The Arun-class lifeboat was a fast all-weather lifeboat designed by the Royal National Lifeboat Institution (RNLI) for service at its stations around the coasts of the United Kingdom and Ireland. They were operated by the RNLI between 1971 and 2008. Many have been sold to see further service in the lifeboat and coastguard services of other countries.

The class takes its name from the River Arun in Sussex, England.

The RNLI's first lifeboat capable of speeds in excess of 10 knots (19 km/h; 12 mph) was the 14-knot (26 km/h; 16 mph) Waveney-class boats introduced in 1967. This was based on an American design, but in 1971 it was supplemented by the Arun class which was designed by the RNLI and gave vastly improved accommodation and increased the speed to 18.5 knots (34.3 km/h; 21.3 mph).^[1]

The first prototype boat entered service at St Peter Port in 1972 but moved on to Barry Dock where it was stationed until 1997. Two more boats were introduced in 1973 and 1974 and then full production started in 1975 although small numbers of Waveney-class boats were still built until 1982. By 1990, 46 Arun-class boats had been launched. The following year saw the launch of the first 25 knots (46 km/h; 29 mph) Severn- and Trent-class boats.^[2]

The three prototype boats were withdrawn between 1994 and 1997, the third of which then went on display at the National Lifeboat Museum. The production series boats were taken out of regular service between 1998 and 2007. While a few have found new uses around the coast of Great Britain, the majority have been sold to other lifeboat operators around the world, predominantly in China, Finland and Iceland^[2] and some further boats were built new for service in Canada and Greece.^[3] Those travelling long distances go as deck cargo on larger ships but those going to closer harbours are generally sailed across under their own power. The first boat to go to Iceland, Richard Evans, was loaded as deck cargo on a container ship but was washed overboard during the passage – the only Arun to have been lost at sea.^[4]

After their RNLI service, 35 boats joined other fleets to continue their service as lifeboats. As of Dec 2023, it is thought that 12 are still in service, with 2 under restoration.

The design was developed for the RNLI by J.A. MacLachlan working for naval architects G.L. Watson of Glasgow. Initially proposed with chines along the hull to disperse the spray and improve stability when underway at speed, this caused a high deck above water which proved difficult when trying to get people aboard from the water, so the chines were dispensed with on the second boat and the deck curved down nearer the water.^[5] The first three boats were built with wooden hulls^[6] and were respectively 51 feet 7 inches (15.72 m), 52 feet (16 m) and 54 feet (16 m) long, the extra length due to a rounded transom. All differed in superstructure design, 52-01 having a raked back superstructure front with the flying bridge at the after end. 52-02 introduced forward raked wheelhouse windscreens, had a unique wheelhouse side window pattern and retained the aft mounted flying bridge. On 52-03, the flying bridge was moved to the forward end of the wheelhouse, the front of which was rounded. From 54-04, the forward flying bridge was standardised and the forward raked wheelhouse windscreens were reintroduced. The side windows were recessed from the superstructure sides. 54-04 had a Glass Reinforce Plastic hull and this was standardised for future boats. 54-04 to 54-07 retained the rounded transon but from 52-08 a square transom was reintroduced. The final external change came with 52-11 which had flush sided forward wheelhouse windows and this was a feature of all subsequent boats. In 1986 52-030 (ON 1100) became the only steel-hulled Arun. It was regarded as the best of the class for seakeeping, although the slowest.^{[citation needed]} The GRP hulls were moulded in blue material in the outer skin. After a while it was found that water was accumulating in the fibreglass which added up to two tons to the weight. The boats had the coloured layer stripped off and replaced by a new one made with clear gel which reduced the amount of water absorbed.^[5]

The large watertight cabin gave it self-righting capability. Two survivor cabins are situated below decks where first aid and emergency equipment is stowed. The hull is divided into 26 watertight compartments as protection against sinking should it be holed. There is also a flying bridge above the main cabin with an auxiliary steering position which can be used when additional height or visibility is required during an operation. The boat won a Design Council award in 1982.^[6]

Two Caterpillar D343 460 hp (340 kW) diesel engines were fitted to boats up 52-14, subsequent boats being fitted with two Caterpillar 3408TA 485 hp (362 kW) . Fuel tanks have 620-imperial-gallon (2,800 L; 740 US gal) capacity which gives an operating range of 250 nautical miles (460 km; 290 mi).^[6]