Now, I have always assumed that they have these really awesome cutting machines in the plywood factory, calibrated with some kind of ultra high-tech, atomic, quantum-flux process so that every sheet comes out to a perfect 90 degrees.
Indeed, I have built many a wall with OSB and never doubted its squareness. In retrospect, maybe I should have done.
This Georgia-Pacific B-C pine plywood (PS1-09, I think they call it) that I got at Lowes was rife with dimensional inconsistencies.
Of the six sheets I brought home to make the sides of the boat with, five needed some kind of correction with a hand plane.
This end, 1/8" heavy. (except for the top veneer)
This end 1/4" heavy.
That 1/8" in 4 feet becomes 3/4" projected out over a 24 foot boat. No small error!
Even squared, they were different lengths. This one, 1/8" heavy, this one 3/16". I mixed and matched the sheets for the two sides and they came out the same, thankfully.
Then there is the bow in them. I found the curviest sheets impossible to weigh down enough.
Here they are, all glassed and waiting to set up.
Even the amount of weight you see here was not enough to get them to lie flat.
This caused one of my joints to come out all wonky
Luckily, it was alright on one end of the joint and didn't start to get bad until the end you see above. I had to cut that joint apart half way and fix it
Then, someone told me that by wetting one side and drying the other, you can induce the plywood to flatten out. This made the second side go much smoother.