Extreme Max 3005.3837 Single-Post Horizontal Bunk Guide-On System - 18" Bunk Includes a pair of carpet-padded boat trailer guide-ons perfect for 14' to 16' boats; Fits bunk or roller-style trailer frames up to 3" wide x 4.25" tall* 18" carpet padded bunks are vertically adjustable from 19" to 25" and sit on 96 angled vertical arms with up to 19" (3" trailer frame) or 21" (2" trailer frame) of total horizontal adjustment Zinc-coated for freshwater corrosion-resistance, not intended for use in saltwater or brackish water The size and shape of trailer frames can vary drastically, please measure your boat on the trailer before ordering to ensure a proper fit * Hardware to fit taller trailer frames (5.25" and 6.25") is available, see long description below