Frogmen will begin operating over the next few weeks from the pressurised capsule fitted to the hull of HMS Spartan, a nuclear-powered hunter-killer vessel.

The hangar, installed at Rosyth dockyard in Scotland last year, will allow a dozen or more SBS troopers to swim undetected from Spartan while it remains underwater and travel many miles to their objectives using one of three new mini-submarines, called swimmer delivery vehicles (SDVs), acquired from the Americans.

At present submarines have to surface quietly miles offshore to enable special forces to deploy collapsible kayaks and fast rigid raider boats for a night dash to the coast, or for frogmen to swim slowly but silently underwater.

??The hangar effectively gives the SBS their own little long-range submarine,?? said one navy source.

??There??s much less chance of being detected if you don??t have to surface. They can store much larger equipment on the deck instead of having to squeeze it through a hatchway in the boat??s hull, and they can operate there more comfortably out of the way of the crew. The Americans have been doing this for years. We??re now catching up.??

The hangar, nicknamed Project Alamanda, has been allowed to continue even as the navy prepares to implement the government??s defence cuts, particularly in destroyers. The hangar project has survived because of the emphasis of the Ministry of Defence (MoD) on rapid reaction forces equipped to take on terrorists and enemies in coastal areas.

It is detailed in the navy??s official annual review, Broadsheet, which states: ??A dry hangar will be fitted to HMS Spartan to provide a covert projection and insertion capability for swimmers and swimmer delivery vehicles.??

The MoD does not disclose information about the hangars, but they are believed to be similar to the ??dry deck shelters?? used on several American submarines.

Each measures about 9ft wide by 9ft high and 40ft long and has three compartments, one for the SDV, kayaks, rigid raiders and troopers, another to allow passage into the main submarine and a forward compartment for decompression and treatment of divers.

It is not known if the British hangar is detachable and able to be flown out in an emergency to Spartan ?? as the Americans can do ?? or if the vessel will regularly sail from its home port of Faslane carrying the facility piggyback on its deck.

The navy??s first three SDVs were obtained from America in 1999. They are 22ft long and have battery-powered engines. A pilot, navigator and up to four SBS men ??ride?? the craft, wearing breathing apparatus.

The navy has had experience of such vessels since the second world war, when two-man craft ?? called chariots ?? sneaked through heavy defences to mine enemy ships at anchor.

Lord Ashdown, the Liberal Democrat peer and a former Royal Marine, was a test driver for an early form of SDV developed from a torpedo, but the Americans have gone much further than the submersible mini-tugs made popular by the underwater battle scene in the James Bond movie Thunderball.

Spartan, commissioned in 1979, is due to be retired in 2006, but navy sources said it has just had a refit. With Astute, its replacement, years behind schedule in construction, it may continue as the main SBS submarine. The Astute class, three of which are on order, is designed to accommodate dry hangars from the start.

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