This kit by Shipshape/Ce Smith Co. is designed to help you get your boat centered on your trailer. The kit comes with everything you need to install a set of the guide rollers on your boat trailer. It uses rubber rollers that are pre-attached to the 1 1/2 inch x 1 1/2 inch square guide posts to guide your boat onto your trailer. These things are really handy if you load your boat by yourself. Each post has a total height of 21 inches and they can adjust up to 13 inches out from the frame on each side. You can mount them above or below your trailer frame to reach the exact height that you need. The kit will fit rectangular/square tube or channel iron frames as long as your frame is 3 inches wide x 4 inches tall or smaller. The guide posts are galvanized while the mounting u-bolts are zinc plated to help keep rust from forming. All the mounting hardware is included along with detailed instructions. This roller style of bunk guide allows your boat to be easily directed up onto your trailer without resistance or damage to your boats hull.