Building a Strongback for the Stripper

The strongback is the foundation upon which the cedar strip boat, canoe or kayak is built. These are commonly called "Strippers". There are different variations of strongback for different types of boat. My strongback is simply a long "box" with legs, above which the forms will be attached.
The strongback needs to be straight and level. Accuracy in this step will ensure that the boat will be built on an accurate foundation.

Here is what the strongback should look like when complete (without it's legs).
Since the longest piece of plywood available is 2440mm and the longest "stringer" length is 3.6m, I need to create joins all over the length. These joins need to be staggered to preserve rigidity.

So easy ...
... or so it seems.
Simply buying plywood, having it cut to size and slapping it all together is no guarantee that you will end up with a straight strongback.
Ask me, because I did it.

1. I had the two plywood sheets (2440mm x 1220mm) cut into strips of 200mm x 2440mm each (twelve strips).
2. I bought the straightest meranti "stringers" (22mm x 44mm x 3000mm) that they had. The pine was so crooked, that the 20% extra price of the meranti seemed a bargain. Even so, the meranti planks were definitely not straight.
3. Figuring that I would "pull" the meranti strips straight by ensuring they mount flush to the edges of the plywood sheets was my intention. I did this with both the sides.
4. Then flush-mounted the covering sheets on the top and bottom, closing the box.
   

I ended up with a mess. On the best "face", the curve in the box meant that from one end to the other, the "Centerline" curved by 30mm.

Firstly, the suppliers did not square the sheets properly before cutting. I did not check this, I assumed they would do a good job since they cut wood every day, all day long.
Secondly, of the twelve sheets, three sheets were "curved" along the edge. Perhaps this picture explains it best as I do not know the terms used to describe this type of flaw.How on earth do they cut like this? That saw they have is so amazing, I cannot imagine how skilled you need to be to get a cut like this. *cough*
This is the main cause of the "bend" in my strongback.
Thirdly, whilst I asked for sheets to be cut to 200mm, some sheets varied from 201mm at one end to 197mm at the other end of the sheet.
Fourthly, neither of the ends of the sheet were squared. When you lay them end-to-end you will never get them aligned straight unless you leave gaps.

So unless I have equipment to do the cutting myself, I am at the mercy of having to deal with "substandard" lumber.

I am now in the process of fixing the strongback.
I have had to resort to purchasing one extra sheet of plywood to replace the rounded sheets. I cannot fix them on my table saw because I am just not equipped with proper fences and slides. If I build a holding jig I would probably be able to fix the curved sheets, but that would mean buying a sheet of plywood to build a jig. Catch 22.
This 3rd sheet I had cut whilst I "supervised" the cutting to ensure he did what I wanted. (The cutter was not very happy with my interference but I had permission from the manager).
Still I have to rely on substandard materials, but with these lastest boards, I have enough straight pieces for two sides. Together with the best of the "substandards" I am confident I can fix the strongback.

My strategy is slightly different this time.
Each "side" of the strongback is made up of either 3 or 4 boards of 200mmx 12mm plywood (varying lengths to ensure staggered joints).
This time I have a center line marked on each board prior to fitting.
I am using a 2m long aluminium straight edge to align the edges of these boards before any screws go in.
Clamp them in position, screw it tight. All edges flush (or as flush as I can get it).
Checking squareness and level, after every screw goes in. It's slow ... painfully slow ... and my back hurts, but if it ends up with a square strongback, I will smile.

I cannot yet say this will resolve my issue, but thus far things seem to be straight and I am 2/3rds done. I hope for the best.