Join Date: May 2016
Location: Deltona, Florida
Your 1/2 inch plywood probably only has three plies. The top and bottom ply grain are the same direction with the middle ply lying cross grain to them.
Even plywood will warp, if the two plies top and bottom want to warp in the same direction.
That's what I'd guess happened.
Funny story along the lines of Steve's comment about glue not curing/drying all the way through. Many years ago, I mounted a seat in my boat, using 5200 Marine Sealant to glue the aluminum of the bench to a plywood base the swivel was mounted to. I'd used the boat a couple of times (maybe three weeks had passed) before taking Wife out for a trip. It was the rear seat. My use was to sit in it and run the motor, I rarely lean back.
Wife is sitting back there, fishing out the back of the boat. Feet up on the transom, leaning back in the chair. I am facing forward when I hear, "Oh, ****!" and this sucking/ripping sound. I get turned around just in time to see Wife and seat fall into the bottom of the boat.
Fortunately, the seat back supported most of her, and she didn't get hurt. That sealant had only cured about an inch or so. I guess it sealed off the air, and prevented any further curing. The ripping sound had been the plywood coming apart for that inch or so, and the uncured glue was the sucking sound.
Funny now, scary for a few minutes then.