The watertight superstructure provides buoyancy to initiate self righting if the boat should capsize. The hull is based on a design by A McLachlan of Watsons of Glasgow and the fit out and superstructure were designed by the RNLI's own Technical Department. The first Arun, launched in 1971, continued the pattern of introducing faster lifeboats into the RNLI fleet which started with the Waveny class in 1966. Lifeboats with displacement hulls and protected propellers, and limited to a speed of 8-9 knots. The first three Aruns were built with cold moulded wooden hulls.
Subsequent boats have glass fibre hulls with aluminium superstructures.
One Arun class was built in steel in 1985. She is stationed at Aith in the Shetlands. The hull is divided into 26 watertight compartments. Void spaces are filled with buoyant materials, giving reserve buoyancy if the boat is holed. Powerful twin diesel engines provide the propulsion.