You drill a hole in the wood (although not needed) and the tip will drill into the aluminum of the rib or your flat and screw in. It fast and easy and will be the right size hole.
Great boats. I have done floors on two of them. Used 3\8 and just screwed it down to the braces. Its will form to fit even in the front if needed. My dads was done this way better than ten years ago and its still going strong.
I plan on using those coated self tapping deck screws made for this application. They have wings that bore a hole through the plywood, then snap off when the reach the aluminum and tap thier own hole into the metal. They also sink themselves in the plywood for a flush mount. Check them out! You can get them at any lowe's or home depot. They are called TEK brand decking screws. Also youtube them for a demonstration. Cheers :beer: