Most of these types of construction are monocoque and necessarily lightly built, in many cases carbon skins seperated by a constructional foam core. If you screw a fitting down it is being held by less than half a millimetre of carbon and some extremely brittle polyeurathane core - ie by not much at all and the result is very predictable! The specific issue on our project is that sailboats generate massive loads, and skiffs have very big sails!
The solution isn't new - Bolt or screw into a solid patch of carbon fibre, or indeed marine plywood placed where you need the fitting to go. However, simply glueing a patch of good quality marine plywood to the surface is NOT enough (Note: read is NEVER enough!). As loads are applied and released, the fitting will move back and forth - and the result of that is the foam core will slowly get crushed under the pad you have stuck down. In the end the fitting loads will be doing massive damage to your pride and joy - literally in many ways! Eventually a complete rebuild is required.
The trick is to replace the core at the loading area as well - so use an oversized piece of marine ply cut into the surface, such that is sits on the lower skin of the foam sandwich and ends up slightly proud of the existing skin, giving you a neat base for that fitting or whatever.
It's not over yet though - With the plywood insert trimmed to the shape you need and all smoothed off, laminate over it with the same material the top surface is made from - in our case carbon fibre. How many layers? Well a good rule of thumb with light to general loading is to double the existing skin thickness, and to double that with high loadings. The skin is 2 layers of 200g carbon in most places. We'll be putting down 4 layers of 200g as a minimum.
Here's how to cut in a piece of marine plywood and insert it into the carbon/foam sandwich:
1. Mark out the area to be cut and cut through the top skin - usually you can do this with a sharp knife, sometimes it needs a bit more and we use a hack saw blade.
2. Use a chisel to remove the foam core down to the bottom skin.
3. Ensure a good fit of the block of marine ply - which in this case will stand proud of the surface when we are done to provide a base for a fitting.
4. Glue in the block of 'ply using a 50/50 microfibre and microballoon mix in epoxy (we're using West on this one, but SP106 is just as good)
5. Fillet and use peel ply to hold the fillet still, and to leave a good finish for sanding later.
Next we will laminate over with the carbon and vacuum bag it down.
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