There are two Castle-class Offshore Patrol Vessels:
|Armament||30mm or 40mm close-range gun|
|Aircraft||No hanger space but capable of receiving a Lynx, Merlin or Sea King helicopter on its flight deck.|
The Castle Class vessels are the largest dedicated patrol vessels operated by the Royal Navy. They currently operate in one of three roles: North-Sea oil and gas protection, Fishery Protection and as a Falkland Islands patrol vessel. Usually one remains in UK territorial waters undertaking fishery or North-Sea patrols, and the other operates in the South Atlantic. With only two vessels in this class there are numerous occasions when one or both are undergoing refit.
Although the normal complement of the vessels is 42 (maximum of 50) they can accommodate a Royal Marines platoon of up to 25 members. This would be used when they operate in their anti-terrorist role as protector of the North-Sea oil and gas rigs. If terrorists ever took over one of the rigs it would undoubtedly be members of the SBS who would be sent to recapture it as this is one of the primary roles of that elite squadron.
The Castle class have a secondary-warfare role as minelayers. The Royal Navy does not have any dedicated mine-laying craft, but trials have shown that if necessary this class of vessels may undertake such a role. Whilst it is accepted that mines are a dangerous and to some degree "dirty" weapon, they remain a fundamental part of modern naval warfare and the Royal Navy has therefore sensibly kept at least two vessels capable of laying them.
The Castle class does not have a hangar and so does not normally carry a helicopter. However it has a large flight-deck which means that it is capable of operating a helicopter for a short-term period. The flight-deck is capable of receiving any helicopter up to the size of a Sea King helicopter. The uses of such a helicopter could include, the transfer of a Royal Marines platoon (although when a covert landing is required, it is likely that the Royal Marines would use of the RIB boats) medevac operations, along with general provisioning and transfer operations.