C.E. Smith Post Guide On Boat Trailer Accessory - C.E. Smith Trailer Post Guide-Ons give a clear visual marker to know you're backing, loading, or unloading your boat trailer evenly and properly to avoid costly damage. Unlike plastic guide-ons that fall apart or break with minimal impact, C.E. Smith Post Guide-Ons boats square 16 gauge steel uprights, pre-galvanized for corrosion resistance, and 2-3/8in-dia.