Adding carbon fibre to glass?

[minaret]

Definitely remove the gel. Given the amount of experience I'm seeing here, I'd suggest you pay a pro to do it. Backing plates for a davit should be substantial and aluminum. Never glass in a backing plate of any kind.

[stevensuf]

I have no intention of glassing in the backing plates, only to strengthen the surrounding areas of the transom,

The only reason i was thinking of using carbon fibre/epoxy is i have both spare on board, prices here in antigua for anything boat related or anything full stop are very high.

So far i have removed some glass and used some 5200 to glue on some wood as a kind of stringer to help reduce flex in the panel.

[Roy M]

Think of the installation this way: If you have a lever arm (the davits) being pulled at an angle to the bolts (when the davits are loaded with a dinghy and outboard, etc.), the deck is going to want to flex where the bolts are located. To reduce the amount of flex, you need to stiffen the deck, and to spread the load out as widely around the deck as is reasonably possible. Carbon fiber can be looked at like a piece of steel. When thin, as when epoxied to the underside of the deck, it stiffens it only a bit, but increases the shear strength, not what you really need for this application. You could build up several layers of carbon fiber cloth, at a considerable cost (and applying it upside down, can be challenging), or you can use alternatives to stiffen the deck outwards. Plywood doublers, as large as you can install (both thick and wide), are quite easy to apply with epoxy, and you can use screws from the top or beneath as temporary clamps. Sheet aluminum, say 1/4" thick, can also add considerable stiffness. Long straps of aluminum, or ply, within reason, can spread the load more forward over the existing deck. Just make sure that you bed all your holes with sealant (Sikaflex is good, by the way). I generally place a flexible gasket immediatly between the stantion base and the deck, just to take the pressure that might fracture the glass at the junction of stantion base and deck. It also helps with the expansion/contraction one gets with hot and cold weather on the bolted joint.

[stevensuf]

HI
The davit mounts are vertical on the transom, not through the deck, they go through the transom lockers the glass is about a half inch thick there.

[sailvayu]

Is the flexing side to side or up and down?

[Roy M]

Same deal, just spread the support for the flexing moment over a larger area using the same materials, using the transom instead of the deck.

[zeehag]

what minaret said.

[Andrew Troup]

If you were going to add a high-modulus element to stiffen a panel, it should be spaced away from the panel by some sort of core. Otherwise it's not going to be in a position to do any work.

It need not be a full coverage core: it could be trapezoidal cross-section strips, making a network of stiffening ribs. There would be some advantages in such a configuration.

The ribs would need to be laid out by someone who understood how to design a structure to spread the incoming load without causing stress concentrations, though....

The core should be scarfed into high quality marine ply inserts, in way of the metal backing pads which will be added over the finished layup.

And I endorse what Minaret said, or failing that, get detailed advice from an expert, especially if using high modulus material.

[kirsi]

I agree with what Roy M is saying. He has obviously done this. My boat is an old 1977 that has been completely redone. I have done this a lot with upgrading to larger winches, adding davits, a tabernacle, inner forestay and more .

There are two backing plates. A wood one and a metal one. The wood one should be glassed in. The metal one is not. If you don't need or want the structural analysis, you can do the work yourself. So these instructions are how to do it with out the analysis, though I am sure the armchair kind will follow...

First remove the wooden backing plates you put in. 5200 cannot be used for this purpose. If you are having flex they are not large enough. By the way, thanks on the silkaflex info. I did my refit 6 years ago and had not heard of it.

As sailvayu said, grind down the gel coat to the epoxy layer to get good adhesion. My boat is fiberglass on the inside, so I am writing as someone who has done it to fiberglass, not to gel coat. Adding disclaimers now.

Use a thick piece of marine plywood, 3/4 inch is good. I cannot tell you how large it needs to be as I cannot see your space available. This new plywood plate should be epoxy and fiberglassed on using regular epoxy.

If you want to use the carbon fiber you can, but not necessary. You could put it on one side of the new wood backing plate.

You would then use epoxy on thick fiberglass cloth (or several thinner layers) like a sandwich filling, between the transom and the new plywood. You want to make sure that the space between the transom and the plywood is filled, so depending on the surface of the transom, you may need several layers to fill this space. The fiberglass should be the same size as said plywood or a little large for each piece (layered). Screw in place, let cure, remove screws.

Next glass over the entire plywood piece extending over the edges by several inches. Your epoxy work is done.
Next goes the metal backing plate. This should be substantial. With good size washers. Finish it off with your silkaflex.

[Lagoon4us]

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.

My crossbow would argue with that point, it sports a carbon fibre bow and bends beautifully.

[Ziggy]

I have epoxied non-structural wood to gelcoat and it holds just fine. However, for structural applications, I would remove the gelcoat.

[kirsi]

A couple of pictures of carbon fiber work as described.

[noelex 77]

Originally Posted by Lagoon4us

My crossbow would argue with that point, it sports a carbon fibre bow and bends beautifully.

So would my fly fishing rod

However that is very low modulus carbon fibre in a flexible resin. This is not what the OP has, and if wants to stiffen the structure, is not what he needs.

Optomising the properties of a composite carbon fibre/ fiberglass mix is not easy. It is particarly troublesome when adding small amount of carbon fibre.

In the early days of carbon fibre thre were some problems with glider wings that broke with lower force than equivalent wing without the reinforcement.
In other words the extra carbon fibre actually made the fibergalss structure weaker. An unusual case, but it does show the pitfalls of simply adding carbon fibre without some careful thought and analysis.

[Roy M]

When I was building my curved cabintop and hatches, I used two 1/4" plywood sheets, with bands of carbon fiber tow, like hanks of black hair, laid as if they were straps of steel between the laminated plywood sheets. If you visualize the carbon fiber as flexible steel straps that harden in the epoxy, it may give you a better idea of how to effectively use graphite materials. They provide moderate stiffness in thin sections, but their real strength is when they are are being pulled. It's like reinforcing steel bars in concrete. Okay in shear, but better in tension. I don't know what the compression strength might be, so that metaphor is probably not completely appropriate. If placed correctly, carbon fiber cloth will provide incredible resistance to bending if it is being pulled, not bent at a right angle.

[stevensuf]

Well I roughened up a couple of areas of the gel coat and epoxied on a couple of sheets of carbon fibre just as a tester, the flex has already reduced significantly, I may try adding another layer or two and reinforcing some other sections.