"Stupid" Epoxy /polyster question...

I know it's one of those "how long is a pice of string" questions, but even so...

On an old Polyester boat with a CSM laminate, if replacing the poly/csm w Epoxi/Biax+mat and wr what would the srenght difference be?

I do realise that it depends on the weights of the fabrics but as I don't know the fiber content/strength on the original laminate I really can't do a proper comparision. So I'd like to have an approximate idea of how much stronger the Epoxi laminate would be.

I know that the answer really isn't as simple as a percentage strength increase as the properties differ in the various modes but anyway...
So please give me a hint.

ie would a new Epoxi Biax+mat and rowing laminate need to have the same thickness as the old one.

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If you think the question is too stupid just SHOOT me ;-P~~~~~

ErikG

 

Laminate Differences

If you're changing to a knitted/woven roving/epoxy laminate from a PE/CSM one, you'll notice quite a difference in strength...stiffness is another issue. Strength is comparable if the thickness and area is the same. The knits and WR will be at least two times as strong as the CSM. The epoxy resin will give you better adhesion to the fibers, elongation, wear resistance, and will contribute little as far as laminate strength (fibers are load carrying).

Stiffness of the laminate depends on the modulus of the materials as well as the moment of inertia (section, or thickness), and this combination is known as the flexural rigidity. A typical 1.5oz CSM hand laid will produce around 0.048" laminate thickness while a typical 1808 knit will be slightly thinner (around 0.045"), and a 24oz. woven roving is about 0.050" thick. The moduli of the knits and the WR is also a bit higher than the CSM, so you might be able to get away with a slightly thinner laminate which will be just as stiff. The thickness of the laminate is a much greater contributor to the flexural rigidity than the modulus so the thickness cannot be that much thinner.

Well, within all my ramblings, it comes down to this: the CSM is weaker, but will be stiffer if it's thicker...

 

Polyester resin is more rigid than epoxy. The information is not enough to calculate what a repair schedule should be. A patch that is more rigid than the rest of the hull will make the joint fail. Also, a more flexible laminate may cause other problems. Why do you want to make a change in the laminate?

 

Well it's already done now so...

One of the reasons for Choosing Epoxi over poly is that the weather was a bit cold, and with West Epoxi you can work at lower temps than w poly. Also Epoxi is a stronger "glue" than polyester.

So, since I choose to use Epoxi and normal mat's don't work w Epoxi, and I had Epoxi and knitted biax/mat from an earlier project, I just needed to get an idea of how thick the new replacement laminate needed to be.
So I ended up doing it the same thickness as the old one, probably creating a somewhat stronger laminate than the old one.

My boat is no a marvel of technology, it's a pretty typical late 70's boat designed for speed and cruising, at least for it's time it did both. Now it'sjust another old boat, only faster than most other boats of the same vintage.
Laminate thickness vary between port and starbord where it should be the same and so on, so it's a boat of it's time.

I know that this does not answer Gonzo's q's. But since I didn't have any other info than that when I started, that was the reason for my question.

Well well

 

I think with a low tech boat like yours the repair is just fine. They are overbuilt anyway. I thought you might've been mixing laminates in some exotic high tech design, which could cause you trouble. Sailing season is started, so have fun

 

In a hurry

I do realise I might have gotten better answers if I was more specific but hey, you live to learn (or do we learn to live?)

Yes the sailing season has started, and I have quite a way to go before the boatinterior is ship shape. And the weather sucks again! The rain is poring down, and I dont have a shed...

Erik