Strongback - initial work

While waiting for the weather to warm up enough to safely glue the final pieces of frame D’, I decided to move on to the next major step: building the strongback.

Strongback foundation (cross beams P, A, B, D and E)

The first challenge was simply finding enough wood. There were no suitable leftover boards at the boat yard, so I took a calculated risk and used some of the pine I had originally bought for the boat itself. After working out the numbers, I’m confident I’ll still have enough material. When I purchased the pine, I was forced to buy more than strictly necessary because the boards were sold in bundles, so there should be some margin. Once the hull is flipped and the strongback is no longer needed, I can disassemble it and reuse the wood elsewhere.

Cutting wood for the strongback

Although I’m not using the CNC kit, the plans include drawings for the CNC strongback, so I decided to loosely base my build on that design. I started by cutting all the boards for the outer rectangular perimeter and the cross beams. By now, I’m reasonably proficient with the table saw, and I was able to get accurate and repeatable cuts. I had bought 6×70 mm construction screws specifically for this task, and I used them to assemble the perimeter and fasten the cross beams.

I placed cross beams at the positions of frames P, A, B, D, and E, which gives a fairly uniform distribution along the length of the strongback. One detail that requires attention is which side of the reference line each beam is fixed on, since every frame sits at a specific offset relative to the plan’s baseline. I made a mistake or two here, which meant backing out some screws and repositioning a couple of beams.

Marking the crossbeam positions (both sides at the same time)

Once everything is screwed together, you do get a rectangular shape—but that doesn’t automatically mean it’s square. It’s very easy to end up with a parallelogram. To avoid that, I carefully measured the diagonals and adjusted the structure until they matched. I started at one end, corrected the first section, and then used weights to hold it in place while I moved on to the next section. An alternative method is to mark a centerline on each cross beam and use a laser level to check that all the beams are aligned; again, weights are useful to keep each corrected section from moving while you work through the rest.

Measuring diagonals and using weights to prevent further movement

After the alignment looked good, I added plywood corner gussets as recommended in the plans. These stiffen the structure, but even so, I still noticed some tendency for certain sections to bend slightly. Because of that, I kept rechecking alignment as I went along and continued to do so later when adding the frames themselves—a topic I’ll cover in the next post.

I was pleasantly surprised by how much faster this phase went compared to building the frames. After the long, detailed work of cutting, fitting, and glueing individual frame components, the strongback felt relatively quick and straightforward. With the base structure in place, it also became much easier to visualise the actual size of the boat.

Easy to visualise the boat size

With the strongback ready, the next step is to start mounting the frames in their correct positions. That’s where things really start to come together, and I’ll describe that process in the next entry.