My slot came out so perfect I am
posting pictures with commentary to help other people in the same
boat. This is what the bulkhead looked like after I peeled off the delaminated
wood, filled in the original holes with thickened
epoxy, and built up the bulkhead with
fiberglass to its original thickness. (Half the
fiberglass strips go up and over (under the
plywood reinforcement) onto the underside of the
deck. I also reinforced the other side of the bulkhead with fiberglass.) It was relatively easy to find center and perpendicular - the green lines.
The surface needs to be flat and smooth so I covered a board with waxed paper, smeared a shot of
epoxy on it, and bolted it to the bulkhead until it hardened.
You can see how uneven the fiberglassed surface was - darker red is thicker epoxy. BTW, that is the same batch of resin and hardener that I used for the fiberglass - the hardener changes color quickly.
I started with the biggest, stiffest blade I have - it has the maximum amount of bearing surface and is easy to control.
I went as deep as I could, then switched to a short, stiff blade, 1.5", which performed very well, and went as deep as I could with that, then changed to the extended reach blade to finish the cut. OMG, size matters! I quickly discovered why the long blades are not common. For those of you who haven't used one of these tools yet, the blade oscillates back and forth very quickly, it looks like the attachment is vibrating. So the longer the blade the longer the travel path for the tip of the blade - it quickly got smoking hot! Burnt the epoxy inside the cut!! I bought three blades so I changed blades once a minute to keep things cool. In the picture below you can see the heat stains on the blades - given the position where the burn marks start it looks like 1.5 inches is the max optimum length, but that wasn't enough to finish the job.
Also, the tips came square - the action wore the side teeth down to nubs so they look rounded.
Here is the first cut - this angle of the
photo shows the importance of having a smooth, flat bearing surface for the blades to slide on. The green ticks on the sides are the width of my new chain plate - three inches.
The two holes are test holes I drilled before making my initial post - one hole I drilled trying to use a square as a guide, and the other hole my neighbor eyeballed the drill bit to help me get it aimed straight down. Like the original slot, both are too far aft, and, for most people, it's not good to go there.
Here it is on
deck, the cut, the two test holes, and the scar from the original slot. (The bubbles are because the epoxy got too hot.)
To make the second cut I used a 3/8" piece of
aluminum for a spacer. I was a lazy @ss and just taped it to the bulkead, you can just see the tape along the bottom of the picture. The buzzy marks all over it are from it vibrating under the saw blade. Shoulda drilled some holes in it and bolted it to the bulkhead.
I made the second cut the same way as the first, here it is on deck, thank goodness it was too far forward instead of too far aft.
The green lines are marks to finish the cuts - they show how bad the second cut was. I marked center by inserting a long, thin, stiff straight-edge piece of plastic up the slot, using my center marks on the bulkhead as a guide. Then measured out the rest of the slot from there. Using my 20/20 hindsight I'm not sure it was necessary to make the second cut from below since I also used the piece of plastic through the first slot to orient my drill bit. Drilled a 3/8" hole on each end. (Practiced some language when the drill bit walked on the starboard side.)
Finished the cut from the deck side, and the chain plate penetrated the slot perfectly.
On deck - that's a perfect fit!
Thanks to everybody - especially Vamonos.
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