Re-Coring discussion with photos, by Mark Weinheimer - Inner Banks Sails and Canvas (North Carolina).
Here are some process pictures. After picking the boat up off the keel, we turned it over in rope slings suspended from the hoist. I have an improved system in mind for the rollover which I’ll photo document.
I used a router and Dremel tool to remove the damaged core. Use a single cutter bit set to just slightly less than ½”, the thickness of the balsa. Cut the outline of the bad core and score repeatedly across the whole area without removing any material until you have cross hatched down to 1-2” blocks. These will pop off the outer skin with a chisel fairly easily. Clean up the underside of the outer skin with a grinder to smooth the old putty so you can have a good bond with the new core.
Use paper to pattern the new core to the shape required. It’s not necessary to be exact as you will have to use putty to fill around the edges anyway. Pre-wet the edges of the old balsa and the deck with epoxy, pre-wet the new core on the mating face and spread mayo consistency putty on the skin. Press the new core firmly into the putty. Cover with a sheet of plastic and weight it down with anything handy – I have several boxes of small lead pieces that work well. Sand bags, bricks or similar will work fine. Put thin plywood against the plastic, then the weight – it will spread the load more consistently. Let this kick completely.
After removing the weight, grind a taper back around the patch for the new inner skin and lightly smooth out the balsa surface.
The next set of procedures is most easily done with an assistant for mixing epoxy. Using the paper pattern from the core cut the glass for the new inner skin. I used 2 layers of 6 oz fiberglass cloth, which seems plenty strong and leaves a smooth finish for paint. I cut the first layer about 1” bigger than the core and the second about 2” bigger. Mix some more “mayo” putty and pour it into any voids around the new core, thicken it enough to stand and fair in any edges you might have so the glass doesn’t turn any sharp angles. Pre- wet the balsa and ground area and roll the glass in, ridge rolling/squeegeeing the bubbles out and floating excess resin to the top. Immediately set the second layer. This is where having help is nice – one person to work the cloth and the other to mix more resin as needed. Work carefully but quickly – the faster you get the epoxy out of the can and spread out, the longer working time you’ll have. A resin rich surface is fine as it makes the final finish cleaner, but make sure you have rolled the glass firmly down against the core – that’s where the strength comes from. While this was still wet, I added a deck beam to the foredeck and 2 across the cabin top. The deck is pretty flat and as long as I was into this I felt it was smart to stiffen the areas. Ahead of time, I cut trapezoidal strips of Styrofoam 1” thick, 1” wide at the bottom and tapering to ¾” at the top. Roll the top edges with sandpaper for a nice radius. For the deck beam, I used 2 layers of 11 oz unidirectional tape offset from each other so that one layer goes down on the deck on each side and 2 layers overlap on the top of the beam. Finish with a layer of 6 oz tape for a smooth finish. Make sure your last layer is the biggest – less grinding in the end.
Always taper off the ends of the beams to minimize stress concentrations.
For the cabin top, some extra attention seemed appropriate. The core in this area had been soaked and crushed, actually breaking the weld on the plate at the top of the compression post. I took out the aluminum cage and took it to a welder for the new plate. After gluing down the new core, I drilled up through mast step bolt holes and used a 1” hole saw to remove the balsa at each bolt. These were poured full of putty when I puttied the rest of the edges of the core. They will be solid compression posts under the step. (pic#004) The cabin top is ready for sanding and paint.
I’m also going to holesaw and putty all the hardware attachment bolt holes in the deck while I have it upside down. I bought all my materials from Jamestown Distributors in Rhode Island (jamestowndistributors.com) and are available from other sources as well.
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