Mountbatten class hovercraft
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The Mountbatten class hovercraft or SR-N4 was built by BHC, the British Hovercraft Corporation. BHC had been formed by the merger of Saunders-Roe and Vickers Supermarine in 1966, work on the SR-N4 had begun in 1965 and the first trials had taken place in early 1968.
The SR-N4 was the largest hovercraft built to that date, designed to carry 254 passengers and 30 cars it was 40 m long, weighed 190 tons and was capable of 83 knots and could cruise at over 60 knots.
The craft entered commercial service in August 1968, with the Princess Margaret initially operated between Dover and Boulogne but later craft also made the Ramsgate (Pegwell Bay) to Calais route. The journey time, Dover to Boulogne, was roughly 35 minutes, with six trips a day at peak times. The fastest crossing was made in 1995 at just 22 minutes.
In 1972 the first SR-N4s were converted to allow for seven further car spaces and 28 more passengers. From 1976 two SR-N4s were refitted with new deep skirts and stretched by almost 17 m, increasing capacity to 418 passengers and 60 cars at the cost of an weight increase to almost 265 tons. To maintain speed the engines were upgraded to four 3,500 shp Rolls-Royce gas turbines fitted with enormous 21 feet diameter propellers. The work cost around £5 million for each craft and they were designated Mark IIIs, the improvements allowed them to operate in seas up to 3.5 m high and with 50 knot winds.
The propellers fitted to the Mark IIIs were the largest in the world, at the time of writing. However, many ships have larger "screws". Technically, these screws should not be called "propellers".
The two main commercial operators merged in 1981 to form Hoverspeed, which operated six SR-N4 of all marks. In all operations, while the craft were occasionally damaged there was loss of life only once when on March 30, 1985 the Princess Margaret was blown onto a breakwater at Dover and four passengers were killed. The last of the craft was withdrawn from service in October 2000.