The RAM-231 RAM Mount Powder Coated Marine Grade Aluminum Rail Base has a 1.5 inch diameter Rubber Ball designed to sit at right angles to the mounting rail. The Stainless Steel U-Bolt and nuts included will fit a circular rail with diameter from 3/4 to 1.0 inch. 

To mount the rail base, place the top of the base on the rail and then slide the u-bolt around the rail, into the holes in the base top. Now tighten the screws onto the u-bolt until the base is clamped firmly. If you desire, you can now slot the black plactice covers over the exposed ends of the u-bolt.