This is not strictly sailing but still boat related and I figure there would be a lot of knowledge here that would be able to offer assistance.

I have a single rowing shell that requires a repair. The hull is carbon fiber and I recently discovered that there must have been some movement / flex in the rigger (for those not familiar with rowing shells the rigger is the metal fitting attached to the side of the shell that holds the gate that the oar sits in) so that where the bolt goes through the hull it has worn the hole. The bolt hole is now an oval shape rather than a circle (approx 15mm long by 5mm high). Wish I had taken a photo when I was last at the boat but hopefully you get the idea.

My question is about the best way to repair the hole so that it can be re-drilled to receive the bolt. Options I am considering are as follows:

1) Fill the hole with thickened epoxy then re-drill the hole

2) Fill hole with epoxy and fiber glass (can you used fiber glass on carbon?), then re-drill the hole.

3) Fill hole with an epoxy filler (eg International Epifill), then re-drill the hole.

My concern is the limited surface area available for whichever option is used to adhere to. The carbon side of the shell would be about 5mm thick so that's not much to bond to. The rigger needs to sit flat against the outside of the shell and on the inboard side is a brace that runs across the shell to the point where the other rigger attaches. So this also needs to be flush with the inboard surface of the shell. So there are very limited options for using the flat surfaces surrounding the bolt hole to provide additional strength / support to whatever is used to fill the hole itself, but I think the inboard side could accommodate a thin build up of something if required.

If I get down to the boat over the weekend I will take a photo or two and add them. But hoping the knowledge here will be able to make some helpful suggestions.

Thanks for your help.

Hi Matt coming from background of building epoxy windsurf boards years ago but still handy with resin.
Please don't fill with resin or filler you really need to use fiberglass or carbon cloth.
Do you know what resin is used? Epoxy or polyester?
Either way sand with 80 grit where you are going to glass on both sides then your cloth you can cut in circles say 3 staggered layers per side. Wet out your patches with plastic squeegee on hard surface with plastic on top then transfer to repair area.

Good luck

The hull is carbon fibre so it will be all epoxy. Do not have fibreglass resin anywhere near it. I would just use a thickened epoxy using silica or micro balloons with heaps of brass filings or SS filings. Use enough silica to keep the filings from sinking to the bottom. I'm not sure how much material you intend to add. If it's not much then a tube of JB-weld might suffice or even glue a brass tube in with JB Weld.

Is it foam core or solid carbon? Using filler is fine. Just use the right one. A high density load bearing filler. Look up west system epoxy additives and read all the different types of additives and the properties.
Epifil says medium to high density and can be drilled and taped so it should be ok.
Tape a thin plastic to the outside of the shell. Cut up an ice cream container, milk bottle or something. Fill from the inside. Again tape over the filler to get a good finish. Use PVC tape. This may be enough. A layer of glass or carbon on the inside if you want to be sure and the piece will still fit.
I imagine the clamping of the 2 metal pieces holds it in place. Not so much the sheer load on the bolt/carbon. A pic or 2 would be good.

I agree with 515, you need to rebuild the carbon fibre shell as he advised then re drill.

Given the original carbon fibre construction has worn over time there is no way just using fillers will last.

Here is a pic of 2 types of core. Balsa on-top and foam on the bottom. It is normal in boat building to drill a larger hole and fill it with epoxy filler. Then drill the hole. This stops water getting into the core and is stronger in compression and sheer. These have been glassed over and have a backing plate underneath as they are for deck hardware but I have seen and done plenty that have just a solid epoxy filler. Just make sure the washer or fitting is larger than the epoxy plug and you use high density filler or additives. Put a glass patch either side of you want.
It would be good to see a pic or 2. Is it foam core or solid carbon and the fitting.

I'd also go along the lines of 515, but you should feather the existing material so it is as strong as original. For that you need a feather of 1:10. i.e. if the material is 4mm thick you need to feather to 40mm. Definitely don't just fill to a an edge where the hole was. Since it's a nice carbon hull, maybe just a small amount of feathering on the outside. As per tarquin, don't have any core exposed to the hole you redrill, so make sure that is all solid. I'd do it in two stages to make it look nice, first bog up the hole with some filler (add graphite or black dye plus high strength filler). Don't worry about the inside, just get the outside good, you can sand it later. Then after it is set, grind most of that filler out from the inside and feather the area around the original hole. Then glass up per 515. The reason I say to do in two goes is that is is hard to do both sides of a hole at the same time neatly and strongly so you have something to squeeze the resin out from the glass.
By doing the feathering you will also be able to get the repair flush with the existing rather than having a lump.

See below, you might want to be a bit more restrained than I was in repairing this dinghy with your shell, but the same principle, a feathered edge will be as strong as original.

