Posted by JimM on July 18, 2002 at 12:16:47:
In Reply to: structual stiffness posted by jim hamm on July 17, 2002 at 14:14:32:
I wasn't aware that jon boats are so flexible. It seems to me that the sides of the boat should provide plenty of torsional stiffness, unless the boat is deteriorated. I wonder if the sides have adequate gunwales, to resist in-plane flexing? Or maybe the smallest ones have low freeboard so the sides aren't wide enough to be rigid.
Best way to stiffen a rectangular structure is to make a "box beam" out of the bottom. The bottom would be a sandwich of two panels separated by lots of longitudinals which are thoroughly bonded to the panels. If not bonded, the whole thing will twist easily. Imagine a sheet of plywood, which can easily be twisted. Then imagine two sheets of plywood stacked with a slab of foam but not bonded - still pretty flexible. Then imagine bonding them together - VERY stiff. I'm leary of structural foam in a marine environment - tends to soak up water and de-bond. Thus the use of longitudinals made of plywood, 'glass or aluminum, epoxy bonded or even riveted to the panels.
Many boats have box-beams in the bottom. There's a large stitch-n-glue design I want to build which uses this method. Finally, that's how airplane wings are built!