Adding carbon fibre to glass?

[stevensuf]

I have added davits to the back of the boat, though i dont like the flex I am seeing on the panels, I have some carbon fibre weave and epoxy, How do i go about adding it, can i just put it on top of the gel coat or do i need to take it back to the glass matt?

[kirsi]

Are you adding it on the deck? Best way is to use them under the deck. If you are already seeing flex, it would be best to use a plate underneath to spread out the stress. The strands are okay, but best is a woven carbon fiber cloth. You could then epoxy it between the deck and the plate.

You would need to rough up the surface well to ensure that the epoxy would stick in addition to cleaning well.

[stevensuf]

The bolts are going through lockers on the transom, with wooden backing plates but the whole panel has a little flex on it, no cracks on the gel coat as yet. I have a 1 meter sheet of carbon fibre cloth, i was going to cut it and put a layer on inside the locker, to hopefully strengthen it and reduce the flex?

[kirsi]

You would use the carbon fiber fabric just as you use fiberglass. If the wood backers are already epoxied in place, you can lay the carbon fiber right on top glassing them in place with as much surface areas possible.

I have used it in multiple areas on my boat and it works great. Great, unless I did not do good enough surface prep. It must bond well to the surface. I would extend it out past the plates as far as you can.

[stevensuf]

do i need to remove the gel coat inside the locker, or just clean/roughen it up?

[kirsi]

How are the backing plates attached? Are they large and much larger than the base of the davits?

I would remove the gel coat,down to the glass. I have never put epoxy on gel coat, but I would think it would not give good adhesion. It is too smooth. Maybe someone with gelcoat\epoxy experience will chime in soon.

[stevensuf]

The backing plates are held on with bolts and a layer of sikoflex as well.

[kirsi]

Silkaflex? Does that have marine construction qualities? I looked it up and it is for roofing and flashing.

Can you remove the old backing plates? What are they made of? How much larger are they than the davit base?

[mcarling]

You want to put the carbon fiber on the side to which the panel is flexing. Putting it on the side away from which the panel is flexing would be less effective.

[stevensuf]

Sikaflex is a very common marine bedding/bonding compound, will come off fairly easily, as for putting the fibre on the outside , that is a no no, would look ugly as sin, just need to know how far into the gel coat i need to go or remove it completely?

[noelex 77]

Carbon fibre is stiff, but brittle. It will break if its forced to flex
There is a danger of adding just a thin layer of carbon fibre without some structural analysis.
The carbon will stop the deck flexing, but in doing so it will take most of the force. If the carbon is thick and strong enough this is not a problem, but if the carbon is only capable of supporting a a small portion of the force it will fail.

[mcarling]

Originally Posted by noelex 77

Carbon fibre is stiff, but brittle. It will break if its forced to flex
There is a danger of adding just a thin layer of carbon fibre without some structural analysis.
The carbon will stop the deck flexing, but in doing so it will take most of the force. If the carbon is thick and strong enough this is not a problem, but if the carbon is only capable of supporting a a small portion of the force it will fail.

Yes, that's correct, depending on the resin that's used. Use a flexible resin and you should be fine.

[stevensuf]

so maybe vinyl ester instead of epoxy would be a better idea?

[kirsi]

Get the flexible epoxy. Works great.

I would rough up the gel coat, and do a small trial piece to see how it sticks. Give it a few days and then check if it really sticks. You can always then grind it off the sample place and take it down more if it doesnot stick.

I have used carbon fiber beams that work well.

[sailvayu]

I do not think just adding a layer of carbon fiber will do you much good in this application. Carbon fiber is great for tensile strength put not as good for shear. Stiffness is more of a result of thickness increasing the moment of inertia. (think I beam) Personally I would look into adding stiffeners in the form of a core or extra thickness to increase the moment. 3 layers of 1808 will be better than 1 layer of carbon. Carbon fiber has to be used with careful consideration to engineering it properties or you might as well just use S glass. Also I would recommend laminating with the same resin as the base. in other words if the boat is built with polyester use polyester. And yes if you are talking interior gel coat remove it. Interior gel has wax added to help cure so you want all of this off before laminating over.