To mount footstraps on the hull of my I20, I cut 1" square pieces out of a 3" diameter piece of PVC pipe and used that as backing for the bolt instead of flat washers (last time I checked, the inside of my hull was not flat). I superglued a piece of bicycle innertube to the piece of PVC and applied silicone to that. The footstap connections are strong enough to lift the stern of the boat with (I would not try that with just washers, and it does not leak a drop.