This 12 inch Hot-Dipped Galvanized Keel Roller Bracket is designed to fit 12 inch Keel or Straight Rollers. You will commonly see these mounted to cross-members along the center line of the boat trailer, both towards the rear and front of the boat. You will see a picture of what this bracket could look like once mounted.
The U-Bolts you would typically use for a 3 inch x 3 inch cross member are 1/2 inch x 3 inch x 4 5/16, available above in both Hot-Dipped Galvanized or Stainless Steel. If you are needing a different size check out the Boat Trailer U-Bolts and Trailer Bolts.
Also adjust the height of the roller to raise or lower for best positioning on your trailers cross member. This support roller bracket can be added to a boat trailer that does not currently have them as well. It is not solely a replacement piece. You can used it to help guide your boat trailers keel.