Working on the Transom
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Getting the layout
The first thing to do was to figure out where to make the cut. While most often the cut is make on the inside of the hull, on the Whaler this wasn't feasible. The stern is made up of molded engine well, bench like section and all that is molded and built into the the hull. That would be a major undertaking to cut away from the hull and then reinstall.
Removing the outer skin
Once the layout of the transom was determined it was time to cut out the outer skin. An 1 1/2 tab was left all the way around to have something to connect the layup to. Since we had to cut out the top of t he transom, we would be able to slide the new ply into place.
Digging out
Frankly, the hardest and most tedious part of this part of the project was digging out the old plywood. Since so much of it was waterlogged and soft initially it went pretty quickly. Unfortunately there was about 1/3 of it that required work with a chisel and a firm talking to.
Using plywood
I decided to use marine plywood as the transom core replacement. You may ask why I didn't use Coosa for this. First, the cost was much more than the plywood by a factor of 2 1/2. Secondly, the Coosa did not seem like it would bend all that easily to match the curve. Thirdly, the original plywood lasted some 45 years so new plywood, properly sealed will easily last for as long as I'll need it.
The layout
Here is the transom with the layout of the interior marked on it.
Tools of the trade
The main cut was done using a 41/2 inch grinder with a cutoff wheel. Remaining cuts were done with a multi tool. The multi tool came in handy for peeling back the layers of old plywood. As you can see it was in pretty bad shape. And of course there are the chisels for reaching in those deep areias.
Bulk of plywood removed.
With the bulk of the plywood removed, you can see all the holes in the transom. None of those had ever been properly repaired. They were simply filled with caulking compound.
Surprise surprise
The plywood did not go all the way down to the lower hull, It stopped about 3 inches shy and the area below it was filled with foam. Not only that, but as we dug things out we ran across a bunch of rubber bands!
Digging out the sides
So on this Whaler, the transom ply goes in behind the pontoon like structures on the side. That all had to be dug out as well. To get to it, another area was cut out of the top on both sides. This allowed me to access the wood to chisel it out. What I found worked best was to use the multitool to cut along the sides to free it up from the fiberglass, and then dig it out. This was a slow process, but it worked and all the old rotted wood was out of the transom.
The curve
Once all the old plywood was removed, it was time to plan for cutting the plywood to replace the old. The transom is about 2 inches thick. There is about a 1/4 inch of layup on each side with a an 1 1/2 - 13/4 layer of plywood. My initial though was to use 2 layers of 3?4 inch marine plywood. But the problem was that the transom is not straight across but has a curve to it. Since there was no way 3/4 inch would bend I opted to go with 3 layers of 13 mm plywood.
Making a template
The local big box store sells thin 3 layer plywood for underlayment. I have found with other projects that it works well for making templates. Once all the old plywood was dug out of the transom I went to work with strips of that plywood and a hot glue gun to make up a template in the shape of the transom. I then traced the shape on the new marine plywood and cut out 3 matching layers to build up the transom.
The layup.
Since there were so many holes in the transom. I cut out pieces of fiberglass cloth and epoxied them over the holes from the inside. This would prevent the epoxy used to lay up the first sheet of plywood from leaking out. After everything was cured, I would then go and fill in those holes with some 1708 patches and fair them out.
I used thickened Total Boat 2:1 epoxy for the layup of the plywood and the 1708. Once the first sheet was in place it was clamped up to match the curve of the hull. I've discovered over time that you can never have too many clamps. I was fortunate that the local discount tool store (HF) had some very deep throated C clamps. Notice the wedges along the bottom edge and up the side to hold the plywood tight.
The result
Three layers of plywood, glued with thickened epoxy, 5 layers of 1708 glass cloth laid up with epoxy and the transom is finished. And it is back to patching more holes!