Frame #2 is situated where the hull sections bolt together so I made four. If it were a one piece hull you would only need two of these frames. The bulkheads are of 4mm okoume and the frames made out of 3/4″ sapele. Once again, I cut laps in the sapele frame pieces then everything is glued together with epoxy resin.
After the glue had cured and the clamps removed I drilled the 3″ holes in the upper corners of the frames with a drill press and 3″ hole saw. I have a $99 Delta drill press that I sometimes load up in the van for working on the road, hardly an industrial tool, it does a decent job in most applications for cheap. Even spinning a relatively large drill. If you have problems with your drill press bogging down, slow the speed which will increase the torque and hopefully get you through to the other side. I shortened the pilot bit in the arbor of the hole saw so it was just sticking out about a 1/4″ past the saw teeth, this way I could nestle the hole saw right up in the corner of the frame for a perfectly laid out hole. The drill press and hole saw will tend to spin the work piece around so I brace one side of the frame against the post on the drill press before I start drilling. Clamping the work piece to the drill press table is recommended.
Then I laid out the holes for the bolts that hold the three sections together using the measurements from the plans. I used a spring loaded metal punch to make a dimple for the drill bit to start at.
I started a very laborious process where I took the frame I had just laid out, clamped it to one of the other three frames and drilled an 1/8″ hole at each corner then proceeded to the next frame until each frame was drilled out to 1/8″. Then I took each frame individually and drilled them out to 1/4″ and then 5/16″, the final hole dimension.
Careful layout, believing in your layout, and cutting to the line really pay off in situations like this. That said, my fabrication of these four frames, and drilling the bolt holes was not as precise as I would have liked, so… I chose one of the frames to be my template for the other four. I cleaned up the four edges of the template frame, located one of the other three frames back to back (as they would be bolted together on the complete hull) and slid bolts through the bolt holes not allowing them to protrude through the template frame. I used a large pattern bit in my router with the top bearing riding on the template frame and cutting the work piece to the exact dimensions as the template frame.
Now the hull sections should line up near perfect when bolted together.