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Ocracoke 20b glass


Markjo
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Been going through the archives looking for an answer to a question. There is some banter back and forth but I want to know. Is 12 oz. Biax sufficient or is 1208 required? I've laid plenty of biax and woven but have never laid  1208 or 1708 using epoxy. Seen plenty of opinions though over the years in regards to csm and epoxy. 

I realize that 1208 is easier to source through numerous suppliers compared to a 12 oz. Biax.

Edjumacate me.

 

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I'll take a stab at this since no one else has yet.

 

I don't know what is sufficient for the application because there is no application information... Then I still probably wouldn't.

 

CSM, as a mat, usually has a binder that holds all the pieces together and the binder is dissolved by styrene, a product not contained in epoxy.  Therefore the binder is never dissolved using epoxy and the fibers never come loose to make fitting work.  There is also some sizing compatibility that causes the resin/fiber bonding to be less than designed.  Stitch mat and some combination products don't have that problem because they're not glued and are therefore compatible with epoxy.  Most combination products that my supplier provides are epoxy compatible.

 

I don't think you will see a big difference laying 1208.  It's a little bit stiffer but harder to pull out of shape.  

 

Can you use 1200 where 1208 is specified?  I had this discussion with Graham.  I heard from others in the business that combination textiles are hold overs from polystyrene products and the mat has no place in epoxy construction.   Graham's response was that while it's true that you don't need the csm for peel strength, it does help getting glass fibers packed closer together and it provides some off axis strength.  But in the end he said that if the csm offends me I could use 1800.

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My GENERAL and LIMITED knowledge of glassing suggests to me that the main advantage you get out of 1208 vs just 12oz biaxial would be the thicker skin of the composite.

 

Essentially, all composite hull construction is about beam theory...you have the fibreglass skins and the hull sandwiched between them.  Just like an I-beam, the skins are taking most of the load, the inner one in compression and the outer one in tension.  If you want to strengthen the structure, you can either thicken the skins, or you can place them further apart by thickening the wood between them.

 

The mat would probably do a bit of both...the skin would get thickened but also the biaxial would be separated a little more by the thickness of the layer of mat underneath. Plus you would get some random directional stiffness, I guess - I don't know enough about CSM to comment very intelligently on that but I'm sure Graham knows what he's talking about.

 

I wouldn't replace 1208 with just 12 because the calculations done on the hull may have shown that it needed 20oz total of glass.  But 18oz biax would probably replace it really well.  Two layers of 12 would be overkilling the replacement.

 

I have seen many situations in which a design specified something like "24 oz total biax over chine inside and out" but the designer didn't really care if you used 2x12oz, 4x6oz, 1x24oz, etc etc.  Well, 1208 is 20oz/yd stuff.  I think as long as you are close to that 20 number, you are very likely fine (given that you are not using woven in place of biaxial or something).

 

Hope this helps.  Also hope this is actually correct because it's not like I do this professionally.

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I can vouch for the CSM being a bit tougher to work with. I used it on the outside of my current build under 10 OZ finishing cloth. It worked fine on the bigger areas but on the chines it was a little tough to get the air out and keep it out. 

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