The Rhino-Rack Nautic Stack is the most space conscious kayak loading option on the market. With up to 4 kayaks that can be stacked on your vehicle's roof, one can securely stack these in multiple ways. The vertical design means you can carry up to 4 kayaks fit on a single vehicle's roof. The arms fold when not in use, allowing for optimal room for other multiple car accessories. The bar is made from high strength anodized aluminum alloy, while the base is made from glass reinforced nylon for exceptional strength, low weight, and maximum durability.Compatible with these roof rack crossbars: Rhino-Rack Vortex, Thule: Aero, Wingbar (Aeroblade); Yakima Heavy Duty, Whispbar. Engineered in Australia, built with high quality materials and includes a 5-year warranty. The Nautic Stack includes 2 installation styles: a universal clamp style and c-channel nut and bolt. The clamp system is temporary for rapid application & removal, while the c-channel nut and bolt system is for long term installation. Includes 2 Nautic Stack folding kayak arms (18.1" tall/46 cm), mounting hardware (nuts, bolts, washers, quick release knobs), 2ea 18 ft (5.5m) Rapid Straps with Buckle Protector, 2ea 14.7 ft (4.5m) Rapid Straps with Buckle Protector.