Each side is 1 1/4" angle.The angle goes inside the gunwales instead of the outside.Each have one hole drilled right through the center and through the gunwales.Two 1/4" bolts with wing nuts hold the whole thing on.It's solid as if it had been welded to the boat.The front is 1 1/4" flat and it's not bolted.
Catfish, the steel itself isn't that much heavier than aluminum piece for piece (1x1x16ga. vs 1.5x1.5x.125 alu). Also plywood is about the same weight as a .125 sheet of aluminum but a heck of alot cheaper. The main way you would save weight by using aluminum is by using less bracing and using the top as part of the structure. For the weight savings is it worth the extra cost? For me when I built the deck NO, but someday I would like to go all alu. anyways.