screws allow full access and removal of the tank.
You look to have joists over the fuel tank. It would be best if the cover didn't need them. My SeaRay had a 20-24"(not sure exactly) plywood cover over the fuel tank. It was reinforced with fiberglass cloth and resin to give it additional strength.

 

[76SeaRay]

Your cross pieces is what I had in mind. Mine is a 24 inch span. My original tank cover was glassed into place and not removable. The tank was foamed in so there was foam supporting the floor over the tank. My tank I am installing is a brand new custom fabricated aluminum tank that is epoxy painted and I am not foaming it in place. So not sure why I would ever need access to it as long as I have access covers at the fuel line in the back and fuel fill/sending unit area up front.

 

[Lectro88]

There are dozens of ways to do almost anything.
You could add some support to this lid, as in almost stringers or maybe ribs is another way to say it, attached to the bottom of lid and then you could almost, probably get by with 1/2" ply.. an inch or 2" rib stiffens things up pretty quick... heck 1/2" ribs make alot of difference in the right application.
Using 3/4" Ply, spans 2'(feet) pretty easily in building applications. without the above mentioned scabbing or ribs. And then again I'm notorious for over-building.

 

[mickyryan]

Fiberglass cloth and resin do add a lot of strength, plus waterproofing. Not sure where you get your info.

1/2" plywood is usually strong enough for boat decks. Remember, it is the span of the plywood across the stringers and frames, that dictate the thickness required. Most stringers are less than 2' apart.

i got my info from experience my fat ass cracked a deck that had 1/2 plywood over stringers with csm and woven it cracked ... 5/8 plyweood nope no cracking sounds jumping up and down on it

 

[mickyryan]

they also say just about everywhere that csm does not add strength to anything its the layers of woven and really it depends on direction whether it adds str or not

 

[Chris1956]

CSM is strong in all directions. Not as strong as the equivalent weight of cloth, provided it's fibers are laid at a bias.

Clearly 5/8 plywood with CSM is stronger than 1/2 plywood with the same CSM layer.

 

[kcassells]

Fiberglass Mat
Chopped strand mat is very economical and offers good stiffness. However, for greater strength
and a lower weight product biaxials and woven cloth fabrics are generally used.
All the following mats are compatible for use with polyester resins and vinyl ester resin.

Fiberglass Mat

Wholesale/retail sales of composite and fiberglass materials, including epoxy, adhesives, carbon fibers, urethane foam, urethane casting rubbers

www.uscomposites.com

Deck should be a cloth.