Flanged seacock installation is improved with the use of a backing block between the valve flange and the hull. Each size fits multiple seacock sizesPress threaded inserts into the hull side of the backing block into the holes that match the seacock flange1/2" thickness is rigid enough for a secure installation but flexible enough to conform to minor hull curvatureWill not swell or absorb moistureMade from an environmentally friendly PVC foam specially formulated for marine applications