This is a Black Brant III duckboat that hit a rock and tore a hole in the bottom of the hull.
The wood you see in the center of the hole is the bottom face of the cockpit sole leaving only 1/2" to 2" of area to work between the hull bottom and sole.
I ended up cutting a hole with my cordless skill saw that was about 7"x18" to get out all the damaged wood. The hull thickness is 3/8" so I will add a 1/2" plywood backing plate that is a couple inches larger than the hole I cut out. Because it is an area I can not just screw the block to, I ripped the piece in half and put them in one at a time screwing them in place with 3/4" screws from the bottom of the hull. In the picture below you can see the backing plates dry fit in place.
I then cut a piece of 3/8" plywood to fit the cut out and screwed it to the backing plate. When screwing pieces together in an application like this, it is important not to screw through the bottom piece because there is no way to access it to seal the puncture. That is why I used 1/2" plywood for the backing plate so I had some meat to fasten to. Structurally it did not need to be that thick.
Below are the two backing plate pieces, they need to be well sealed with epoxy prior to install since there is no way to get to them after installation.
The two backing plates are installed with epoxy thickened with cab-o-sil. I screwed several long fasteners into the two pieces (making sure not to go through) so I had something to move and hold them in place with.
Cab-o-sil thickened epoxy is applied to the backing plates as well as the fill piece and the fill pieces are screwed in place. Clean up the squeeze out and let cure.
After the epoxy has cured pull the fasteners and sand the area smooth. Sand through the paint down to epoxy 3" to 4" inches around the repair. Cut a piece of tooling cloth 2" larger than the repaired area and apply. After the epoxy has cured sand and re-seal, repeat if needed. Sand, prime, paint and get it back in the water.
The damaged went through the fiberglass and needs to be re-glassed. The paint and primer around the damaged area needs to be sanded down to epoxy. Glassing over paint should be avoided. There was a gouge in the wood so I filled it with thickened epoxy before applying the fiberglass.
I cut small pieces of glass and applied them with epoxy using a bristle brush, adding pieces to the area until it was all covered with at least two layers of glass. Sand and seal twice before re-painting.
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