Seaspeed was a British hovercraft operator which ran services in the Solent and English Channel between 1965 and 1981, when it merged with a rival to form Hoverspeed.

In 1981, in response to intensifying competition and rising costs, Seaspeed merged with its historic rival Hoverlloyd to create the combined Hoverspeed.

[4] Having approached French railway operator SNCF, the two train companies decided to jointly establish a new venture to enter the market with.

[6] According to authors Robin Paine and Roger Syms, the selection of the SR.N4 had been a foregone conclusion and the predominant reason for the company's formation was to operate such a vehicle.

However, management was keen to align with existing Sealink services (also owned by British Rail) so that, in the event of a vessel's failure (or bad weather conditions), it could be more readily substituted for.

Folkestone was eliminated due to its small harbour, unable to cater for large hovercraft, and high exposure to southerly winds.

[9] SNCF, being keen to support the initiative, offered to build a railway line and station adjacent to the proposed Boulogne hoverport at its own expense.

[10] In August 1968, Seaspeed commenced its cross-Channel services from the Eastern Docks at Dover, England to Boulogne-sur-Mer, France using its first SR.N4 The Princess Margaret.

Despite this, the N500 was beset by design and operational problems that would never be successfully resolved; following a somewhat sporadic service on the cross-Channel route, the N500 was permanently withdrawn during 1983 and later scrapped.

Seaspeed was not the only hovercraft operator that decided to move on the cross-Channel market at the time; a rival company, Hoverlloyd, was also established in the 1960s and launched its own competing route between Calais and Ramsgate.

[12] Despite this rivalry, considerations were made towards mutually beneficial cooperation; at an early stage, Seaspeed's management considered conducting SR.N5 services out of Hoverlloyd's hoverport at Ramsgate.

[19] In early 1976, the British Rail board approved a plan to drastically upgrade the two SR.N4s by lengthening them, achieved via the insertion of an almost 17.1 metre (56.1 ft) long section.

According to Paine and Syms, rival operator Hoverlloyd was interested in the modification, but did not have such work done to their own SR.N4 fleet, reportedly due to a lack of available finances.

[25] Each SR.N4 was powered by an arrangement of four Bristol Proteus gas turbine engines; while these were marinised and proved to be one of the hovercraft's more reliable systems, they were relatively fuel-hungry, consuming significant amounts of aviation-grade kerosene.

[26] As the worldwide oil crisis of the 1970s caused fuel prices to rise sharply, the operation of the SR.N4 became increasingly uneconomic, especially in comparison to slower, diesel-powered ferries.

These combined factors gradually worsened Seaspeed's balance sheet as time progressed and demanded economies of scale, and consolidation of operations.