I am still trying to figure out why you think you need 1/4" for the floor. My floor in my jonboat is made of old street signs. It flexes a little between the ribs, but in several years has yet to break (and i weigh 250 on a good day). I would think 1/8" would be more than enough.
I am 6' 5" and 345 pounds
Was thinking 1/4 but may hold off for a better idea .
I'm in the business of working with metal and personally I wonder if 1/4 might be over kill, and hot in the sun if left uncovered. I have wondered about sheets of ABS plastic for this purpose But I think that stuffs pricey also and I really don't know much about it.
!/8" should be plenty, that's .125" and to add support between the ribs just throw down some 1 1/2" foam.
My main fishing buddy tops out over 350 and he was surprised how rigid the floor and the decks are in my boat. I have 2" ribs (old MonArk) so I have 2" foam. If you rivet it down with large head rivets it'll stiffen the entire boat up. Mine rides a lot better in rough water (or barge wakes) and is a lot quieter .... Instead of Bang Bang Bang it's more of a subdued Thud Thud Thud
I'll never go back to wood in an aluminum boat although it's fine if you are on a tight budget. Aluminum is a lot more expensive and takes a bit more skill to work than wood ... But it never rots so I figure I get it all back in longevity and lower maintenance.
Skipping the 1/4" and going thinner , but for sure want to get away from the plywood . After checking the place that my bro in law told me 1/4" was 99.00 a sheet , I found he was a bit off on price but still under 400.00 a sheet . Here is the place .
Yeah 1/8" is plenty , I took out the middle bench of my 16' jon and put a pc 7' 8''x 4' and it is fine . just rivited down no foam did put a pt 2x4 under where the bench was cuz the ribs were spaced farther there . were the floor meets the side i put a pc of that mesh ridge vent for architectural shingles , it lets water thru but not your tackle . As far as price goes I lucked out a friend of a friend works at a scrap yard and he hooked me up with a brand new 4x8 sheet for employee scrap rate it was like $40 . Its amazing how much waste their is in this country but he said people scrap brand new stuff all the time , so I would start looking at scrap yards and the road signs work very well also me and my friends have gone that route too .