I do not have access under the deck. I have old back to back seats now. The floor feels strong, no sagging or anything. I'm not sure if it is marine plywood under there or molded fiberglass. The boat is a 1988 Sunrunner 220. Has anyone used the toggle bolts by Garelick and what has been your experience. Any other things to watch out for? It is a 15'' fixed pedestal. I am 270pounds.
If you have the pedestal stand you could mark the first hole and drill a small hole to see if it's wood or fiberglass. If wood you could use ss wood screws. The ss wood screws might wook for the fiberglass, but I'm not sure about that.
I have seen a two part plate for mounting your seat. You make a hoe in the center and then slide these 2 half moon backing plates in. It gives you great backing. I am not sure that in the long run regular screws would be a good idea for 240 lbs plus any weight you might throw up against it say a wave hit the boat and you were thrown against it
I'd drill a few holes, in the area were the base would be, and check the drill shavings to really see if the wood is rotten, if it is good wood, I'd start with screws into the plywood decking, with coating the hole and screws with a good grade of epoxy, but the better system is using the half moon backing system,provided the florr decking is not rotten or wet