As 515 says, feather the damage out and use fibreglass or carbon to build out, then grind/sand back. Not knowing what resin has been used, or what the core is made of, i'd stick with epoxy as it will stick to most materials, and not be prone to melting foam core material.

i'd also suggest posting your question here:

www.seabreeze.com.au/forums/Windsurfing/General

for Mr Hooper/Mark Australia to see, as they do construction and repair with composite materials all the time. They might have more advice to offer. There are others on there who may offer opinions, some good some bad, but those two actually know what they're talking about.


edit: buy resin from a marine chandlery or a specialist resin/composite shop, not bunnings. Resin manufacturers often offer hardeners for the resin that cure at different rates. Always stick with the standard (or slow hardener if you live somewhere hot) only go for the fast hardener if you live somewhere cold (below 15 degrees consistently) the fast hardener can heat up that much that it will melt/deform whatever is around it in a warmer climate, and give you not much working time.

Lots of good advice here about going filler AND glass. I would also be looking at the other outrigger and all the outrigger bolts. If one is elongating the hole on one side I reckon the other bolts will be elongating too.
Epoxy glue is great at gluing and taking strain, but it can crack easily. Glass is great at handling shock loads. So put the two together and you get a strong fix. It seems to me that the shell thickness is not great enough under the outrigger bolts. In normal boatbuilding we increase the laminate under fittings (often by putting plates of some kind under the laminate which does a similar thing). You could make up little ovals of carbon or glass and have two or three little ovals, each one smaller than the underneath one and make the hole area of the hull skin much more capable of handling the point load.
Carbon is great for global loads but not always nice for point loads in light laminates. The stress concentration of an outrigger and bolt requires some good design and reinforcing. The equivalent in a sailboat would be a chain plate and we never bolt a chainplate to a hull skin without adding extra laminate to the skin behind the chainplate.

Good advice on the epoxy.
From the original description you give there is a fitting on the outside and the support inside that goes from one side to the other. These two are bolted together through the shell. This is where all the strength is and the bolting together of these two pieces strengthens the shell. It is the clamping effect of the bolt or hopefully bolts that holds things in place. Not the sheer load or bolt pushing sideways on the shell. I bet the only reason it is worn is because it came loose and the clamping effect was lost allowing the bolt to push against the carbon. If you try and grind out and build up it will be very difficult to get a flush surface for the fitting to fit flush again. If you don't get it flush you will load this area up and possibly crack the shell. You could easily weaken the structure if it's not done properly.
Cover both pieces in cling film. Put epoxy filler in the hole and bolt back together again. If you can hold or get someone to hold the piece on the outside so you can fill against it that would help. You should be able to fill pretty cleanly with a plastic filler blade. You can tape off the area so you don't get epoxy everywhere. This will give you a slight overfill due to the thickness of the tape,which is what you want. Remove tape before bolting the pieces together.
Make sure you spray the bolt with silicon spray or Wd40 so you can get it back out. If you really want put a layer of glass or carbon on the inside.
Been working on sailboats for over 30 years and have spent lots of time bedding fittings down and repairing elongated holes in various types of structures.
Again pics would help.

Thanks for all the responses, it's been helpful to get the different suggestions.

In answer to some of the questions:

There is no core material in this part of the shell. It's just solid carbon.
That part of the rigger is actually under tension as the pressure is applied to the gate (rowlock) during the stroke.
As Tarquin1 said, the whole rigger assembly is connected and the rigger end plate fits against the outside of the hull with the bolt going through it, the hull and the brace the connects to the other side of the boat where there is a mirror image of this arrangement with the other rigger.

I agree pics would assist. I just haven't been able to get back to the boat to take any.

Sounds like the consensus is the use of building up the whole with small layers of fiber and epoxy resin. I think I will be able to get at least one layer of cloth over the inside of the whole that spreads wider than the hole itself without disrupting the fitting of the brace.

Just one question - does it have to be carbon fiber used to fill the whole or can you use glass fiber over carbon? It's just that I have some glass and would need to find / purchase the carbon and I only need the smallest amount.

Thanks.

Select to expand quote

MattM14 said..
Thanks for all the responses, it's been helpful to get the different suggestions.

In answer to some of the questions:

There is no core material in this part of the shell. It's just solid carbon.
That part of the rigger is actually under tension as the pressure is applied to the gate (rowlock) during the stroke.
As Tarquin1 said, the whole rigger assembly is connected and the rigger end plate fits against the outside of the hull with the bolt going through it, the hull and the brace the connects to the other side of the boat where there is a mirror image of this arrangement with the other rigger.

I agree pics would assist. I just haven't been able to get back to the boat to take any.

Sounds like the consensus is the use of building up the whole with small layers of fiber and epoxy resin. I think I will be able to get at least one layer of cloth over the inside of the whole that spreads wider than the hole itself without disrupting the fitting of the brace.

Just one question - does it have to be carbon fiber used to fill the whole or can you use glass fiber over carbon? It's just that I have some glass and would need to find / purchase the carbon and I only need the smallest amount.

Thanks.

Glass should do, no need to lash out on carbon. You won't have any bonding issues. Glass will be harder wearing than carbon.